Steel Bumper Systems for Passenger Cars and Light Trucks

Transcription

1 Sixth Edition January 2019 Steel Bumper Systems for Passenger Cars and Light Trucks An in-depth report on steel bumper systems, including information on: Material Properties Manufacturing Product Design

2 Copyright Steel Market Development Institute This publication is for general information only. The information in it should not be used without first securing competent advice with respect to its suitability for any given application. The publication of the information is not intended as a representation or warranty on the part of Steel Market Development Institute - or any other person named herein - that the information is suitable for any general or particular use or freedom from infringement of any patent or patents. Anyone making use of the information assumes all liability from such use. First Edition, June 1998 First Edition (revision), March 2001 Second Edition, February 2003 Third Edition, June 2006 Fourth Edition, May 2010 Fifth Edition, May 2013 Sixth Edition January 2019

3 CONTENTS Contents...i Figures... v Tables... ix Preface... x Introduction... xi Objective... xii 1. Bumper systems and components Bumper systems System selection Metal facebar system Plastic fascia and reinforcing beam system Plastic fascia, reinforcing beam and energy absorbing system Bumper components Plastic fascia Energy absorbers Facebar Reinforcing beam Types of bumper beams Steel reinforcing beams Steel facebars Plastic reinforcing beams Aluminum reinforcing beams Bumper design concepts Sweep (roll formed sections) and depth of draw (stampings) Tailored products Latest benchmark bumper beams Current steel bumper design passenger cars Typical bumper design - North American passenger cars Typical bumper design - North American and Europe passenger cars Current steel bumper design pick-ups, full size vans and sports utilities Auto/Steel Partnership high speed bumper design - North American passenger cars Quantech design criteria for high speed steel bumper system Flow chart for high speed system Bumper design for pedestrian impact Impact tests EuroNCAP leg to bumper impacts with a leg-form impactor Government regulations Design approaches i

4 CONTENTS Design solutions Localized buckling solutions for thin-gauge UHSS bumper beams Roll-formed bumper Hot-stamped bumper Hot-formed bumper Engineered bumper beam solutions for IIHS small overlap tests One piece design solution (Figure 2.9) Two piece design solution (figure 2.10) Steel materials Introduction Automotive steels Conventional steels, interstitial-free and mild steels High-strength steels (HSS) high-strength low alloy (HSLA) steels Advanced high-strength steels (AHSS) Typical properties of steel grades for facebars Typical properties of steel grades for brackets, supports and reinforcing beams Future steel vehicle materials portfolio for automotive applications Elongation versus tensile strength Elongation versus after fabricated yield strength Elongation versus tensile strength for hot-formed steel Yield strength versus strain rate Sheet steel descriptors SAE J2329 low-carbon sheet steel Steel grade Types of cold rolled sheet (table 3.3) Types of hot rolled sheet (table 3.4) SAE J2340 dent resistant, high-strength and ultra high-strength sheet steel Steel grade Steel type Hot rolled, cold reduced and metallic coated sheet Surface conditions for cold reduced and metallic coated sheet Conditions for hot rolled sheet SAE J2947 categorization and properties of dent resistant, structural, high strength low alloy and recovery annealed sheet steels Scope Rationale SAE J1562 zinc and zinc-alloy coated sheet steel Galvanizing processes ii

5 CONTENTS Types of coatings Coating mass Surface quality Coated sheet thickness Coating designations SAE J1562 zinc and zinc-alloy coated sheet steel Carbon sheet steel Boron sheet steel Grade list maintenance SAE J405 wrought stainless steels SAE specification and ordering descriptions ASTM A463/A463M-15 aluminized sheet steel Scope Product use Manufacturing processes Stamping Stretching Drawing Bending Bending and straightening Forming limits Roll forming Hydroforming Hot forming Manufacturing considerations Forming considerations Guidelines for roll forming high-strength steel Guidelines for roll forming ultra-high-strength steel General guidelines for stamping high-strength and ultra-high-strength steels Guidelines for hat sections stamped from high-strength or ultra high-strength steels Rules of thumb for high-strength steel stampings Welding considerations Introduction Gas metal arc welding (GMAW) Resistance welding High frequency welding Laser welding iii

6 CONTENTS 6. Relevant safety standards in North America and Europe United States National Highway Traffic Safety Administration (49C.F.R.), Part Bumper Standard Requirements Vehicle Pendulum corner impacts (See Figure 6.3) Pendulum longitudinal impacts (See Figure 6.3) Impacts into a fixed collision barrier Canadian Motor Vehicle Safety Regulations Section 615 of Schedule IV Requirements United National Economic Commissions for Europe ECE Regulation Requirements Test vehicle Impact device Longitudinal test procedure Corner test procedure Insurance Institute for Highway Safety: Bumper Test Protocol (Version VII) Requirements Test vehicles Impact barrier Full-overlap impact Corner impact Research council for automotive repairs (RCAR) low-speed offset crash test (Low-speed structural test) Requirements Test vehicle Front impact Rear impact Research Council for Automotive Repairs (RCAR) Bumper Test Requirements Bumper barrier Full overlap impact Summary/Conclusions References iv

7 FIGURES FIGURE TITLE PAGE FIGURE 1.1 COMMON BUMPER SYSTEMS 1-3 FIGURE 1.2 COMMON REINFORCING BEAM CROSS SECTIONS 1-6 FIGURE 2.1 DEFINITION OF SWEEP 2-2 FIGURE 2.2 DEFINITION OF DEPTH OF DRAW 2-3 FIGURE 2.3 EXAMPLES OF TAILOR WELDED BLANKS 2-5 FIGURE 2.4 BENCHMARK STEEL BUMPER BEAMS 2-6 FIGURE 2.5 TYPICAL BUMPER DESIGN FOR PASSENGER CARS AND MINIVANS 2-10 FIGURE 2.6 AUTO/STEEL PARTNERSHIP BUMPER DESIGN FOR HIGH SPEED SYSTEM NORTH AMERICAN PASSENGER CARS FIGURE 2.7 EuroNCAP PEDESTRIAN TESTS (2010 CRITERIA) 2-17 FIGURE 2.8 EuroNCAP LEG FORM IMPACTOR 2-18 FIGURE 2.9 EuroNCAP LEG FORM IMPACT CRITERIA (2010) 2-19 FIGURE 2.10 OPTIMIZED ONE-PIECE BUMPER SYSTEM 2-21 FIGURE 2.11 OPTIMIZED TWO-PIECE BUMPER SYSTEM 2-21 FIGURE 3.1 REPRESENTATIVE MICROSTRUCTURE OF CONVENTIONAL AND HIGH STRENGTH STEEL FIGURE 3.2 REPRESENTATIVE MICROSTRUCTURE OF DUAL PHASE AHSS 3-3 FIGURE 3.3 REPRESENTATIVE MICROSTRUCTURE OF TRIP AHSS 3-4 FIGURE 3.4 ELONGATION VERSUS TENSILE STRENGTH 3-11 FIGURE 3.5 INCREASE IN YIELD STRENGTH THROUGH WORK HARDENING (WH) 3-12 FIGURE 3.6 EXAMPLE PROCESSING STEPS FOR DIRECT HOT FORMED COMPONENT 3-13 FIGURE 3.7 STRESS VERSUS STRAIN AT DIFFERENT STRAIN RATES FOR DP 600 (THE DATA AT 1000 s-1 WERE OBTAINED USING THE SPLIT HOPKINSON BAR (SHB) METHOD) FIGURE 4.1 TYPICAL CIRCLE GRID PATTERN 4-1 FIGURE 4.2 TYPICAL CIRCLE GRID PATTERN 4-2 FIGURE 4.3 TYPICAL FORMING LIMIT DIAGRAM 4-4 FIGURE 5.1 a) RULES OF THUMB - SPRINGBACK 5-5 FIGURE 5.1 b) RULES OF THUMB SPRINGBACK 5-6 FIGURE 5.1 c) RULES OF THUMB - SPRINGBACK 5-7 FIGURE 5.2 RULES OF THUMB - DIE FLANGE STEELS 5-8 FIGURE 5.3 RULES OF THUMB - HAT SECTION 5-9 FIGURE 5.4 RULES OF THUMB - RADIUS SETTING 5-10 FIGURE 5.5 a) RULES OF THUMB - COMBINATION FORM & FLANGE DIE 5-11 FIGURE 5.5 b) RULES OF THUMB - COMBINATION FORM & FLANGE DIE 5-12 FIGURE 5.6 RULES OF THUMB - FORMING BEADS 5-13 FIGURE 5.7 RULES OF THUMB - FORMING AN EMBOSS 5-14 v

8 FIGURES FIGURE TITLE PAGE FIGURE 5.8 RULES OF THUMB - EDGE SPLITTING 5-15 FIGURE 5.9 RULES OF THUMB - PART DESIGN 5-16 FIGURE 5.10 RULES OF THUMB - DIE CONSTRUCTION 5-17 FIGURE 5.11 RULES OF THUMB - DEVELOPED BLANKS 5-18 FIGURE 5.12 RULES OF THUMB - TRIMMING 5-19 FIGURE 5.13 RULES OF THUMB - DIE SHEAR 5-20 FIGURE EXAMPLES OF BUMPER BEAM WELDS 5-23 FIGURE EFFECT OF SHIELDING GAS ON WELD PROFILE 5-26 FIGURE MICROHARDNESS PROFILE OF A 1.6-MM DP 980 GMAW WELD JOINT 5-28 FIGURE STATIC TENSILE TEST RESULTS OF TRIP 780 LAP AND BUTT JOINTS 5-29 FIGURE STATIC TENSILE TEST RESULTS OF DP 780 BUTT JOINTS 5-29 FIGURE DYNAMIC TENSILE TEST RESULTS OF TRIP 780 LAP JOINTS AND BUTT JOINTS FIGURE DYNAMIC TENSILE TEST RESULTS OF DP 780 BUTT JOINTS 5-31 FIGURE RSW 5-33 FIGURE RESISTANCES ASSOCIATED WITH STEEL RESISTANCE SPOT WELDS 5-34 FIGURE COMMON RESISTANCE WELDING PROCESSES 5-35 FIGURE RESISTANCE WELDING LOB CURVEE 5-35 FIGURE FIGURE FIGURE SCHEMATIC WELD LOBES FOR AHSS, HSLA, AND MILD STEEL WITH A SHIFT TO LOWER CURRENTS WITH INCREASED STRENGTH RSW WITH AHSS, CURRENT RANGE FOR VARYING ELECTRODE FORCE (Cap Type B 16/6, 6-mm tip diameter, single pulse, 340-ms weld time, 250- ms hold time, plug failures) RSW WITH AHSS, CURRENT RANGE FOR VARYING WELD TIME (Cap Type B 16/6, 6-mm tip diameter, single pulse, 3.5-kN electrode force, 250-ms hold time, plug failures) FIGURE EXAMPLE OF A FULL BUTTON NUGGET 5-38 FIGURE FIGURE EXAMPLES OF COMMON FAILURE MODES IN PEEL OR CHISEL TESTING OF AHSS EXAMPLE WELD SCHEDULE SHOWING CURRENT PULSING, TEMPERING, AND TWO STAGE FORCE FIGURE PULSED CURRENT PROFILE AND WELD LOBE OF 1.6-MM Q&P FIGURE WELD SPOT MICROGRAPH AND MICROHARDNESS OF 1.6-MM DP FIGURE WELD GROWTH MECHANISM OF OPTIMIZED THREE-PULSE WELDING CONDITION vi

9 FIGURES FIGURE TITLE PAGE FIGURE FIGURE FIGURE FIGURE FIGURE SIMULATION RESULTS WITH MICROSTRUCTURES AND HARDNESS DISTRIBUTION FOR SPOT WELDING OF 0.8-MM DC06 LOW-CARBON STEEL TO 1.2-MM DP 600 STEEL LOAD-BEARING CAPACITY OF SPOT WELDS ON VARIOUS COLD-ROLLED STEEL (Steel type, grade, and any coatings are indicated on the bars) REPRESENTATIVE MINIMUM SHEAR TENSION STRENGTH VALUES FOR GROUP 3 STEELS REPRESENTATIVE MINIMUM SHEAR TENSION STRENGTH VALUES FOR GROUP 4 STEELS MINIMUM CROSS-TENSION STRENGTH (CTS) VALUES FOR GROUP 2, 3, AND 4 STEELS FIGURE TENSILE SHEAR STRENGTH OF SINGLE SPOT WELDS 5-48 FIGURE TENSILE-SHEAR SPOT WELD FATIGUE ENDURANCE CURVES FOR VARIOUS HSS (The data are normalized to account for differences in sheet thickness and weld size. Davidson data in the plot refer to historical data) FIGURE EXAMPLES OF HYDROGEN EMBRITTLEMENT (HE) FAILURE IN RSW 5-49 FIGURE ACTUAL EXAMPLE OF THE LME PHENOMENON IN ZINC COATED AHSS 5-50 FIGURE BASIC JOINT DESIGNS FOR HF WELDS IN PIPE AND TUBE 5-51 FIGURE EFFECT OF FREQUENCY ON DEPTH OF PENETRATION INTO VARIOUS METALS AT SELECTED TEMPERATURES FIGURE WELD HARDNESS OF A HF WELD IN A DP 280/600 TUBE 5-54 FIGURE LASER WELDING 5-55 FIGURE POWER DENSITIES OF VARIOUS WELDING PROCESSES 5-55 FIGURE COMPARISON OF TYPICAL WELD PROFILES 5-56 FIGURE KEYHOLE MODE WELDING 5-57 FIGURE LASER BEAM WELDING 5-58 FIGURE FOCUSING OF THE LASER BEAM 5-59 FIGURE IMPACT OF LASER WELD DESIGN OPTIMIZATION ON FRACTURE TYPE 5-60 FIGURE ABSOLUTE STRENGTH AND DUCTILITY OF DP 800 AND DP FIGURE CROSS-TENSION TESTING OF DP 800 AND DP FIGURE MICROHARDNESS PROFILE OF A 1.6-MM Q&P 980 LASER WELD JOINT 5-62 FIGURE ERICHSEN TEST RESULT OF 1.6-MM Q&P 980, LASER WELDED vii

10 FIGURES FIGURE TITLE PAGE FIGURE 6.1 IMPACT PENDULUM (20 Impact Height) (Source: Reference 6.8) FIGURE 6.2 PENDULUM (20-16 Impact Height) (Source: Reference 6.8) 6-5 FIGURE 6.3 SAMPLE IMPACT APPARATUS (Source: Transport Canada, Safety and Security) FIGURE 6.4 IMPACT DEVICE (Source: Reference 6.10) 6-9 FIGURE 6.5 IIHS IMPACT BARRIER (Source: Reference 6.4) 6-12 FIGURE 6.6 STEEL BUMPER BARRIER (Source: Reference 6.4) 6-12 FIGURE 6.7 STEEL BACKSTOP (Source: Reference 6.4) 6-13 FIGURE 6.8 OVERLAP FOR FRONT CORNER TEST (Source; Reference 6.4) 6-13 FIGURE 6.9 RCAR FRONT CRASH PROCEDURE (Source: Reference 6.13) 6-16 FIGURE 6.10 RCAR REAR CRASH PROCEDURE (Source: Reference 6.13) 6-17 FIGURE 6.11 RELEVANT BUMPER ENGAGEMENT (Source: Reference 6.14) 6-19 FIGURE 6.12 BUMPER BARRIER (Source: Reference 6.14) 6-20 FIGURE 6.13 BUMPER BARRIER WITH BACKSTOP AND ENERGY ABSORBER (Source: Reference 6.15) viii

11 TABLES TABLE TITLE PAGE TABLE 2.1 SWEEP NUMBERS (CAMBER, X, MILLIMETERS) 2-3 TABLE 2.2 LATEST BENCHMARK BUMPER BEAMS 2-7 TABLE 3.1 TABLE 3.2 STEEL GRADES FOR POWDER COATED, PAINTED & CHROME PLATED FACEBARS TYPICAL PROPERTIES AS-SHIPPED FROM THE STEEL MILL STEEL GRADES FOR BRACKETS, SUPPORTS AND REINFORCING BEAMS TYPICAL PROPERTIES AS-SHIPPED FROM THE STEEL MILL TABLE 3.3 North American AHSS Materials Portfolio (updated ) 3-9 TABLE 3.4 TABLE 3.5 TABLE 3.6 TABLE 3.7 TABLE 3.8 TABLE 3.9 COLD-ROLLED STEEL SHEET, COATED AND UNCOATED, MECHANICAL PROPERTIES (1)(2) 3-17 HOT-ROLLED STEEL SHEET, COATED AND UNCOATED, MECHANICAL PROPERTIES (1)(2) 3-17 REQUIRED MINIMUM MECHANICAL PROPERTIES (1) OF DENT RESISTANT SHEET STEEL 3-18 REQUIRED MECHANICAL PROPERTIES (1 ) OF HIGH STRENGTH AND HSLA HOT-ROLLED AND COLD-REDUCED UNCOATED AND COATED SHEET STEEL 3-19 REQUIRED MECHANICAL PROPERTIES (1) OF HIGH STRENGTH RECOVERY ANNEALED COLD-REDUCED SHEET STEEL (2) 3-19 REQUIRED MECHANICAL PROPERTIES (1) OF HIGH STRENGTH RECOVERY ANNEALED COLD-REDUCED SHEET STEEL (2) 3-20 TABLE 3.10 RECOMMENDED COATING MASS FOR GALVANIZED STEEL SHEET 3-25 TABLE 3.11 SAE J403 CARBON STEEL COMPOSITIONS FOR SHEET 3-27 TABLE 3.12 SAE J405 CHEMICAL COMPOSITIONS OF WROUGHT STAINLESS STEELS, % (Maximum unless a range is indicated) TABLE SPOT WELDING PARAMETERS FOR LOW-CARBON STEEL >700 MPa (AHSS) 5-42 TABLE AHSS BARE-TO-BARE, BARE-TO-GALVANIZED, GALVANIZED-TO-GALVANIZED RSW PARAMETERS FOR PULSATING AC 60 Hz TABLE TRANSVERSE TENSILE TEST DATA FOR HFIW DP 280/600 TUBE 5-53 TABLE STIFFNESS PERFORMANCES COMPARISON FOR SEVERAL JOINING DESIGNS ix

12 PREFACE This publication is the sixth revision of Steel Bumper Systems for Passenger Cars and Light Trucks. It is a living document. As experience in its use is gained, further revisions and expansions will be issued. The standards discussed in this document refer to the editions of the standards as of January Please note in the event these standards are replaced by newer editions, users of this document are encouraged to investigate the possibility of using the most recent standards. In some cases new vehicles may adopt new edition standards, while current vehicles may continue to use the standard edition in place at the time of vehicle development. This publication brings together materials properties, product design information, manufacturing information and cost information. It has been prepared to suit the needs of OEM bumper stylists, bumper engineers and bumper purchasers. It is also intended to suit the needs of the Tier 1 and Tier 2 bumper suppliers and steel industry marketing personnel. This publication was prepared by the Bumper Project Group of the Steel Market Development Institute. The efforts of the following members are acknowledged: AK Steel Corporation AGS Automotive Systems ArcelorMittal Benteler Automotive Cosma International FCA Group Flat Rock Metal Inc. Flex-N-Gate Ford Motor Company General Motors Company Insurance Institute for Highway Safety Multimatic Engineering Services Nucor Corporation Shape Corporation Steel Market Development Institute January 2019 x

13 INTRODUCTION As automakers have faced increasing challenges to lightweight vehicles to accommodate for increasing fuel economy regulations, meet safety requirements, and further develop driving technology, the steel industry has continued to collaborate with the auto industry to deliver innovative new solutions. The steel industry has answered the challenge with the increased use of ultra high-strength steels (UHSS) which make steel bumpers more mass competitive while also making it more difficult to justify the additional cost of alternate materials. To provide UHSS lightweight solutions, the Steel Market Development Institute (SMDI) Steel Bumper Team has addressed numerous technical challenges including; (a) localized buckling with optimized weight savings, (b) corrosion protection of thin gage UHSS material and (c) two side shallow off-set crash performance. Bumper systems have changed dramatically over the past 30 years. More demanding government safety regulations, independent crash performance ratings and aggressive styling concepts have driven the need for new bumper system designs. Steel bumper systems fall into two categories: beams and facebars. Bumper beams are either roll-formed, hot-stamped or use a combination of both manufacturing processes. For example, the 2011 Ford Mustang bumper beams have rollformed closed sections that are subsequently hot-stamped and direct water quenched. Unlike bumper beams, facebars are exposed and have an internal supporting structure. They are typically stamped with an occasional bumper being sheet hydroformed. Roll-formed bumpers are the most common type in North America. They are usually manufactured from cold-rolled uncoated UHSS with a tensile strength range of 860 to 1500 MPa and a thickness range of 1.1 to 2.0 mm. Hot-stamped bumpers continue to gradually gain market share with increased hot-stamping capacity. Hot-stamped bumpers can be manufactured from either aluminized coated or uncoated MnB steel with minimum tensile strength of 1500 MPa after hot-stamping. Both hot- and cold-rolled MnB steels are used for hot-stamped bumpers with a thickness range of 1.0 to 4.0 mm. Hot-stamped bumpers have the lowest average mass of all steel bumper systems. Facebars are most commonly used on light-, medium- and heavy-duty trucks. Facebars are typically stamped from mildor high-strength low-alloy steels with tensile strengths up to 500 MPa and a thickness range of 1.6 to 2.3 mm. Since xi

14 facebars are exposed, cold-rolled steel is typically used to improve surface quality and coating appearance. Facebars are polished either prior to or after stamping, or both, and then chromed or painted on the exposed surfaces, depending on customer preference. The steel bumper market is important to the North American steel industry. For this reason, SMDI s Automotive Applications Council (AAC) established a Bumper Project Team. The Bumper Project Team is a group of experts from the steel industry, Tier-1 bumper suppliers and automotive OEMs. The Team is dedicated to keeping steel the material of choice for bumper applications. They accomplish this by sharing information related to bumper manufacturing processes, steel grades, regulations, solving problems associated with steel bumper development and completing R&D projects that address new design challenges for bumpers and/or make them more cost and mass efficient. The Bumper Project Team prepared this technical information bulletin to provide fundamental background information on North American bumper systems. The SMDI AAC Bumper Team has recently completed the following technical projects to address issues limiting the use of UHSS material above 1500 MPa. Localized buckling of the steel beam during low speed tests and computer simulation: By providing shape and grade changes, the team developed optimized steel bumpers (roll-formed, stamped and hot-stamped) that met the localized buckling requirement at significant weight savings. Corrosion of bumpers: In general, corrosion in steel bumpers will be a concern when gages drop below 1.2 mm. Working with post forming coating suppliers, the team identified numerous solutions to meet the future 15- year corrosion requirements. Small Overlap Rigid Barrier (SORB): Historically, bumper requirements were limited to low speed impact performance. However, the Insurance Institute for Highway Safety (IIHS) has introduced a high speed small (25 percent) overlap test that vehicles must score well on in order to receive the institute s highest safety rating. The SMDI bumper team developed two steel bumper solutions that will enable a Good rating in the IIHS SORB test on both the driver and passenger sides of the vehicle. xii

15 OBJECTIVE The purpose of this publication is to increase the reader s understanding of passenger car and light truck bumper systems. It is an overview of an automotive component system, which has undergone significant change in recent years. The information provided is aimed at automotive industry design, manufacturing, purchasing and safety related staffs; and steel industry sales and marketing groups. The emphasis is on materials, design, manufacturing, government regulations and cost. This document is intended to give the reader in depth knowledge regarding the bumper industry. While the attempt is made to cover all materials, manufacturing methods and bumper designs, some information may not be present. An emphasis has been placed on presenting the most common practices and materials, however additional information has also been presented to give the reader some ideas for possible future bumper designs, manufacturing methods and materials. It is a living document, and revisions and additions will be made as experience is gained. The Bumper Project Group hopes this publication will increase the reader s knowledge of bumper systems and help overcome engineering challenges. xiii

16 1. Bumper systems and components 1.1 Bumper systems System selection There are several factors an engineer must consider when selecting a bumper system. The most important factor is the ability of the bumper system to absorb enough energy to meet government regulations and the OEMs internal bumper standard. Weight, manufacturability and cost are important factors engineers must consider during the design phase. The formability of materials is important for high-sweep bumper systems. Another factor considered is recyclability of materials, which is a definite advantage for steel. As shown in Figure 1.1, there are five bumper systems in common use today: A. Metal facebar B. Plastic fascia and reinforcing beam C. Plastic fascia, reinforcing beam and mechanical energy absorbers D. Plastic fascia, reinforcing beam and foam or honeycomb energy absorbers E. Plastic fascia, reinforcing beam foam and mechanical energy absorbers Metal facebar system A metal facebar system, as shown in Figure 1.1 A, consists of a single metallic bumper that decorates the front or rear end of a vehicle and acts as the primary energy absorber in a lowspeed collision. The bumper regulations in the United States require passenger cars to withstand a 2.5 mph (4 km/h) impact at the curb position, plus or minus two inches (50 mm), with no visual damage and no damage to safety related items. The North American OEMs voluntarily design their passenger car bumpers to withstand a 5 mph (8 km/h) impact with no visual damage and no damage to safety items. Current facebar systems can only withstand a 2.5 mph (4 km/h) impact at the curb position plus or minus 2 inches (50 mm) with no visual damage and no damage to safety items. For this reason, the use of current facebar systems is restricted to light trucks, often to meet voluntary internal OEM design standards. The aesthetics of facebars match the styling trend for full size vans, pickups and sport utilities. Thus, most facebars are presently being applied to these vehicles. 1-1

17 In addition to low-speed impact tests, the bumper manufacturer needs to pass the step load test and meet the durability performance specified by the customer. Usually, the step load tests are static and are easier to pass, but durability remains a challenge since a vehicle s vibration response to road excitation sources is body and chassis dependent. So, even for identical road inputs, different vehicles will respond differently and the bumper manufacturer will need to comprehend that while preparing to provide tabulated material fatigue properties for the durability analyses. If the voluntary internal OEM design standard for light truck bumpers were to rise to the 5 mph (8 km/h) voluntary passenger car standard, then the facebar systems used on full size vans, pickups and sport utilities would have to be redesigned. For the reason of weight, such redesigns would likely revert to systems that employ a reinforcing beam. 1-2

18 1-3 FIGURE 1.1 COMMON BUMPER SYSTEMS

19 1.1.3 Plastic fascia and reinforcing beam system This system, as shown in Figure 1.1 B, consists of a plastic fascia and a reinforcing beam fastened directly to the vehicle frame or motor compartment rails. It is primarily used for rear bumper systems in passenger cars since the crash requirements are less severe and there is less need for mechanical energy absorbers and foam Plastic fascia, reinforcing beam and energy absorbing system Bumper systems with a plastic fascia, reinforcing beam, and energy absorption systems are the most common type of bumper system in North America. They are used on both front and rear bumper systems and readily meet the 5 mph (8 km/h) voluntary bumper standard set by North American OEMs. While most passenger cars, SUVs, crossovers and minivans have this type of bumper system, the energy absorption method varies. During low speed crash events the reinforcing beam always absorbs a significant amount of energy while additional energy is absorbed by mechanical energy absorbers (Fig. 1.1C), foam or honeycomb (Fig. 1.1D), or both (Fig. 1.1E). 1.2 Bumper components Plastic fascia Energy absorbers Bumper fascias (Figure 1.1) are designed to meet several requirements. They must be aerodynamic to control the flow of the air around the car and the amount of air entering the engine compartment. They must be aesthetically pleasing to the consumer. Typical fascias are styled with many curves and ridges to give bumpers dimension and to distinguish vehicles from competing models. Other requirements of bumper fascias are light weight and ease of manufacture. Virtually all fascias are made from one of three materials: polypropylene, polyurethane or polycarbonate. Energy absorbers (Figures 1.1C, D, and E) are designed to absorb a portion of the kinetic energy from a vehicle collision. Energy absorbers are effective in a low speed impact, where the bumper springs back to its original position. Energy absorber types include foam, honeycomb and mechanical devices. All foam and honeycomb absorbers are made from one of three materials: polypropylene, polyurethane or low-density polyethylene. 1-4

20 1.2.3 Facebar Mechanical energy absorbers, also referred to as crush cans, are metallic and sometimes resemble shock absorbers. Although mechanical energy absorbers have several times the weight of a foam energy absorber, they are also capable of absorbing several times the energy. Most front bumper systems use mechanical energy absorbers due to higher energy absorption requirements Reinforcing beam Facebars (Figure 1.1A) are usually stamped from steel with plastic or stainless steel trim to meet styling requirements. Steel facebars, for formability reasons, are usually made from steels with a low to medium yield strength. Higher strength steels are being investigated for facebars to reduce the thickness and weight. After stamping, steel facebars are chrome plated or painted for appearance and corrosion protection reasons. The reinforcing beams (Figures 1.1B, C, D and E) are key components of the bumper systems. Reinforcement beams help absorb the kinetic energy from a collision and provide protection to the rest of the vehicle structure. By staying intact during a collision, beams preserve the frame. Design considerations for reinforcing beams include strength, manufacturability, weight, recyclability and cost. Steel reinforcing beams are usually roll-formed or hotstamped using ultra high-strength steel. Typical cross sections are shown in Figure 1.2. Roll-formed beams are the most common but hot-stamped beams have the lowest average mass of all steel bumper systems and are becoming more popular as a result. The most common cross section for roll-formed beams is the B-section and the most common sections for hot-stamped beams are box and hat sections. Sometimes a stamped or roll-formed face or back plate is welded to a roll-formed or hotstamped C-section to create a boxed section. Additional reinforcements are sometimes welded to reinforcing beams, such as pole protectors and bulkheads. All steel reinforcing beams receive corrosion protection. Some beams are made from hot-dip galvanized or electrogalvanized sheet. The zinc coating on these products provides excellent corrosion protection. Other beams are protected after fabrication with a paint system such as E-coat. Since steel reinforcing beams are becoming stronger and lighter with thinner gauges being used, more beams are using both zinc coating and E-coating to meet corrosion protection requirements. 1-5

21 FIGURE 1.2 COMMON REINFORCING BEAM CROSS SECTIONS 1-6

22 1.3 Types of bumper beams Steel reinforcing beams Steel facebars Steel reinforcing beams are produced using the coldstamping, hot-forming or roll-forming processes. The tensile strength of cold-stamped and roll-formed beams ranges from MPa. While the tensile strength of hot-stamped beams, after heating and quenching, range from MPa. All steel beams have an elastic modulus of 207,000 MPa. Steel reinforcing beams are protected from corrosion by zinc coatings, aluminum coatings and E-coating during painting. After mounting to a vehicle frame, reinforcing beams are covered by cosmetic or energy absorbing fascias. Steel facebars are typically cold-stamped from low-carbon and high-strength steels having tensile strengths from MPa and an elastic modulus of 207,000 MPa. They are either chrome plated or painted for corrosion protection and appearance before being mounted to a vehicle s frame. Most facebars are dressed up with plastic trim Plastic reinforcing beams There are two types of plastic beams glass reinforced plastic or unreinforced plastic. Examples of glass reinforced plastic beams include polypropylene (compression molded), unsaturated polyester (compression molded) and polyurethane (reaction injection molded). Examples of unreinforced plastic beams include polycarbonate/polybutylene (injection or blow molded), polyethylene (blow molded) and polypropylene (blow molded). Plastic beams have tensile strengths up to 275 MPa and flexural moduli up to 15,000 MPa Aluminum reinforcing beams Typically, aluminum beams are made by stretch or press forming extruded shapes made from the 6000 and 7000 aluminum series. After forming and heat treating, the beams have tensile strengths up to 550 MPa and an elastic modulus of 69,000 MPa. 1-7

23 2. Bumper design concepts 2.1 Sweep (roll formed sections) and depth of draw (stampings) The current styling trend for vehicles is toward rounded, aerodynamic shapes. This trend has impacted bumper design and challenged bumper manufacturers to provide the highly rounded shapes desired by vehicle stylists. Steel bumper manufacturers have met the challenge and are providing the contours required for both reinforcing beams and facebars. In some instances the designs require multiple radii incorporating both large (gentle) and small (tight) radii. These can also be accommodated with the appropriate manufacturing methods. A convenient way of defining the degree of roundness for a stamped or roll-formed reinforcing beam is to use the concept of sweep. Sweep expresses the degree of curvature of the outer bumper face, or the face farthest removed from the inside of the vehicle. Sweep is defined in Figure 2.1 and Table 2.1. Sweep in the camber, X, for a 1524 mm chord length, L, of a given circle of radius, R. Sweep is expressed as the number of 3.18 mm (1/8 inches). For example, if X is 127 mm for an L of 1524 mm, the sweep would be 40. Table 2.1 indicates that a sweep number of 40 corresponds to a radius of curvature of 2350 mm. Table 2.1 also lists camber for cord lengths smaller than 1524 mm. For example, if the camber is 68.9 mm and the chord length is 1016 mm, the sweep number is 50. The concept of sweep applies well to a reinforcing beam when it has a near constant radius of curvature and no wrap arounds at the end of the reinforcing beam. Depth of draw is often used to describe the amount of rounding and wrap around on a bumper section, and in particular, a stamped facebar. As shown in Figure 2.2, depth of draw is the distance, X, between the extreme forward point on a bumper and the extreme aft point on a bumper. This distance has a physical significance in that it cannot exceed the opening available with a given stamping press. X is usually stated in inches (millimeters). 2-1

24 2-2 FIGURE 2.1 DEFINITION OF SWEEP

25 TABLE 2.1 SWEEP NUMBERS (CAMBER, X, MILLIMETERS) FIGURE 2.2 DEFINITION OF DEPTH OF DRAW 2-3

26 2.2 Tailored products There are two types of tailored products used for bumper beams: laser welded blanks and tailor rolled blanks. A laser welded blank joins two or more flat steel blanks together with laser welding prior to forming. The blanks can have different strengths and thicknesses so that the formed end product has extra thickness and/or strength where it is needed. Examples of laser welded blanks are shown in Figure 2.3. A tailor rolled blank is created by sending a steel coil through a tailor rolling process where the thickness is reduced in certain areas with compressive rollers. The variable thickness coil can then be blanked to create a tailor rolled blank. The tailor rolled blank can then be stamped or hot-formed into a component that has extra thickness where it is needed. Now it is even possible to send a tailor rolled coil through a roll forming line to produce roll formed parts with variable thicknesses. Both laser welded blanks and tailor rolled blanks have been implemented into production for bumper beams and are considered a viable method of mass reduction for steel bumper systems. 2-4

27 2-5 FIGURE 2.3 EXAMPLES OF TAILOR WELDED BLANKS

28 2.3 Latest benchmark bumper beams Examples of recent bumper beams are given in Table 2.2 and Figures 2.3. The examples clearly illustrate that steel bumper beams readily meet the challenges faced by bumper designers -- styling, weight, cost and structural integrity. FIGURE 2.4 BENCHMARK STEEL BUMPER BEAMS 2-6

29 TABLE 2.2 LATEST BENCHMARK STEEL BUMPER BEAMS DEFINITIONS XF - High-strength low-alloy (HSLA). Designation number is yield strength in ksi. XLF - High-strength low-alloy (HSLA) with low carbon. Formability of this quality is superior to XF quality. Designation number is yield strength in ksi. T - Martensitic quality MPa - Mega Pascal MnB - Manganese Boron CR - Cold-Rolled UHSS - Ultra High-Strength Steel 2.4 Current steel bumper design passenger cars A flow chart for designing passenger car bumpers is shown in Figure 2.4. There are two paths -- one path is for vehicles sold only in North America and the other path is for vehicles sold in both North America and Europe. Two types of standards influence bumper design: mandatory government standards and voluntary insurance industry standards. In the U.S., the federal standard regulating bumper design is referred to as the National Highway Traffic Safety Administration (NHTSA) standard (see Section 6.1).The federal standard that regulates bumper design in Canada (see Section 6.2) allows the use of the NHTSA standard. Thus, the NHTSA standard covers vehicles to be sold in both Canada and the United States. 2-7

30 The Economic Commissions for Europe standard (see Section 6.3), which is similar to the NHTSA standard, regulates bumper design. In addition, front bumpers must conform to Pedestrian Protection regulations (see Section 2.7). The Insurance Institute for Highway Safety (IIHS), in an effort to reduce the cost of passenger vehicle bumper repairs, has developed a test standard that simulates a broader range of impacts occurring in actual on-the-road crashes (see Section 6.4). The voluntary IIHS tests are more severe than the NHTSA tests. The IIHS standard provides a weighted damage estimate that is used when determining overall rating for a vehicle to be sold in North America. This target is used when designing the vehicle s bumpers. Similar to IIHS, the European insurance industry publishes two voluntary tests to prevent unnecessary damage in low speed crashes. These tests are referred to as the RCAR Structural Test (see Section 6.5) and the RCAR Bumper Test (see Section 6.6) Typical bumper design - North American passenger cars In Figure 2.4, the designer s first step is to determine the OEM Internal design requirements. For example, are the IIHS tests to be included in the design process? Are there OEM requirements, such as packaging, that are not included in the flow chart? If the answer to the latter question is yes, the designer must modify the flow chart. If there are no IIHS requirements, the designer moves directly to a NHTSA Base Design. Thus, it is suggested the corner impact be used to establish the base design. The designer then moves on to the longitudinal pendulum and barrier impacts. If the NHTSA damage and A+B planes force criteria have been satisfied, a final design has been reached. If the OEM has specified IIHS requirements, it is suggested the designer start by satisfying the OEM IIHS requirements. Usually, these requirements are more demanding than the NHTSA criteria, especially if the IIHS target is a zero or minimal damage estimate. The designer may be designing a front bumper, a rear bumper or both bumpers. If only a front or rear bumper is being designed, the designer must establish the IIHS damage estimate desired by the OEM for the bumper. If both a front and rear bumper are being designed, the designer must establish the desired IIHS weighted damage estimate. In the flow chart, the only difference between the front or rear and the front and rear paths is the acceptance criterion. The criterion for a single bumper is the damage estimate for that bumper. The criterion if both bumpers are being designed is the weighted damage estimate, which is calculated using the damage estimate for each of the two bumpers. 2-8

31 Once an acceptable IIHS design has been achieved, the designer verifies that the NHTSA criteria have been met before reaching a final design Typical bumper design - North American and Europe passenger cars In Figure 2.4, the designer s first step is to determine the OEM internal design requirements. For example, are the IIHS and RCAR tests to be included in the design process? Are there OEM requirements, such as packaging, that are not included in the flow chart? If the answer to the latter question is yes, the designer must modify the flow chart. In general, the NHTSA and ECE requirements are similar as are the IIHS and RCAR Bumper Test Requirements. However, the requirements associated with the RCAR Structural Test are more demanding than the NHTSA, ECE and RCAR Bumper Test requirements. For this reason, plus the fact a European front bumper must have pedestrian protection, the flow chart goes through the European path before the North American path. A European front bumper must meet Pedestrian Protection requirements. Thus, a design concept that will provide the required Pedestrian Protection must be selected and it is logical to commence the design process here for a front bumper. After preparing a Base Design that satisfies Pedestrian Protection requirements, if there are no RCAR requirements, the designer addresses the ECE requirements. Often, the pendulum corner impact is the most demanding ECE case. Thus, it is suggested the corner impact be used first to verify the Base Design. After the designer has satisfied the ECE requirements, the designer would proceed through the North American bumper path as outlined in Section to reach a Final Design. For a front bumper, if RCAR requirements are to be met, it is suggested the RCAR requirements be addressed before the ECE requirements because the RCAR requirements are more demanding. The RCAR Structural Test is more demanding than the RCAR Bumper Test. Thus, if the RCAR Structural Test is a requirement, it should be addressed before the RCAR Bumper Test. Once a design that is acceptable from the RCAR point of view has been achieved, the designer moves through the ECE requirements and then the North American bumper path as outlined in Section to reach a Final Design. A rear bumper would essentially follow the same path as a front bumper. However, one major difference is that Pedestrian Protection is not a requirement and this step in the design process is bypassed. 2-9

32 2.5 Current steel bumper design pick-ups, full size vans and sports utilities There are no federal regulations in the United States or Canada for bumpers on pickups, full size vans or SUVs. These bumpers are designed to meet OEM internal specifications. Thus, a designer should develop a design flow chart using Figure 2.5 as a model. FIGURE 2.5 TYPICAL BUMPER DESIGN FOR PASSENGER CARS AND MINIVANS 2-10

33 2.6 Auto/Steel Partnership high speed bumper design - North American passenger cars The Auto/Steel Partnership (A/SP) commissioned Quantech Global Services to conduct a study on the front-end of a four-door, mid-size sedan. The objective was to reduce the cost and mass of the front end structure through the use of advanced high-strength steels. The study included the development of a high speed bumper system. Current North American passenger cars have low speed bumper systems. Thus, Quantech s first task for the high speed bumper system was to establish design criteria and a design process. Sections and outline the results of Quantech s research into these two areas Quantech design criteria for high speed steel bumper system Quantech, in consultation with A/SP, established the design criteria for a high speed bumper system as: 1. No bumper damage or yielding after a 5 mph (8 km/h) frontal impact into a flat, rigid barrier. Note: This criterion does not apply to low speed bumpers, where controlled yielding and deformation are beneficial. 2. No intrusion by the bumper system rearward of the engine compartment rails for all impact speeds less than 9 mph (15 km/h). 3. Minimize the lateral loads during impacts in order to reduce the possibility of lateral buckling of the rails. 4. Full collapse of the system during Danner (RCAR), NCAP and IIHS high speed crash without inducing buckling of the rails. 5. Absorb 1 percent of the total energy every millisecond. 6. Absorb 15 percent of the total energy in the NCAP crash, including engine hit. 7. Use the front-end crush space efficiently. 8. Meet the air bag sensor requirements in low, medium and high speed impacts. 9. No detrimental effect on baseline body-in-white static or dynamic stiffness. Bumpers should protect car bodies from damage in low speed collisions - the kind that frequently occurs in congested urban traffic. The IIHS Low Speed Crash Test Protocol (see Section 6.4) addresses this issue. For marketing reasons, many current bumper systems are designed to ensure no or minimal cost of repair after the IIHS 5 mph (8km/h) barrier impact. A/SP believes all future vehicles should meet this requirement. Thus, Criterion 1 was set to achieve zero damage and no or minimal cost of repair after the IIHS 5 mph (8 km/h) barrier impact. 2-11

34 Criterion 4 addresses three high speed load cases: percent-9 mph Danner (RCAR Test - see Section 6.6 and Reference 6.12). This load is a 9 mph (15 km/h) impact at a 40 percent offset into a rigid barrier. The A/ SP objective is to have no damage to the radiator and other costly equipment in the front-end and to have no damage to the rail beyond 300 mm (12 inches) mph NCAP (NHTSA New Car Assessment Program, Reference 5.2). This load is a 35 mph (56 km/h) impact into a rigid barrier. The A/SP objective is to maximize the energy absorbed in the bumper system percent-40 mph IIHS (Reference 5.3). This load case is a 40 mph (64 km/h) impact at a 40 percent offset into a deformable barrier. The A/SP objective is to ensure the bumper system does not break and is capable of transferring the load to the right rail, thereby minimizing the damage. A major objective of A/SP is to reduce vehicle weight using steel as the material of choice. Criterion 6 addresses this objective. Traditional bumper systems absorb about 8-11 percent of the energy in the 35 mph (56 km/h) NCAP crash. If bumper systems were to dissipate higher levels, there would be an opportunity for mass savings in the front end structure. To capitalize on this opportunity, A/SP set 15 percent energy absorption as a stretch goal for future bumper systems Flow chart for high speed system Steel is the material of choice for future and current bumper beams because of low cost with lightweight. The flow chart in Figure 2.6 outlines the design process developed by Quantech for a high speed bumper system having a steel beam. The process is a logical route to satisfying the design criteria outlined in Section First, a base design is prepared. It is checked against the IIHS low speed, 5 mph (8 km/h), flat frontal barrier load case. If there is damage or yielding, the base design is modified. If not, the three high speed load cases are analyzed in the following sequence percent offset 9 mph (15 km/h) Danner mph (56 km/h) NCAP percent offset 40 mph (64 km/h) IIHS. The results from the analyses of the three high speed load cases are compared to the design criteria in Section If all of the criteria are met, the designer assesses the amount of energy absorption. Energy absorption should be maximized because the higher the amount, the greater the opportunity to reduce mass in the front end structure. 2-12

35 If the designer believes energy absorption has been maximized, a viable design has been captured. If not, the learning from the three high speed load cases is used to improve the base design and reach a viable design. Usually, three or four viable design alternatives are developed using the above process. The designer then selects one of the alternatives as the Preferred Design. The Preferred Design should be lightweight and easy to manufacture. Also, it should be easy to assemble and integrate with the rails. Cost is also a consideration when selecting the Preferred Design. FIGURE 2.6 AUTO/STEEL PARTNERSHIP BUMPER DESIGN FOR HIGH SPEED SYSTEM NORTH AMERICAN PASSENGER CARS Source: Auto/Steel Partnership and Quantech Global Services 2-13

36 2.7 Bumper design for pedestrian impact Impact tests Pedestrian safety is a globally recognized safety concern. Efforts towards modifying vehicle designs to offer some protection for pedestrians began in earnest in the 1970s. At the same time, test procedures to evaluate the performance of new designs developed. Pedestrian safety has improved significantly since then. The Steel Market Development Institute wished to learn how pedestrian safety might affect steel bumper systems. Thus, it retained Dr. Peter Schuster, California Polytechnic State University, to study this topic. The following information is based on his work (Reference 2.4). The European Union has been subjecting select vehicles to a battery of tests (frontal, side and pedestrian) as part of its new car assessment program (EuroNCAP, Reference 2.5). The EuroNCAP pedestrian tests (Figure 2.7) consist of: leg to bumper impacts with a leg-form impactor, upper leg to hood edge impacts with an upper legform impactor, head to hood top impacts with two different headform impactors The European Union typically subjects a vehicle to three leg-to-bumper impacts, three upper leg-to-hood edge impacts and up to 18 head-to-hood top impacts. The results are reported with a four-star rating system, similar to that used in the United States NCAP program. Japan s NCAP program includes tests that simulate pedestrian head to hood top impacts. However, leg- to-bumper and upper leg-to-hood edge impacts are not included. North American NCAP programs do not currently include pedestrian requirements. However, the high number of pedestrian accidents in North America and the trend to global vehicle design likely mean that pedestrian impact requirements will come to North America in the longer term EuroNCAP leg to bumper impacts with a leg-form impactor This test significantly influences bumper design. Thus, a brief discussion of the requirements is in order. First, it should be stated that the purpose of this test is to reduce severe lower limb injuries in pedestrian accidents. The most common lower limb injuries are intra-articular bone fractures, ligament ruptures and comminuted fractures. 2-14

37 In this test, a leg-form impactor is propelled toward a stationary vehicle at a velocity of 40 km/h (25 mph) parallel to the vehicle s longitudinal axis. The test can be performed at any location across the face of the vehicle, between the 30 bumper corners. The leg-form impactor is shown in Figure 2.6. It consists of two semi-rigid 70 mm (27.6 inches) diameter core cylinders (the tibia and femur ) connected by a deformable knee joint. This core structure is wrapped in 25 mm (1 inch) of foam flesh covered by 6 mm (0.24 inches) of neoprene skin. The performance criteria proposed for 2010 are shown in Figure 2.6. The maximum acceleration of the tibia is intended to prevent fracture of the tibia due to bumper contact. The maximum knee bend angle and shear deformation are intended to prevent severe knee joint injuries such as ligament ruptures and intra-articular bone fractures Government regulations As of June 2005, there were no government regulations for pedestrian impact. However, the European Union and major vehicle associations have negotiated an agreement (Reference 2.6). The agreement states that new vehicles will achieve a limited level of pedestrian impact performance starting in 2005, with an increased performance in The limits shown in Figures 2.8 are the targets for For 2005, the leg-to-bumper targets are: knee bending < 20 knee shear < 6 mm (0.24 inches) acceleration < 200 g Design approaches There are two general approaches to designing a front bumper system for pedestrian safety: Provide front end vehicle components to cushion the impact and support the lower limb Provide sensors and external airbags to cushion the impact and support the lower limb Cushioning the impact Cushioning reduces the severity of bone fractures. It is directly related to the acceleration impact criterion shown in Figure 2.8. Limiting the lower limb acceleration to 150 g requires a bumper stiffness lower than that usually provided to satisfy the damageability criteria associated with low-speed, 8 km/h (5 mph), vehicle impact. Thus, a pedestrian-friendly bumper system must be capable of limiting leg-form acceleration without sacrificing vehicle damageability in a low speed impact. 2-15

38 Supporting the lower limb Design solutions Supporting the lower limb reduces the risk of knee joint injuries such as ligament ruptures and intra-articular fractures. It is directly related to the knee bend angle criterion in Figure 2.7. Enough support must be provided below the main bumper to limit the bending angle to 15. Any support provided must not conflict with styling requirements or result in unacceptable low speed, 8 km/h (5 mph), impact damage. As bumper systems meeting the requirements of pedestrian leg impact are only beginning to hit the marketplace in Europe, Australia and Japan, it is too early to identify the most popular designs. However, a thorough review of articles and patents does indicate the most popular design solutions for passenger cars. There is limited production of vehicles with exposed bumper beams (facebars) in these areas. Hence, there has been little activity devoted to adapting facebars to meet pedestrian impact requirements. For passenger cars with reinforcing beams, the most commonly proposed design solutions are: 1. Front End Vehicle Component Solutions: a. Lower stiffener: A new component called a stiffener or spoiler may be located below the bumper system to prevent the lower part of the leg form from moving further toward the vehicle than the knee. The stiffener may be a fixed component or a component that deploys based on impulse or speed. b. Energy absorbers: To cushion impact, an energy absorber may be placed between the bumper beam and the pedestrian. Alternately, an energy absorber may be placed behind the bumper beam. The most commonly proposed energy absorbers are plastic foams (single or multi-density) and molded plastic egg crates. However, several proposed design solutions incorporate spring steel, composite steel/foam and crush can absorbers. c. Beam design: A tall front-view bumper height may be used to provide leg support. d. Bull-bars : Structures may be added to the front of an existing bumper system to provide energy absorption and to support the lower limb. 2-16

39 2. Sensor and Airbag Solution: Any current bumper system may be covered with an airbag. In this way, the energy absorption capability of the bumper becomes irrelevant. The key disadvantages to this design approach are cost and sensor capability. All of the Front End Vehicle Component Solutions listed above may be used in conjunction with steel reinforcing beam bumper systems. The Sensor and Airbag Solution would appear to have the greatest potential for use with steel facebar bumper systems such as those used on pickup trucks. FIGURE 2.7 EuroNCAP PEDESTRIAN TESTS (2010 CRITERIA) 2-17

40 2-18 FIGURE 2.8 EuroNCAP LEG FORM IMPACTOR

41 FIGURE 2.9 EuroNCAP LEG FORM IMPACT CRITERIA (2010) 2.8 Localized buckling solutions for thin-gauge UHSS bumper beams Acceptance and application of thin gage UHSS (>1500 MPa) is limited due to the localized buckling phenomenon during the IIHS 10 km/hr curved impact barrier tests. SMDI worked with Detroit Engineered Products (DEP) as an FEA consultant to redesign three current bumper systems that exhibit localized buckling failures during simulation. In addition to addressing the localized buckling failures, DEP was able to optimize each bumper system to provide significant weight savings Roll-formed bumper By optimizing the design of the bumper with both an improved geometry and an engineered dimple pattern; DEP was able to meet the IIHS curved barrier test with no localized buckling. By upgrading the material grade from the baseline DP780 to MS1500; DEP was also able to achieve a 29 percent mass savings in the bumper beam. 2-19

42 2.8.2 Hot-stamped bumper Similarly, for the hot-stamped bumper, the DEP optimized bumper passed the curved barrier test with no localized buckling. By upgrading the material from the baseline PHS 1300 to PHS 1900; DEP was able to achieve a bumper beam mass savings of 23 percent Hot-formed bumper Similarly, for the hot-formed bumper, the DEP optimized bumper passed the curved barrier test with no localized buckling. By upgrading the material from the baseline PHS 1300 to PHS 1900; DEP was able to achieve a bumper beam mass savings of 23 percent. 2.9 Engineered bumper beam solutions for IIHS small overlap tests The Insurance Institute for Highway Safety (IIHS) introduced a passenger side 40 MPH small overlap (25 percent off-set) test after they found that numerous vehicles with good driver side ratings had poor performance on the passenger side. Historically, bumper system designs are required to meet low speed impacts without damage to the vehicle front end structure and related components. The SMDI along with DEP decided to evaluate the importance of a steel bumper beam solution in meeting the passenger side 40 mph small overlap test. Using advanced engineering design optimization tools including MeshWorks, the team was able to provide two lightweight solutions One piece design solution (Figure 2.9) The optimized bumper system design was 804 grams (10 percent) lighter than the baseline system mass. Using MeshWorks, the optimized bumper system (new bumper beam and blockers) achieved sections within the allowed design space. The optimized results satisfied the criteria of maintaining a gap between the bumper and the condenser, radiator and fan module (CRFM) during low speed impact of the full vehicle. The design protected other related vehicle subsystems without damage. The optimized bumper system design met the SORB performance requirements for both the driver and passenger side. 2-20

43 FIGURE 2.10 OPTIMIZED ONE-PIECE BUMPER SYSTEM Two piece design solution (Figure 2.10) The optimized bumper system design was 800 grams (10 percent) lighter than the baseline system mass. Using MeshWorks, the optimized bumper system (new bumper beam and blockers) achieved sections within the allowed design space. The optimized results satisfied the criteria of maintaining a gap between the bumper and the CRFM during low speed impact of the full vehicle. The design protected other related vehicle subsystems without damage. The optimized bumper system design met the SORB performance requirements for both the driver and passenger side. FIGURE 2.11 OPTIMIZED TWO-PIECE BUMPER SYSTEM 2-21

44 3. Steel materials 3.1 Introduction 3.2 Automotive steels Flat rolled steels are versatile materials. They provide strength and stiffness with favorable mass-to-cost ratios and allow high speed fabrication. They also offer excellent corrosion resistance when coated, high energy absorption capacity, good fatigue properties, high work hardening rates, good formability, aging capability, excellent paintability and complete recyclability. In addition, to these characteristics, the availability of advanced high-strength and ultrahighstrength steels have made sheet steel the material of choice in the automotive industry. The steel industry provides more than 200 automotive sheet steel types and grades which offer designers a wide choice for any given application. Bumper steels with elongations up to 50 percent facilitate forming complex shapes. Bumper steels with tensile strengths over 1900 MPa facilitate mass reduction. Automotive steels are classified in several different ways. Common industry designations include conventional steels (interstitial-free and mild steels); high-strength steels (carbonmanganese, bake hardenable and high-strength, low-alloy steels); and AHSS (dual phase, transformation-induced plasticity, twinning-induced plasticity, ferritic-bainitic, complex phase and martensitic steels). Additional higher strength steels for the automotive market include hot-formed, postforming heat-treated steels and steels designed for unique applications such as improved edge stretch and stretch bending. Third generation steels (retained austenite) with high strength and ductility are also available Conventional steels, interstitial-free and mild steels These steels are widely produced, and with their exceptional formability (elongations of percent), are typically used for complex shapes, including vehicle exterior painted surfaces, such as doors, fenders and deck lids. Conventional steels have an essentially ferritic microstructure. 3-1

45 3.2.2 High-strength steels (HSS) high-strength low alloy (HSLA) steels These steels are medium strength (approximately MPa) and used in various body structure, suspension and chassis parts and wheels, where strength is needed for increased in-service load. HSLA steels are essentially singlephase ferritic microstructures strengthened primarily by the addition of micro-alloying elements (Figure 3.1). FIGURE 3.1 REPRESENTATIVE MICROSTRUCTURE OF CONVENTIONAL AND HIGH STRENGTH STEEL Advanced high-strength steels (AHSS) These steels are high-strength (generally greater than 500 MPa) and applied in the body structure, including beams and cross members, sill and pillar reinforcements and other energy-absorbing components. These steels provide the automotive design engineer with high value, lightweight solutions with the required stiffness (for improved noise and vibration performance as well as ride and handling characteristics), crash energy management (to absorb front and rear crash energy) and strength (to provide anti-intrusion during side or roll-over accidents). The principal difference between HSS and AHSS is the microstructure. HSS are single-phase ferritic steels with a potential for some pearlite in carbon manganese grades. AHSS are primarily steels with a microstructure containing additional phases for example martensite, bainite, austenite and/or retained austenite in quantities sufficient to produce unique mechanical properties. In addition to 3-2

46 controlling the chemistry, AHSS require control of the cooling rate to help create the desired microstructure, either on the hot mill runout table (for hot-rolled products) or in the cooling section of the continuous annealing line furnace (continuously annealed or hot-dip coated products) Dual phase (DP) steels These steels range in strength from 500MPa to 1200MPa and obtain their properties from the introduction of a martensitic phase into the ferrite microstructure (Figure 3.2). The ferrite phase provides excellent formability, while the martensitic phase provides the improved strength (higher ultimate tensile strength compared to conventional steel with similar yield strength). Dual phase steels are used increasingly in safety-critical auto body structural components because its higher ultimate tensile strength (UTS) provides much greater energy absorption over conventional HSS with the same yield strength (YS) Safety/ahss_repairability_phase1_study2.pdf FIGURE 3.2 REPRESENTATIVE MICROSTRUCTURE OF DUAL PHASE AHSS 3-3

47 Transformation induced plasticity (TRIP) steels These steels have a similar range of strength as DP steels, 500MPa to 1200MPa, while providing improved formability. The improved formability is obtained with the introduction of additional phases (austenite and bainite) into the microstructure (Figure 3.3). These phases improve the work hardening properties of steel and provide additional energy absorption characteristics. FIGURE 3.3 REPRESENTATIVE MICROSTRUCTURE OF TRIP AHSS Ultra high-strength steels (UHSS) These steels are AHSS which are ultrahigh in strength, greater than 980 MPa and are used in areas where exceptional strength and anti-intrusion are needed, including the A-pillars, B-pillars, rockers and roof rails Third generation advanced high-strength steels (3rd Gen AHSS) Third-generation advanced high-strength steels were developed to provide a high-value steel solution to bridge the properties gap between the already developed first-generation AHSS and second-generation AHSS (predominantly stainless steel grades). This new generation of steel shares the high-strength properties of AHSS, while also having a higher total elongation similar to high-strength steels, enabling automakers the continued use of their current stamping and assembly infrastructures. These grades are mainly multi-phased (MP) steels containing retained austenite which provides the increased ductility. 3-4

48 3.3 Typical properties of steel grades for facebars The steel grades that are commonly used for facebars are shown with their typical properties in Table 3.1. Most facebars are made from high-strength steel (minimum yield strength higher than 240 MPa). Although dual phase steels are not listed in Table 3.1, successful trials have been completed and facebars are expected to switch over to this grade for mass reduction. For comparative purposes, Table 3.1 also includes similar SAE grades. The Society of Automotive Engineers (SAE) designates SAE steel grades. These are four digit numbers which represent chemical composition standards for steel specifications. It is important to note that the similar SAE grades are not equivalent grades. That is, there are minor differences between the SAE grades and the common grades to which they are similar. The differences might be significant in some applications. Some OEMs specify grades that can be proprietary in nature. As a result of their depth of draw and complex shape, facebars are produced by the stamping process. Steels of high formability are required and all of the grades shown in Table 3.1 are available to meet the demanding requirements of a facebar stamping. Facebars are either powder coated, painted or chrome plated and a high-quality surface is required on the steel sheet. In addition, the majority of the sheet steel used for plated facebars is flat polished prior to the stamping operation. 3.4 Typical properties of steel grades for brackets, supports and reinforcing beams The steel grades that are commonly used for brackets, supports and reinforcing beams, are shown with their typical properties in Table 3.2. Most reinforcing beams are made from advanced and ultra high-strength steel (minimum tensile strength greater than 550 MPa). For comparative purposes, Table 3.2 also includes similar SAE grades. It is important to note that the similar SAE grades are not equivalent grades. There are minor differences between the SAE grades and the common grades which are similar. The differences might be significant in some applications. All of the high-strength steel grades in Table 3.2 are available with sufficient formability for the production of stamped brackets, supports and reinforcing beams. They can also be readily roll formed into reinforcing beams. 3-5

49 The advanced and ultra high-strength steel grades in Table 3.2 generally have less formability than the high-strength grades listed. However, they offer significant weight reduction opportunities and are commonly used for less severe stampings and roll-formed reinforcing beams. Grades 120XF and 135XF have sufficient ductility to produce stampings, including reinforcing beams, provided the amount of draw is minimal. Grade 140T has a relatively low as-delivered yield strength, which facilitates stamping and roll forming operations. An advantage of this grade is the fact it work-hardens significantly during forming. In fact, the yield strength after forming approaches 965 MPa. Thus, 140T offers sufficient formability to produce roll formed beams with a large sweep and it provides high yield strength in the finished part. Grades 140XF and M130HT through M250HT have low formability and their use is generally restricted to roll formed reinforcing beams since roll forming is a process of gradual bending without drawing. The Carbon-Boron grades can be used to produce complex parts through the hot stamping process. After quenching, the parts have yield strengths up to 1300 MPa and tensile strengths up to 2000 MPa. The stainless steel grades are readily stamped or roll-formed. Their excellent corrosion resistance eliminates the need for protective coatings. TABLE 3.1 STEEL GRADES FOR POWDER COATED, PAINTED & CHROME PLATED FACEBARS TYPICAL PROPERTIES AS-SHIPPED FROM THE STEEL MILL NOTES HR CR Hot-rolled sheet Cold-rolled sheet 1008/1010 Low-carbon commercial quality (CQ). Mechanical properties are not certified. SS Stainless steel 3-6 DR Dent resistant quality. Strength increases due to work hardening during forming. Designation number (e.g. 210) is minimum yield strength in MPa. XLF Microalloy quality. Strength is obtained through small quantities of alloying elements such as vanadium and niobium. Designation number (e.g. 50) is minimum yield strength in ksi.

50 TABLE 3.2 STEEL GRADES FOR BRACKETS, SUPPORTS AND REINFORCING BEAMS TYPICAL PROPERTIES AS-SHIPPED FROM THE STEEL MILL NOTES HR Hot rolled sheet CR Cold rolled sheet HDG (CR) Hot-dip galvanized (cold rolled base) sheet EG (CR) Electrogalvanized (cold rolled base) sheet Aluminized (CR)Hot-dip aluminized (cold rolled base) XLF Microalloy quality. Strength is obtained through small quantities of alloying elements such as vanadium and niobium. Designation number (e.g. 50) is minimum yield strength in ksi. SS Stainless steel CB (M) Carbon-Boron quality (Modified). Properties are for the steel as-shipped from the steel mill. Strength is achieved through heating and quenching. After quenching, the yield strength is about 1140 MPa (165ksi) CB Carbon-Boron quality. Properties are for the steel as-shipped from the steel mill. Strength is achieved through heating and quenching. After quenching, the yield strength is about 1140 MPa (165ksi). 3-7 XF Recovery annealed quality. Strength is achieved primarily through cold work during cold rolling at the steel mill. Designation number (e.g. 120) is minimum yield strength in ksi. 140T Dual phase quality. Structure contains martensite in ferrite matrix. Properties are for the steel as-shipped from the steel mill. Designation number (e.g. 140) is the minimum tensile strength in ksi. M...HT Martensitic quality. Strength is determined by carbon content. Designation number (e.g. 130) is the minimum tensile strength in ksi. N/A Not applicable. The Carbon-Boron steels listed are intended for hot forming. The Recovery Annealed and Martensitic steels are primarily used in roll forming operations. However, they may be used for stampings provided the amount of draw is minimal. The n value for dual phase steels is very dependent on the range over which it is calculated.

51 3.5 Future steel vehicle materials portfolio for automotive applications In 2011, WorldAutoSteel released the final results of the Future Steel Vehicle (FSV) project. As part of the project, a materials portfolio (Reference 3.1) was developed that summarizes steel grades considered in the design of FSV. The North American steel suppliers continue to develop new AHSS grades that provide greater strength and/or ductility than those originally introduced as part of the FSV study. These steel providers have developed a similar matrix of the material available and can be found in Table 3.3. All are commercially available now or in the near future. The AHSS family of products in the portfolio provides a key tool for future automotive applications. The combination of new design technologies along with emerging steel grades and advanced steel processing technologies enable optimal component and vehicle lightweighting. AHSS grade development has been driven by the need to achieve better performance in crash energy management with material gauge reduction and subsequent lower mass. Table 3.3 shows the steel grades available in North America and their generalized properties available for vehicle design including facebars, brackets, supports and reinforcing beams. Some steel manufacturers can supply these grades globally. 3-8

52 TABLE 3.3 North American AHSS Materials Portfolio (updated ) 3-9

53 3.6 Elongation versus tensile strength The AHSS (advanced high-strength steel) Guidelines published by World Auto Steel ( (Reference 3.2) provide a comparison between the various families of steel products in the form of tensile strength versus percent total elongation (Figure 3.4). The latter is a good measure of the formability for each material class. Each bubble in the graph represents the typical properties of all steel products in each category of steels, as produced by most of the major steel makers around the world. The steel grades shown in the bubbles are: IF (interstitial free) products Mild (mild steel) products BH (bake hardenable) products CMn (carbon-manganese and carbon-boron) products HSLA (high-strength low-alloy) products TRIP (transformation induced plasticity) products DP, CP (dual phase, complex phase) products AUST. SS (austenitic stainless steel) MART (martensitic) products Boron (hot stamped steel) TWIP (twinning-induced plasticity) 3rd Gen. AHSS The above bubbles may be placed into four groups: Conventional HSS (high-strength steel), Stainless Steels, AHSS (advanced high-strength steel) and UHSS (ultra highstrength steel). See section 3.2 above for further detail on each grouping. Steels within a fifth group, 3rd Generation AHSS are currently developed and will offer ultra highstrengths with higher elongation. It is clear from the graph that most of the traditional steel products obey an inverse relationship between strength and ductility. Bucking this trend are the dual phase and complex phase families of steel products. These products have recently attracted the attention they deserve for their excellent combination of higher strength and very good ductility, making them suitable for energy-absorption applications. Carrying this concept a step further are the TRIP (TRansformation Induced Plasticity) steels. TRIP steels provide further enhanced potential for energy absorption at thinner gauges, thus making it possible for a vehicle structure to provide improved safety at lower mass. 3-10

54 FIGURE 3.4 ELONGATION VERSUS TENSILE STRENGTH 3.7 Elongation versus after fabricated yield strength The above data are all based on tensile properties obtained from under formed materials. In actual service the steel sheets are strained during fabrication, which is known to increase their strength and decrease their ductility. Many of the formed parts are also subsequently painted and baked to cure the paint. Although not all steels respond to the straining and baking process many of them do. Key grades include Bake Hardening (BH), Dual Phase (DP) and TRIP steels. Figure 3.5 shows the yield strength increase from straining and baking for several steel grades. This has no significant effect on forming of the steel but it can certainly affect its performance in service. The effect is usually beneficial as straining and baking increase the stress levels (yield stress) at which permanent deformation begins. 3-11

55 FIGURE 3.5 INCREASE IN YIELD STRENGTH THROUGH WORK HARDENING (WH) 3.8 Elongation versus tensile strength for hot-formed steel The implementation of press-hardened applications and the utilization of hardenable steels are promising alternatives for optimized part geometries with complex shapes and no springback. Hot-stamped or press hardened steels typically use blanks that are heated up, formed in a press and rapidly cooled. Hot-formed (HF) steel is typically boronbased, containing percent boron, and is usually referred to as boron steel, (Reference 5.3). The processes used to produce boron steel bestow a unique combination of properties. Direct hot-forming may be used to deform the blank in the austenitic state (at high temperatures) or indirect hot-forming may be used to heat and finish the piece after most forming is completed at room temperature. In either case, the steel undergoes a series of transitions in elongation and strength (as shown in Figure 3.5 above), finishing with a rapid cooling to achieve the final desired microstructure and associated mechanical properties. In direct hot-forming, the boron-based steel is blanked at room temperature (1) and then heated to high enough temperature for austenization (2). The steel is then formed (stamped) while hot and quenched (3) in the forming tool, developing the martensitic microstructure. Some special post-forming work may be required to finish the pieces, which are exceptionally high-strength. For indirect hot-forming, the steel is blanked and pre-formed at room temperature. 3-12

56 The part is then heated and forming is completed while the steel is in this low-strength, high elongation state. A final quench in the die produces the final properties and shape. Parts made from boron steel benefit from several material advantages, including ultra high-strength and improved (reduced) springback. The part remains in the die through the cooling phase so springback is virtually nonexistent. The use of hot-formed boron steel is growing rapidly due to its ultra high-strength and good forming properties. FIGURE 3.6 EXAMPLE PROCESSING STEPS FOR DIRECT HOT FORMED COMPONENT 3.9 Yield strength versus strain rate More recently, consideration was given to the impact of the rate of straining of a particular material or component on its performance. Since steel is a strain rate sensitive material, its yield and tensile strength increases as the loading rate increases. This provides further benefits in its ability to sustain and absorb higher loads and higher input energy, such as in the case of deformation of a bumper or other structural component. Again, this is not a new discovery but it was only through the introduction of the Ultra-Light Steel Auto Body-advanced vehicle concepts (ULSAB-AVC) phase development that this benefit of steel began to be introduced in structural design of automobile components. Considerable effort was then expended in various laboratories around the world to generate tensile data at straining rates ranging from quasi-static (0.001/sec) to 1000/sec for many of the 3-13

57 above steel grades. The effect of the higher strain rate on the strength and ductility for DP 600 steels is provided in Figure 2.4. The data for these steels and other products of interest for use in the design of bumpers and other energy-absorbing components are available from many steel producers. Use of the tensile properties of steels at higher rates of loading has begun in automotive design and is expected to be universally used as more data for more steel grades become available and as automotive designers become more comfortable with the reliability of this data. FIGURE 3.7 STRESS VERSUS STRAIN AT DIFFERENT STRAIN RATES FOR DP 600 (THE DATA AT 1000 s-1 WERE OBTAINED USING THE SPLIT HOPKINSON BAR (SHB) METHOD) 3-14

58 3.10 Sheet steel descriptors Sheet steel is a complex product and there are many methods used to describe it. The following descriptors are often associated with automotive sheet steel: Type - Chemical composition, microstructure processing method or end-use are all used to describe the type of steel. Examples include low-carbon, dent resistant, microalloy, highstrength low alloy, recovery annealed, dual phase, bainitic and martensitic sheet. Grade - Physical properties such as yield strength, tensile strength or elongation are used to denote a grade. Examples include 180 MPa minimum yield strength and 1500 MPa minimum tensile strength. Steel Product - The final process that steel receives prior to shipment from a steel mill is often used to describe a steel product. Examples include hot rolled, cold rolled and coated sheet. Metallic Coating - The process used to apply a metallic coating or the type of metal in the metallic coating are used to describe steel. Examples include hot-dip galvanized, electrogalvanized and zinc coated sheet. Surface Condition - Surface smoothness is used to describe sheet steel. Examples are exposed, semi-exposed or unexposed body sheet. In practice, when specifying sheet steel, most (if not all) of the above descriptors are required to fully describe the desired steel product. Published documents, such as those of the Society of Automotive Engineers (SAE) greatly facilitate the correct specification of sheet steel. In this context, the relevant SAE documents are: Categorization and Properties of Low-Carbon Automotive Sheet Steels, SAE J2329 (Reference 3.5) Categorization and Properties of Dent Resistant, highvstrength and Ultra high-strength Automotive Sheet Steel, SAE J2340 (Reference 6.4) Selection of Galvanized (Hot Dipped and Electrodeposited) Steel Sheet, SAE J1562 (Reference 3.6) Chemical Compositions of SAE Carbon Steels, SAE J403 (Reference 3.7) Chemical Compositions of SAE Wrought Stainless Steels, SAE J405 (Reference 3.8) Categorization and Properties of Dent Resistant, Structural, High Strength Low Alloy, and Recovery Annealed Sheet Steels, SAE J2947 (Work In Process) 3-15

59 3.11 SAE J2329 low-carbon sheet steel Steel grade This SAE Recommended Practice establishes mechanical property ranges for low-carbon automotive hot-rolled sheet, cold-rolled sheet and metallic-coated sheet steels. It also contains information that explains the different nomenclature used with these steels. It is necessary for both the steel user and producer to know the mechanical properties and the range in these properties. There is a wide variety of parts within the automotive industry, and different levels of specific mechanical properties, e.g., r-value, n-value, yield strength and total elongation may be required for specific applications. It is suggested the steel user and steel supplier consult early in the part and die design process to determine specific grade requirements. In the past, yield strength has been chosen as a major discriminator of the categorization system since this property has meaning to both automotive and steel engineers, this document builds on that rationale but also addresses certain minimum elongation, n value and r-value discriminators. In this document, low-carbon sheet steel is classified by 5 grade levels with yield strength, tensile strength, elongation, r-value and n value requirements. In addition, surface quality and/or aging characteristics are an important consideration. Thus, the categorization system is as follows: 1. The first two alphabetic characters will designate hotrolled or cold-rolled method of manufacture. 2. The third numeric character defines grade based on yield strength range, minimum tensile strength, minimum percent elongation, minimum r-value and minimum n value. 3. The fourth alphabetic character classifies the steel type with regards to surface quality and/or aging character. 4. An optional fifth alphabetic character may restrict the carbon content to a minimum of percent. There are five grades of cold-rolled sheet and three grades of hot-rolled sheet. Mechanical properties are shown in Tables 2.5 and 2.6, while chemical composition is shown in Table Types of cold rolled sheet (table 3.3) There are two types of cold-rolled sheet, either in the bare or coated condition: E - Exposed. Intended for critical exposed applications where painted surface appearance is of primary importance. U - Unexposed. Intended for unexposed applications. 3-16

60 TABLE 3.4 COLD-ROLLED STEEL SHEET, COATED AND UNCOATED, MECHANICAL PROPERTIES (1)(2) Types of hot rolled sheet (table 3.4) There are four types of hot-rolled sheet, either bare or in the metallic coated condition: R A coiled product straight off the hot mill, typically known as hot roll black band. F A processed product in coils or cut lengths. The product may be susceptible to aging and coil breaks. N A processed product in coils or cut lengths. The product is non-aging at room temperature but is susceptible to coil breaks. M A processed product in coils or cut lengths. This product is non-aging at room temperature and free from coil breaks. When specifying a hot-rolled sheet, the surface condition should also be indicated. TABLE 3.5 HOT-ROLLED STEEL SHEET, COATED AND UNCOATED, MECHANICAL PROPERTIES (1)(2) 3-17

61 3.12 SAE J2340 dent resistant, high-strength and ultra high-strength sheet steel Steel grade This SAE Recommended Practice defines and establishes mechanical property ranges for seven grades of continuously cast high strength automotive sheet steels that can be formed, welded, assembled and painted in automotive manufacturing processes. The grade of steel specified for an identified part should be based on part requirements (configuration and strength) as well as formability. Material selection should also take into consideration the amount of strain induced by forming and the impact strain has on the strength achieved in the finished part. These steels can be specified as hot-rolled sheet, cold-reduced sheet, uncoated, or coated by hot dipping, electroplating or vapor deposition of zinc, aluminum, and organic compounds normally applied by coil coating. The grades and strength levels are achieved through chemical composition and special processing. Not all combinations of strength and coating types may be commercially available. Consult your steel supplier for details. In Tables 3.6, 3.7 and 3.8 (dent resistant and high-strength steels) grade is the minimum yield strength in MPa. In Table 3.9 (ultra high-strength steels) grade is the minimum tensile strength in MPa. TABLE 3.6 REQUIRED MINIMUM MECHANICAL PROPERTIES (1) OF DENT RESISTANT SHEET STEEL 3-18

62 TABLE 3.7 REQUIRED MECHANICAL PROPERTIES (1 ) OF HIGH STRENGTH AND HSLA HOT-ROLLED AND COLD-REDUCED UNCOATED AND COATED SHEET STEEL TABLE 3.8 REQUIRED MECHANICAL PROPERTIES (1) OF HIGH STRENGTH RECOVERY ANNEALED COLD- REDUCED SHEET STEEL (2) 3-19

63 TABLE 3.9 REQUIRED MECHANICAL PROPERTIES (1) OF HIGH STRENGTH RECOVERY ANNEALED COLD- REDUCED SHEET STEEL (2) Steel type In Tables 3.6 to 3.9, the steel type is defined by one or two letters as follows: A B AT, BT S X 3-20 Non-bake hardenable dent resistant steel in which increase in yield strength due to work hardening results from strain during forming. Bake hardenable dent resistant steel in which increase in yield strength due to work hardening results from strain during forming and an additional increase in yield strength that occurs during the paint-baking process. These types are similar to Types A and B respectively, except that the steel is interstitial free. High-strength steel, which is solution strengthened using C and Mn in combination with P or Si. High-strength steel typically referred to as HSLA. It is alloyed with carbide and nitride forming elements (commonly Nb (Cb), Ti and V) in combination with C, Mn, P and Si.

64 Y High-strength steel similar to Type X, except the spread between the minimum yield and tensile strengths is larger (100 MPa versus 70 MPa). SF,XF,YF These types are similar to types S, X and Y respectively, except they are sulphide inclusion controlled. R DL DH M High-strength steel that has been recovery annealed or stress-relief annealed. Its strength is primarily achieved through cold work during cold rolling at the steel mill. Dual phase ultra high-strength steel. Its microstructure is comprised of ferrite and martensite. The strength level is dictated by the volume of low-carbon martensite. DL dual phase has a low ratio of yield-totensile strength (less than or equal to 0.7). A dual phase ultra high-strength steel similar to Type DL, except it has a high ratio of yield to tensile strength (greater than 0.7). Martensitic ultra high-strength steel, the carbon content of which determines the strength level Hot rolled, cold reduced and metallic coated sheet The steels in Tables 3.6 to 3.9 can be specified as either hotrolled sheet or cold-rolled sheet in either the bare or metallic coated condition. Hot-dipped or electrogalvanized coated sheets are covered by SAE J1562 (Section 2.12). All of the steels shown in Tables 3.6 to 3.9 may not be commercially available in all types of coatings. Consult your steel supplier. Also, hot-rolled sheet for the steels shown in Tables 3.6 to 3.9 may not be commercially available in thicknesses below mm. Again, consult your steel supplier Surface conditions for cold reduced and metallic coated sheet Cold reduced and metallic coated sheet steel is available in three surface conditions: Exposed. Intended for critical exposed applications where painted surface appearance is of primary importance. Unexposed. Intended for unexposed applications. Semi-exposed. Intended for non-critical exposed applications Conditions for hot rolled sheet Four conditions of hot-rolled sheets are available: P: A coiled product straight off the hot mill, typically known as hot roll black band. W: A processed product in coils or cut lengths. The product may be susceptible to aging. 3-21

65 N: A processed product in coils or cut lengths. The mechanical properties do not deteriorate at room temperature. V: A processed product in coils or cut lengths. The mechanical properties do not deteriorate at room temperature. The product is free of coil breaks. When specifying a hot-rolled sheet, the desired surface condition should also be indicated (E, U or Z as per Section 2.1.4) SAE J2947 categorization and properties of dent resistant, structural, high strength low alloy and recovery annealed sheet steels Note: This specification is a work in progress and will be published by SAE when complete Scope Rationale This SAE Recommended Practice defines and establishes mechanical property ranges for six grades of continuously cast high strength automotive sheet steels that can be formed, welded, assembled and painted in automotive manufacturing processes. The specification is being created to bring all high-strength sheet steel grades in concert with J2329 and J2745 definitions of conventional and Advanced High-Strength Steel (AHSS) respectively. The spec will also add grades and requirements which have evolved in the steel and auto industry since the last revision of J SAE J1562 zinc and zinc-alloy coated sheet steel Zinc and zinc-alloy coated steel is used to enhance a structure s protection against corrosion degradation. For the purpose of this SAE Recommended Practice, a galvanized coating is defined as a zinc or zinc-alloy metallic coating. The selection of the optimum galvanized steel sheet product depends on many factors, the most important being desired corrosion protection, formability, weldability, surface characteristics and paintability. The trade-offs of these product characteristics are more complex than is the case with uncoated steel sheet products. This SAE Recommended Practice defines preferred product characteristics for galvanized coatings applied to sheet steel. A galvanized coating is defined as a zinc or zinc-alloy metallic coating. 3-22

66 Galvanizing processes Types of coatings Two generic processes for metallic coated sheets are currently used in the automotive industry: Hot-dip process. A coil of sheet steel is passed continuously through a molten metal bath. Upon emergence from the bath, the molten metal coating mass is controlled by air (or other gas) knives or mechanical wipers before the coating solidifies. This process produces a sheet with a coating on two sides. Electrodeposition process. This continuous coating process uses cells in which the metallic coating is electrodeposited on a coil of sheet steel. This process can produce a sheet with a coating on either one or two sides. The types of commercially produced metallic coatings include: Hot-dip galvanized. Essentially a pure zinc coating applied by the hot-dip galvanizing process. Electrogalvanized. Essentially a pure zinc coating applied by the electrodeposition galvanizing process. Galvannealed. A zinc-iron alloy coating applied by the hot-dip galvanizing process. The coating typically contains 8-12% percent iron by weight. Alloy. Aluminum-zinc silicon alloy (55 percent, 43 percent and 2 percent by weight respectively) and zinc-aluminum alloy (5 percent aluminum by weight) coatings are applied by the hot-dip galvanizing process. Zinc-iron alloy (<20 percent iron by weight) and zinc-nickel (<20 percent nickel by weight) coatings are applied by the electrodeposition process. Zinc coated sheet (hot-dip galvanized and electrogalvanized) offers superior corrosion resistance. Through sacrificial electrochemical action, zinc coatings protect bare (cut) edges. Galvanneal, due to its lighter zinc content, has less corrosion resistance than pure zinc coatings. However, its iron content provides enhanced spot weldability and paintability. Hot-dip galvanized, electrogalvanized and galvanneal are, by far, the most commonly used coatings for vehicle components. Zincaluminum and zinc-nickel coatings have niche applications. For example, zinc-aluminum alloy offers improved corrosion resistance to acids; hence, it is often used for mufflers. 3-23

67 Coating mass Surface quality Coating mass is expressed in g/m2. The approximate thickness of a coating in microns = g/m2 x The approximate thickness of a coating in mils = g/m2 x The heavier the coating mass equals the greater the corrosion resistance of a metallic coated sheet. However, spot weldability decreases with an increase in coating mass. Three surface qualities may be specified: Exposed Semi-exposed Unexposed Coated sheet thickness The thickness of metallic coated sheet steel is determined by measuring, as a single unit, the combination of the base sheet steel and all metallic coatings Coating designations SAE J2329 uses a nine-character designation system to identify the galvanizing process, the E-coating type and mass of each side of the sheet and surface quality. The first and second characters denote the galvanizing process: HD = hot-dip galvanized EG = electrogalvanized (electrodeposition) The third and fourth characters denote the coating mass of the unexposed side in accordance with Table

68 TABLE 3.10 RECOMMENDED COATING MASS FOR GALVANIZED STEEL SHEET The fifth character denotes the E-coating type of the unexposed side: G = pure zinc A = zinc-iron N = zinc-nickel X = other than G, A or N The sixth and seventh characters denote the E-coating mass of the exposed side in accordance with Table Examples of typical specification and ordering descriptions for automotive sheet steel are given in Section The eighth character denotes the E-coating type of the exposed side: G = pure zinc A = zinc-iron N = zinc-nickel X = other than G, A or N The ninth character denotes surface quality: E = Exposed Z= Semi-exposed U = Unexposed 3-25

69 3.15 SAE J1562 zinc and zinc-alloy coated sheet steel This SAE Recommended Practice provides chemical composition ranges for carbon steels supplied to certified chemical composition rather than to certified mechanical properties. SAE J403 uses a four or five character system to designate steel grade: The first two characters are the number 10, which indicate that the grade is carbon steel. The last two characters represent the nominal carbon content of the grade in points of carbon. One point of carbon is 0.01 percent carbon by weight. Five points would be shown as 05, fifteen points as 15, etc. If boron is added to a carbon steel to improve hardenability, the letter B is inserted between the first two characters and the last two characters. Examples of typical specification and ordering descriptions for automotive sheet are given in Section Carbon sheet steel SAE J403 provides compositions for carbon grade sheet steels. Table 3.11 shows the compositions for grades 1006 through SAE J403 provides compositions for grades 1006 through However, grades above 1025 have relatively low formability and weldability due to their relatively high carbon content. Thus, grades above 1025 are seldom used for automotive sheet applications. It is important to note that sheet steels specified or ordered to SAE J403 are not supplied with certified mechanical properties. If certified mechanical properties are required, automotive sheet steel should be specified or ordered in accordance with SAE J2329 (Section 3.11) or SAE J2340 (Section 3.12). 3-26

70 TABLE 3.11 SAE J403 CARBON STEEL COMPOSITIONS FOR SHEET 3-27

71 Boron sheet steel The addition of boron to carbon sheet steel improves its hardenability. For this reason, boron sheet steel is an ideal material for hot stampings. As an example, 10B21 (Modified) is used for hot-stamped bumper reinforcing beams. As received, this steel has a yield strength in the range MPa. Following hot stamping and quenching in liquid-cooled dies, the yield strength is raised to about 1040 MPa Grade list maintenance ISTC Division 1 has developed a procedure which allows for the maintenance of the grade lists in this document. This will involve conducting an industry-wide survey to solicit input. This survey will be conducted at a frequency deemed necessary by the technical committee. Criteria have been established for the addition to or the deletion of grades from the grade lists. New grades will be considered based on the grade meeting a SAE grade designation and chemistry, having a minimum production or consumption of 225 tonnes/year (250 tons/year) and has the sponsorship of at least two individual users or producers. New steel compositions will be considered as Potential Standard (PS) steels, based on the guidelines in SAE J1081, until such time as production of the new steel achieves a level of production or usage qualifying it for consideration as a standard steel. Deletion of grades will be by consensus based on the grade survey. Deleted grades will be archived in SAE J1249. When the cast or heat analysis is requested to be reported to demonstrate conformance to the chemical limits shown in Tables 1, 2, 3A or 3B, in addition to the quantities of carbon, manganese, phosphorus and sulfur, the following elements and their quantities shall also be reported: copper, chromium, nickel, molybdenum and silicon. When the amount of any one of these last five elements is less than 0.02 percent, the analysis may be reported as <0.02 percent SAE J405 wrought stainless steels This SAE Standard provides chemical composition requirements for wrought stainless steels supplied to chemical composition rather than to certified mechanical properties. The standard uses three series to designate stainless steel grades: S20000, S30000 and S S20000 designates nickel-chromium-manganese, corrosion resistant types that are nonhardenable by thermal treatment. S30000 designates nickel-chromium, corrosion resistant steels, nonhardenable by thermal treatment. S40000 includes both a hardenable, martensitic-chromium steel and nonhardenable, ferritic-chromium steel. Table 3.12 shows the chemical compositions for two stainless steel grades that are appropriate not only for bumper facebars but also for bumper reinforcing beams. 3-28

72 TABLE 3.12 SAE J405 CHEMICAL COMPOSITIONS OF WROUGHT STAINLESS STEELS, % (maximum unless a range is indicated) 3.17 SAE specification and ordering descriptions The following examples represent typical specification and ordering descriptions for automotive sheet steel: i. SAE J2329 CR2E Cold-rolled sheet steel, grade 2 (Tables 3.6 & 3.8), exposed surface condition. ii. SAE J2329 HR3MU Hot-rolled sheet steel, grade 3 (Tables 3.7 & 3.8), non-aging at room temperature and free from coil breaks, unexposed surface condition. iii. SAE J2329 CR4C EG60G60GE Cold-rolled sheet steel, grade 4 (Tables 3.6 & 3.8), minimum carbon percent, each side electrogalvanized coated to 60g/m2, critical exposed surface condition. iv. SAE J2329 HR2M 45A45AU Hot-rolled sheet steel, grade 2 (Tables 3.7 & 3.8), non-aging at room temperature and free from coil breaks, each side galvanneal coated to 45g/m2, unexposed surface condition. v. SAE J2340 CR 180A Cold reduced sheet steel, grade HD70G70GZ 180 non-bake hardenable dent resistant (Table 3.9), each side hot-dip galvanized coated to 70g/ m2, semi-exposed surface condition. vi. SAE J2340 CR 250B Cold reduced sheet steel, grade EG70G70GE 250 bake hardenable dent resistant (Table 3.9), each side electrogalvanized coated to 70g/m2, critical exposed surface condition. vii. SAE J2340 HR 340XU Hot-rolled sheet steel, grade 340 high-strength low-alloy (Table 3.10), unexposed surface condition. viii. SAE J2340 CR 1300MU Cold reduced sheet steel, grade 1300 ultra high-strength martensitic (Table 3.12), unexposed surface condition. ix. SAE J1562 EG70G70GE Electrogalvanized sheet having a 70 g/m2 minimum zinc coating (Table 3.13) on each side for an exposed application. x. SAE J1562 HD70G20AE Hot-dip galvanized sheet having a 70g/m2 minimum zinc coating (Table 3.13) on 3-29

73 the unexposed side and a 20g/m2 minimum zinc-iron coating (Table 3.13) on the exposed side for an exposed application. xi. SAE J1562 HD90G90GU Hot-dip galvanized sheet having a 90g/m2 minimum coating (Table 3.13) on each side for an unexposed application. xii. SAE J1562 HD45A45AU Hot-dip galvanized sheet having a 45g/m2 minimum zinc-iron coating (Table 3.13) on each side for an unexposed application. xiii. SAE J1562 EG30N30NE Electrogalvanized sheet having a 30g/m2 minimum zinc-nickel coating (Table 3.13) on each side for an exposed application. xiv. SAE J1562 EG70G00XE Electrogalvanized sheet having a 70g/m2 minimum zinc coating (Table 3.13) on the unexposed side and no coating on the exposed side for an exposed application. xv. SAE J403 HR1010U Hot rolled sheet steel, grade 1010 (Table 3.14), unexposed surface condition. xvi. SAE J403 Hot rolled sheet steel, grade HR1008HD90G90GU 1008 (Table 3.14), having a 90g/ m2 minimum coating on each side for an unexposed application ASTM A463/A463M-15 aluminized sheet steel Scope This specification covers aluminum-coated steel sheet in coils and cut lengths available with two types of aluminum coating applied by the hot-dip process, with several coating weights. The heat analysis of the base metal shall conform to the requirements specified. Structural steel and high-strength low alloy steel shall conform to the mechanical property requirements. Coating weight and bend tests shall be made to conform to the requirements specified This specification covers aluminum-coated steel sheet in coils and cut lengths available with two types of aluminum coating applied by the hot-dip process, with several coating weights [masses]. Product furnished under this specification shall conform to the applicable requirements of the latest issue of Specification A924/A924M, unless otherwise provided herein. 3-30

74 The text of this standard references notes and footnotes which provide explanatory material. These notes and footnotes (excluding those in tables and figures) shall not be considered as requirements of the standard This specification is applicable to orders in either inch-pound units (as A463) or SI units [as A463M]. Values in inch-pound and SI units are not necessarily equivalent. Within the text, SI units are shown in brackets. Each system shall be used independently of the other Unless the order specifies the M designation (SI units), the product shall be furnished to inch-pound units Product use This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Aluminized sheet has an aluminum-silicon alloy on each side applied by a continuous hot-dip process. The coated sheet has the surface characteristics of aluminum with the superior strength and lower cost of steel. One specification, which describes aluminized steel, is ASTM A463 (Reference 3.9). The quality of the sheet steel can be commercial (CS Types A, B and C), forming (FS), deep drawing (DDS), extra deep drawing (EDDS), structural (SS), high-strength low-alloy (HSLAS), high-strength low-alloy with improved formability (HSLAS-F) and ferritic stainless steel (FSS Types 409 and 439). Chemical and mechanical properties are given for all qualities. A463 also defines the type of aluminum-zinc coating and coating weights. For hot-formed bumper beams (see Section 3.4), boron steel with a Type 1 coating is commonly used. The mechanical properties of the boron steel are discussed in Section The Type 1 aluminum coating contains about 10 percent silicon. The coating weight (total both sides) is typically g/m2 ( oz/ft2). 3-31

75 4. Manufacturing processes 4.1 Stamping Stretching The concept of major and minor strain can be used to describe different kinds of sheet forming processes. In cases where the sheet is stretched over a punch, the major strain is always positive. For stretching, the minor strain is usually positive as well. Different punch and clamping configurations can create a variety of major and minor strain levels. In circle grid analysis (CGA), small circles are etched on the surface of the steel sheet prior to stamping (Figure 4.1). After stamping, the deformed circles are compared to the original circles (Figure 4.2). For the condition of plane strain, the deformed circle is an ellipse with the minor strain diameter equal to the original diameter of the underformed circle. A minor strain equal to the major strain is indicated by an original circle, which remains circular after deformation. However, the diameter of the circle after deformation is larger than the diameter before deformation. This condition is called equi-biaxial stretch because the amount of the stretch is equal, regardless of the direction in the plane of the sheet. FIGURE 4.1 TYPICAL CIRCLE GRID PATTERN 4-1

76 FIGURE 4.2 REPRESENTATION OF STRAINS BY ETCHED CIRCLES Drawing Bending When a sheet is pulled into a die cavity and must contract to flow into the cavity in areas such as a corner, or the flange of a circular cup, the sheet is said to be undergoing drawing. Drawing (also known as deep drawing) generates compressive forces in the flange area being drawn into the die cavity. Negative minor strains are generated. In contrast to failures in stretching, failures in drawing do not normally occur in the flange area where the compression and flow of sheet metal is occurring. Instead, necking and fracture occur in the wall of the stamping near the nose of the punch. Failure occurs here because the force causing the deformation in the flange must be transmitted from the punch through this region. If the force required to deform the flange is too great, it cannot be transmitted by the wall without overloading the wall. Bending differs from drawing and stretching, because the deformation present in bending is not homogeneous through the thickness of the material. For pure bending, where there is no superimposed tension or compression on the bending process, the center of the sheet has zero strain. The outer surface is elongated, with a tensile strain equal to t/2r (t=steel thickness, r=bend radius to the midpoint of the steel thickness). The inner surface is compressed, with 4-2

77 a compressive strain equal to t/2r. The strain varies from compressive at the inner radius, through zero at the midpoint of the thickness, to tensile at the outside radius. In pure bending, the compressive and tensile strains are equal. Because the strain varies through the thickness, forming limit analysis (Section 4.1.5) does not directly apply. Materials with very little capacity to be formed can frequently undergo bending operations successfully. The tendency to thin locally, with necking and fracture, is not present in bending. Cold working of the material does take place. However, the amount of work hardening depends on the radius of the bend and the thickness of the material. A sharper radius (smaller r) or thicker material (greater t) causes an increase in strain at the surface. Bending is a plane strain operation. The length of the bend does not change during bending, except for localized distortion at the edge of the sheet Bending and straightening Forming limits As material passes through a draw bead or over a die lip, it is bent, straightened, and sometimes re-bent in the opposite direction. The net strain at the end of this process is small, although cold work has occurred and the material is harder than it was before the process began. As a result, the ability to deform the material in subsequent operations is decreased. The measurement of strain provides an important tool for determining the local deformation that occurs in a complicated process. Sharply changing levels of strain usually indicate a localization of deformation and a higher likelihood of necking and failure during forming. For sheet metal, it has been found that a limit to the major strain exists for each level of minor strain. This phenomenon has been studied in the laboratory and has resulted in the creation of forming limit diagrams. First, flat sheets of a given material are etched with circles as shown in Figure 4.1. The flat sheets are then deformed in a variety of configurations to develop a large range of major and minor strains. If the forming process for any given configuration is continued until failure (as defined by localized necking), the major and minor strains at failure, as shown in Figure 4.2, can be measured for that configuration. By plotting the failure strains of the various configurations, a boundary line indicating the major strain limit for each minor strain is obtained (Figure 4.3). While this limit is not absolute, there is a very high probability of failure above this boundary line and a low probability of failure below this line. 4-3

78 4.2 Roll forming The boundary line is frequently called the forming limit curve, and the entire graph is known as the forming line diagram (FLD). A second forming limit curve, plotted with major strains 10% below those of the boundary line, is sometimes used to provide a safety factor. Each combination of material properties and thickness results in a different FLD. Cold roll forming is a process whereby a sheet or strip of metal is formed into a uniform cross section by feeding the stock longitudinally through a roll forming mill. The mill consists of a train with pairs of driven roller dies, which progressively form the flat strip until the finished shape is produced. The number of pairs of rolls depends on the type of material being formed, the complexity of the shape being produced, and the design of the particular mill being used. A conventional roll forming mill may have as many as 30 pairs of roller dies mounted on individually driven horizontal shafts. Roll forming is one of the few sheet metal forming processes that is confined to a single primary mode of deformation. Unlike most forming operations that have various combinations of stretching, drawing, bending, bending and straightening, and other forming modes, the roll forming process is nothing more than a carefully designed series of bends. In roll forming, metal thickness is not changed except for a slight thinning at the bend radii. FIGURE 4.3 TYPICAL FORMING LIMIT DIAGRAM 4-4

79 The roll forming process is particularly suited to the production of long lengths of complex shapes held to close tolerances. Large quantities of these parts can be formed with a minimum of handling and manpower. The process can be continuous by coil feeding and exit cutting to length. Operations such as notching, slotting, punching, embossing and curving can easily be combined with con- tour roll forming to produce finished parts off the exit end of the roll forming mill. In fact, ultra high-strength steel reinforcing beams, with sweeps up to 50, only need to have the mounting brackets welded to them before shipment to the assembly line. 4.3 Hydroforming There are two types of hydroforming - sheet and tubular. Sheet hydroforming is typically a process where only a female die is constructed and a bladder membrane performs as the punch. High pressure fluid (usually water) forces the bladder against the steel sheet until it takes the shape of the female die. Sheet hydroforming has several advantages versus stamping such as lower tooling costs and less friction during forming. However, it is limited to lower volume applications due to its higher cycle time. In tubular hydroforming, a straight or pre-bent tube is laid into a lower die. The upper and lower dies are then clamped together. Next, conical nozzles are inserted and clamped into each end of the tube. Finally, a fluid (usually water) is forced at a high pressure into the tube until it takes the shape of the die. While tube hydroforming technology has been around for decades, the mass production of automotive parts only became cost effective in about The benefits of hydroforming are usually found via part consolidation and the elimination of engineered scrap. Box sections consisting of two hat sections welded together lend themselves to cost-effective replacement by a single, hydroformed part. Punches mounted in the forming dies are used to pierce holes during forming, eliminating subsequent machine operations. The structural integrity of a hydroformed part made from a single continuous tube is superior to that of a part made from two or more components. Weight savings of 10 to 20 percent can be achieved via both reducing gauge and eliminating weld flanges. If flanges are necessary for attachment, they can be created by pinching the tube during the hydroforming process. High volume tubular hydroformed parts are currently incorporated into automotive components such as subframes, ladder frames, IP beams, roof rails and exhaust components. 4-5

80 4.4 Hot forming Generally speaking, as the strength of steel increases, its ductility decreases. One method used to overcome the reduced formability of ultra high-strength steel is hot forming. Hot formed bumper beams have very high-strength. They offer not only mass reduction but also large and compound sweeps. Highly complex beams can be produced in one piece. The repeatability of dimensions is very good and there is no springback, a phenomenon which is common with cold forming processes. The hot forming process involves the following steps: Blanking Heating Forming/Quenching De-scaling (if required) The typical material used for hot stamping is boron steel having 0.22 percent carbon, percent boron, an asdelivered yield strength of 330 MPa (47.9 ksi), an as-delivered tensile strength of 500 MPa (72.5 ksi) and a percent elongation. The boron steel may be bare or aluminized. If aluminized, a hot dip Type 1 coating (10 percent silicon) and a coating mass of g/m2 ( mils) are common. After heating and quenching, a hot formed part has very high hardness (470 HV). Thus, it is best to punch any required holes into the blank. The developed blanks or pre-formed parts are continuously fed into a furnace. They are heated to austenitizing temperatures, approximately 900ºC (1650ºF). If bare steel is used, the furnace usually has a non-oxidizing atmosphere to suppress scale formation. However, on transfer to the forming/quenching press, some scale will form. If aluminized steel is used, a Fe-Al alloy forms in the furnace on the surface of the steel sheet and scaling is avoided. In the forming/quenching press, the blank/pre-formed section is formed to its final shape using dies maintained at room temperature. The part is held in the die until it is sufficiently quenched. Some tempering is usually required. Tempering may be accomplished by ejecting the part from the forming/quenching dies while it is still fairly hot or by baking the quenched part in an oven. The yield strength of the final hot formed part for a common 10B21 boron steel has increased to about 1140 MPa (165 ksi) and the tensile strength to about 1520 MPa (220 ksi). Elongation has decreased to less than 6 percent. A part made from aluminized sheet has a hard Fe-Al-Si coating system and is scale free, eliminating the need for de-scaling. Further, this coating system provides corrosion protection for the finished part. A part made from bare sheet does have scale and de-scaling is often employed. 4-6

81 5. Manufacturing considerations 5.1 Forming considerations High-strength and ultra high-strength steels have less ductility, and hence less formability, than lower strength steels. Thus, care must be taken in part design and forming method selection. In addition, springback increases with yield strength and it must be accounted for in the process design. Sections through provide Guidelines and Rules of Thumb for the roll forming and stamping processes. The Guidelines and Rules of Thumb are based on practical experience. Their use will help alleviate formability and springback issues associated with the roll forming and stamping of high-strength and ultra high-strength steels Guidelines for roll forming high-strength steel All of the high-strength steels in Table 3.7 can be roll formed, pre-pierced and swept after roll forming. The following Guidelines apply (Reference 5.2): Do: Select the appropriate number of roll stands for the material being formed. Remember the higher the steel strength, the greater the number of stands required on wthe roll former. Use the minimum allowable bend radius for the material in order to minimize springback. Position holes away from the bend radius to help achieve desired tolerances. Establish mechanical and dimensional tolerances for successful part production. Use appropriate lubrication. Use a suitable maintenance schedule for the roll forming line. Anticipate end flare (a form of springback). End flare is caused by stresses that build up during the roll forming process. Recognize that as a part is being swept (or reformed after roll forming), the compression of metal can cause sidewall buckling, which leads to fit-up problems. Don t: Do not roll form with worn tooling, as the use of worn tools increases the severity of buckling. Do not expect steels of similar yield strength from different steel sources to behave similarly. Do not over-specify tolerances. 5-1

82 5.1.2 Guidelines for roll forming ultra-high-strength steel All of the ultrahigh-strength steels in Table 3.3 can be roll formed, pre-pierced and swept after roll forming. The following Guidelines apply (Reference 5.2): 1. The minimum bend radius should be three times the thickness of the steel to avoid fracture. 2. Springback magnitude can range from ten degrees for 120X steel to 30 degrees for M220HT steel, as compared to one to three degrees for mild steel. Springback should be accounted for when designing the roll forming process. 3. Due to the higher springback, it is difficult to achieve reasonable tolerances on sections with large radii (radii greater than 20 times the thickness of the steel). 4. Rolls should be designed with a constant radius and an evenly distributed overbend from pass to pass. 5. About 50 percent more passes (compared to mild steel) are required when roll forming ultra high-strength steel. The number of passes required is affected by the number of profile bends, mechanical properties of the steel, section depth-to-steel thickness ratio, tolerance requirements, pre-punched holes and notches. 6. Due to the higher number of passes and higher material strength, the horsepower requirement for forming is increased. 7. Due to the higher material strength, the forming pressure is also higher. Larger shaft diameters should be considered. Thin, slender rolls should be avoided. 8. During roll forming, avoid undue permanent elongation of portions of the cross section that will be compressed during the sweeping process General guidelines for stamping high-strength and ultra-high-strength steels All of the high-strength steels in Table 3.1 may be stamped into bumper beams. Additionally, some ultra high-strength steels in Table 3.2, such as 120X, 590T, 780T and 140T, may be stamped, bend stretched, drawn and flanged. The following guidelines apply (Reference 5.2): PRODUCT DESIGN Avoid designing parts that require a draw forming operation (i.e., metal must flow or stretch off the binder). Maintain gentle shape changes and constant cross sections wherever possible in part design. 5-2

83 These factors become more important as material strength is increased. Keep the depth of the part to a minimum when the part has excessive sweeps in the plan view or elevation. Avoid designing parts with closed corners that require draw die operations. Keep the flanges as short as possible when there is a deep-formed offset flange. DIE PROCESS Try to form the parts completely to the depth desired in the first forming operation. Minimize stretch and compression of metal to reduce residual strains that cause springback and twist in the part. Use high pressure on the draw binder and balancing blocks. They allow the sheet metal to flow without wrinkling. Keep the side walls perpendicular (90 degrees to the base of the die). Avoid open-angle forming. Overbend the flanges 6 to 10 degrees. On straight channel-shaped parts, consider a solid form die. Pre-forming the sheet steel is a method commonly used to accumulate enough material to ensure that adequate metal is available for forming without splitting or excessive thinning. DIE DESIGN Maintain die forming radii as sharp as possible. Try to fold the metal rather than stretch it over a radius. Folding reduces curl of the sidewalls and springback of the weld flanges. Maintain an even draw depth and length of line. Design robust dies to minimize flexing of the die components. DIE CONSTRUCTION / TRYOUT Sidewalls should be as tight as possible to lessen springback. To reduce shock and press tonnage requirements, a minimum shear of four to six times metal thicknesses is required for cutting dies. This minimum shear also reduces noise on break through. Trim and pierce dies should have 7 percent to 10 percent die clearance. 5-3

84 FIGURE 5.1 a) RULES OF THUMB - SPRINGBACK The techniques shown in Figures 5.1 a) through 5.1 c) can help compensate for springback when forming a 90-degree bend if a sharp radius or a tight flange (see Figure 5.3) is not adequate. Refer to Figure 5.1a. 1. Restrike the flange at an overbend angle between 3 and 7 degrees, depending on the material strength and/or thickness. 2. Set up part in die to allow for overbend. 3. Undercut the lower die steel and let the metal overbend. 4. Pre-form the top part surface prior to flanging and flatten the part using the die pad. 5-5

85 FIGURE 5.1 b) RULES OF THUMB SPRINGBACK Refer to Figure 5.1b) 1. The addition of stiffening darts helps maintain a 90-degree flange. 2. Coining a flange radius as the die bottoms will help maintain form and helps prevent springback. 3. An extension of the upper flange steel allows for extra pressure to be applied on the formed radius. This is a difficult process to control, but it could help in special conditions, particularly on heavier gauge steels. 5-6

86 FIGURE 5.1 c) RULES OF THUMB - SPRINGBACK Refer to Figure 5.1c) 1. The addition of stiffening darts helps maintain a 90-degree flange. 2. Providing a vertical step in the flange stiffens and straightens the flange, stopping sidewall curl as well as springback. 3. Rotary benders are used by many manufacturers to control springback, as the metal is rolled around the radius instead of flanging. Positive comments on this method promote its ability to overbend the flange. 4. Place a 90 durometer urethane behind flanging steels in a free state (not compressed). Clearance holes through the flanging steels allow the screws to hold the urethane in place. Please note the urethane must stay 0.25 inches (6.4 mm) off the bottom of the pocket. This space leaves room for the urethane deflection. Tighten clearance until desired effect is achieved. 5. By adding a horizontal step along the flange, the flange is stiffened, resulting in reduced springback. 5-7

87 FIGURE 5.2 RULES OF THUMB - DIE FLANGE STEELS Typical flange die configuration Refer to Figure 5.2) 1. Flange steel clearance should be 90 percent of metal thickness, but no greater than metal thickness. Maintaining a tight condition helps to prevent springback. 2. Because of the tight clearance, the die steels should be as hard as possible. Therefore, it is recommended that air-hardened tool steel or harder material be used, and a surface coating be applied to increase hardness and improve lubricity. 3. Air-hardened tool steel (D2) is recommended for flange steel (Rockwell on the C-scale). However, other materials may be used as long as they have a surface coating applied which resists scoring. 4. All flanging radii should be as sharp as possible without fracturing the sheet metal during forming. The flange radii should be some- thing less than metal thickness. Start by just breaking the sharp corner and work from there until you can make the flange without splitting the sheet metal. 5-8

88 FIGURE 5.3 RULES OF THUMB - HAT SECTION Refer to Figure 5.3) 1. Maintain a constant depth on hat sections, if at all possible. 2. The size of the radius is to be kept as small as possible, normally less than metal thickness. 3. Form 90-degree side walls on the hat section whenever possible. 4. If the sidewall is not 90 degrees, try to balance the forming with the same angle on the opposite side of the hat section. 5. Unequal residual strain and/or compression on opposite side-walls have a tendency to twist the entire rail. 5-9

89 FIGURE 5.4 RULES OF THUMB - RADIUS SETTING When forming a hat section, the action of the die can aid the retention of shape by setting the corner radii. Refer to Figure As the flange steels make contact with the sheet metal blank, an initial crown is formed. 2. The flange steels then enter over the die-post radii and force the metal to conform to the lower die. The crown remains in the part. It is best if both sides enter simultaneously. 3. The die is now very close to its home position. The crown remains and the lower flanges are starting to form. 4. As the die is closed, the lower flanges are formed with corner radii as sharp as possible. The top corners are forced outward as the crown is hit home by the upper die. If the part retains a crown, then a negative crown can be incorporated to minimize springback. 5-10

90 FIGURE 5.5 a) RULES OF THUMB - COMBINATION FORM & FLANGE DIE Using a combination form-and-flange die is basic to meeting high-strength steel requirements. A general idea of how this die works follows. The die initially forms the contour in the developed blank using the upper pressure pad. The metal is then locked, using the lock beads to prevent feeding the metal in from the ends. The metal is allowed to flow in freely from the sides without restrictions within the ring, a metal thickness apart to stop wrinkling. The flange steels are maintained as sharp as possible, and the side walls are tight. This procedure controls the springback and sidewall curl in order to produce a quality part. If the part is straight, see Figure 5.4 for more information. The four-piece form and flange die shown above incorporates features that lend themselves to the production of hat section parts. Remember that in order for this type of die to work, the finished part must be off the ring when the part is completely formed in order to avoid upstroke deformation. The unique features of this die are as follows: Refer to Figure 5.5a) 1. The upper pressure pad gives the sheet metal blank its initial contour and holds the blank in location. 2. The lower ring (known also as a lower pressure pad) controls the flow of the metal and prevents wrinkling as the part is being formed (See 5 and 6 on Figure 5.5 b). 5-11

91 FIGURE 5.5 b) RULES OF THUMB - COMBINATION FORM & FLANGE DIE Refer to Figure 5.5a) and Figure 5.5b) 1. Flange steels should be kept tight to the lower post to help prevent sidewall curl. 2. A smaller-than-metal thickness radius on the lower post helps prevent springback. 3. Restraining beads are used to restrain the flow in at the ends of the rail. The metal must flow off the ring and on to the die post to prevent the panel from being deformed by the upstroke of the die. 4. Metal thickness clearance between the upper and lower ring under high pressure is needed to allow the metal to flow in from the sides without buckling. 5. Balancing blocks (leveling blocks, kiss blocks or spacer blocks) are used to control the clearance between the upper form steels and the lower ring surfaces in order to adjust for metal flow. 6. If the rail is open-ended, there is no need to restrict metal flow unless stretch is required to help prevent twist. 5-12

92 FIGURE 5.6 RULES OF THUMB - FORMING BEADS Refer to Figure Half-round draw beads are used to control metal flow. They restrict the flow and force the metal to stretch or control feed as required to produce the draw shape of the part. 2. Lock beads are generally used to stop the metal from moving. This condition is pure stretch. In general, it is recommended that this type of bead be avoided in dies used to form high-strength steel material. 3. Start lock bead configurations with radii small enough to shear the sheet metal blank. Then uniformly dress the radii to eliminate cutting, but still locking the metal flow. When the beads need reworking, repeat this procedure. 5-13

93 FIGURE 5.7 RULES OF THUMB - FORMING AN EMBOSS When forming an emboss or surface formation into a relatively flat high-strength steel part, the break lines need to be sharp and crisp. You must coin these lines into the part to set them and reduce any springback or distortion. Sidewalls of the embossment must be 45 degrees or less from the surface. Refer to Figure This formation is totally within the part s perimeter and does not extend to the trim. 2. This example shows the formation open to the part s trim edge. This formation causes excess or loose metal along the edge. Therefore, it is recommended that a short flange and/or small bead be added to stiffen and eliminate this condition. 5-14

94 FIGURE 5.8 RULES OF THUMB - EDGE SPLITTING It is important that the trim quality be maintained to prevent edge-splitting from work hardening. Refer to Figure When forming an outside corner, the trim edge has a tendency to wrinkle. In order to minimize this wrinkling condition, it is recommended that the flange in the area of the wrinkle be as short as possible. 2. Inside corners have a tendency to split. Therefore, try to make the trim line as long as possible by scalloping the edge. 3. A combination of shortening the flange and lengthening the trim line should help stop the splitting. 4. If not, a formation change has to be made to add material to the split area. 5-15

95 FIGURE 5.9 RULES OF THUMB - PART DESIGN Refer to Figure The following are general characteristics of high-strength steel (HSS) that should be taken into consideration during the part design phase: a. HSS will stretch, generally in the range of 2 percent to 6 percent. b. HSS will resist compression due to the hardness of the material. 2. These characteristics of HSS generally require that parts be designed for form and flange die processes rather than draw dies. 3. In some cases, it is necessary to compensate for these material characteristics by designing in darts and/ or notches to equalize the length of line and to help maintain part dimensional integrity. 4. The above diagram shows how these darts and notches could be applied to an HSS part. 5-16

96 FIGURE 5.10 RULES OF THUMB - DIE CONSTRUCTION Refer to Figure Due to the forces exerted during the forming process of high-strength steel, dies must be built with extra strength. Extra strength is necessary to prevent die flexing. The following are ways to compensate for the unwanted flexing in the die: a. Block in or heel cam drivers. b. Use heavy-duty guide pins and bushings. c. Key in the sections and use large fasteners. d. Provide for positive returns. e. Provide heavy-duty die shoes with appropriate reinforcement. 2. Provide for die adjustability during construction. It is important to provide these adjustments because it is undesirable to machine the hardened and coated die details. 3. It is of prime importance to balance the forces exerted on the die during forming. When practical, form two parts at a time, or produce the right and left hand part in the same die. 5-17

97 FIGURE 5.11 RULES OF THUMB - DEVELOPED BLANKS Refer to Figure When using high-strength steel material for BIW (Body- In-White) structural parts, testing has demonstrated that the recommended type of forming is with a flange or form die. This type of die utilizes a developed blank. 2. This blank should be as close to finish trim as possible. A finish trip operation should only be added in areas where the trim is critical. 5-18

98 FIGURE 5.12 RULES OF THUMB - TRIMMING Refer to Figure Since high-strength steel (HSS) is more brittle and harder than mild steel, and because it is not as ductile as a result of the strengthening mechanisms in the metallurgy, it is more difficult to trim. HSS requires approximately the same die clearance between the upper and lower trim steels as mild sheet steel. This clearance is approximately 7 percent to 10 percent of metal thickness per side. The range of the hardness and the thickness determines the exact amount. 2. Dies must be sharpened more frequently when trimming HSS. They also require rigidity to prevent the die from flexing, which can cause dulling of the trim steels. 3. It is recommended that extremely hard cutting edges be provided on trim steels. Therefore, use of S-7 or other shock-resistant steel with a minimum of Rockwell (C-scale) is recommended. 5-19

99 FIGURE 5.13 RULES OF THUMB - DIE SHEAR Refer to Figure Due to excessive shock during blanking or trimming of high-strength steel, a full four times metal thickness shear is recommended to protect both press and die. 2. In order to prolong the die life of either a blank or trim die, die shear must be added. Advantages of the die shear 1. Lessens tonnage requirements. 2. Saves the press; reduces shock on the press. 3. Lengthens the die life between tune-ups and sharpening. 5-20

100 5.1.4 Guidelines for hat sections stamped from high-strength or ultra high-strength steels Basic guidelines for designing and processing hat section parts of high strength or ultra-high-strength steel are (Reference 5.2): Do: Form channels as close to finished shape as possible. Avoid closed ends on channels. Utilize small die radii. A combination of low pad pressure and tight clearance minimizes curl and springback. Allow for extra development time. Don t: Assume high-strength and ultra high-strength steel will behave like mild steel. Depend on traditional die design criteria Rules of thumb for high-strength steel stampings Common concerns associated with the use of highstrength steel in a stamping operation include springback, splitting, tolerances, die design, die life and blank design. The automotive industry routinely produces stamped highstrength steel parts. Over the past several years, many lessons have been learned through extensive practical experience. These lessons have been summarized in the form of Rules of Thumb in Figures 5.1 through 5.13 (Reference 5.2). The application of the Rules of Thumb will alleviate issues associated with high-strength steel at the part design and die design stages. They will shorten die development time and help ensure production success in the stamping of high-strength steel parts. 5.2 Welding considerations Introduction The most common welding processes, most of which are discussed in the following sections, as well as other processes used in niche applications, have been developed with the intent on welding traditional low carbon steels. 5-21

101 However, in the automotive world, especially as it relates to automotive bumper design and manufacturing, the use of low carbon steels is insignificant. New advancements in materials processing has resulted in the formation of modern steels of increasing tensile strength. These steels include transformation induced plasticity (TRIP), dual phase (DP), twinning induced plasticity (TWIP) and boron steels which are part of the current and next generation advanced high-strength steels (AHSS). The global formability diagram for today s AHSS grades is presented in Figure 3.4; which illustrates the relationship of tensile strength and elongation (ductility) as well as the wide variety of steel alloy families available. Such a wide variety of strength and ductility combinations provide automotive manufacturers with the opportunity to customize components in the vehicle with a certain type of steel. Some components of the vehicle must be highly ductile due to the forming operations required, whereas others may need higher strength for weight reduction and a combination of both may be required for crash protection especially in consideration with automotive bumpers. High-strength steels were developed for those primary applications, the reduction in vehicle weight, as well as the improvement in crash performance and occupant protection. The demand for the use of a wide variety of steels has resulted in significant developments in the forming and welding of these steels. All the steels listed and shown in Figure 3.4 are weldable. However, some steels have increased weldability concerns. Weldability concerns are generally found in coated steels, as well as steels of increased strength. Although, care should be taken when selecting a welding process and developing welding parameters, regardless of what steel is chosen. Previous equations for carbon equivalency (CE) used to calculate potential weldability concerns in low carbon steels are no longer valid for AHSS. Researchers are currently in development of weldability calculations similar to CE for applications in AHSS. 5-22

102 FIGURE EXAMPLES OF BUMPER BEAM WELDS Figure shows examples of types of welds used during bumper beam assembly. The geometry of the bumper will dictate the list of potential processes which can be used in welding of the component. Resistance spot welding in most applications requires access to both sides of the joint, whereas GMAW or laser welding does not have this requirement. Resistance spot welding process is used primarily for joining reinforcements or stiffening sections into the bumper system. As indicated in Figure 5.2.1, the crash cans on either end of the beam are welded on with GMAW. GMAW is the preferred process for joining these onto the bumper frame due to potential fit-up concerns, as well as the thickness of the steel being welded. The box or tubular beam has a weld line down the center; there are two potential processes to be used. The first being High Frequency or HF 5-23

103 welding, this would be used if long sections of steel were to be joined and then cut later. The other process is laser welding, used if the beam was already cut to length. AHSS differ from mild steels by chemical composition and microstructure, and it is important to note the microstructure of the steel will change due to welding operations. For example, intensive localized heat associated with some welding processes causes a significant change in the local microstructure, and therefore affect the final properties of the joint. Due to high cooling rates (CR) typical in welding, it is normal to see martensite and/or bainite microstructures in the weld metal and the heat affected zone (HAZ). Considering recent developments of hybrid approaches to welding, there are currently more than 100 types of welding processes available for a manufacturer or fabricator to utilize. Each process has a list of advantages and disadvantages which determine the best use scenario for a given application. Arc welding processes offer advantages like portability and reduced machine cost, but disadvantages such as lower weld speed and increased heat input to produce the weld. High energy density processes, such as laser welding, typically produce low heat inputs and fast welding speeds. However, the equipment cost is high and joint fit up needs to be ideal. Solid-state welding processes offer the advantage of avoiding many discontinuities associated with fusion welding processes, but the equipment is relatively expensive and the process works at limited weld speeds and in limited joint designs possibly not conducive to bumper design. Resistance welding processes are typically fast, and do not require additional filler metals, but are limited to thin sheet applications as well as high-production applications. Bumper systems typically use steels with higher strength than other vehicle structures. Martensitic steels (MS), hot formed manganese boron steels, and even some complex phase (CP) steels are found in bumper systems. These steels all have the characteristic of being greater than 900 MPa strength as well as having less than 20 percent elongation. With increased strength, HAZ softening becomes significantly more of a concern; this section will describe the phenomenon and provide potential solutions to this problem. Another issue relates to the formability of these higher strength steels is spring back, or the tendency of the steel to return to the original geometry after straining. Springback can be a problem in any welding process involving a forging force or forging action. If the forge force is not maintained through the duration of critical cooling, it is possible for the 5-24

104 weld to rupture. The high strength of these steels can cause difficulties in the part fit up critical for welding. Again, certain steels and processes highly susceptible to these issues are outlined and potential solutions provided. Welding processes produce a weld, a metallic bond, using a combination of heat, time and sometimes pressure. Arc welding and high energy density processes generally require no pressure and small to medium lengths of time. Resistance welding processes utilize less heat than arc welding processes, but at significantly higher pressures, and require much less time. The following section introduces some joining methods of AHSS as it relates to automotive bumpers. This information is a culmination of data relevant to welding of automotive bumpers from the WorldAutoSteel AHSS Guidelines, Welding Engineering: An Introduction by David H. Phillips, and Resistance Spot Welding: Fundamentals and Applications for the Automotive Industry by Menachem Kimchi and David H. Phillips Gas metal arc welding (GMAW) GMAW, commonly referred to by its slang name MIG (metal inert gas welding), uses a continuously fed bare wire electrode through a nozzle delivering a proper flow of shielding gas to protect the molten and hot metal as it cools. Since the wire is fed automatically by a wire feed system, GMAW is one of the arc welding processes considered to be semiautomatic. The wire feeder pushes the electrode through the welding torch where it makes electrical contact with the contact tube, which delivers the electrical power from the power supply and through the cable to the electrode. The basic equipment components are the welding gun and cable assembly; electrode feed unit, power supply and source of shielding gas. This set up includes a water cooling system for the welding gun which is typically necessary when welding with high duty cycles and high current. Deposition rates are much higher for GMAW over other arc welding processes (GTAW or SMAW), and the process is readily adaptable to robotic applications. Manual GMAW also requires less welder skill as compared to GTAW. Because of the fast welding speeds and ability to adapt to automation, it is widely used by automotive and heavy equipment manufacturers, as well as a wide variety of construction and structural welding. Relative to SMAW, GMAW equipment is a bit more expensive due to the additional wire feed mechanism, more complex torch, and the need for shielding gas, but overall it is still relatively inexpensive. 5-25

105 GMAW is self-regulating, which refers to the ability of the machine to maintain a constant arc length. This is usually achieved using a constant-voltage power supply, although some modern machines are now capable of achieving selfregulation in other ways. This self-regulation feature results in a process ideal for mechanized and robotic applications. FIGURE EFFECT OF SHIELDING GAS ON WELD PROFILE 5-26

106 Figure provides important GMAW terminology. Of particular importance is electrode extension. As shown, electrode extension refers to the length of filler wire between the arc and the end of the contact tip. The reason for the importance of electrode extension is the longer the electrode extension, the greater the amount of resistive (known as I2R) heating will occur in the wire. Resistive heating occurs because the steel wire is a poor conductor of electricity. This effect can become significant at high currents and/or long extensions. It can result in more energy from the power supply being consumed in the heating and melting of the wire and less in generating arc heating. As a result, significant resistive heating can result in a wider weld profile with less depth of fusion. The stand-off distance is also an important consideration. Excessive distances will adversely affect the ability of the shielding gas to protect the weld. Distances too close may result in excessive spatter. Various gases are being used for shielding the in GMAW process. The most common ones include argon (Ar), helium (He), carbon dioxide (CO2) and combinations of these. Figure illustrates the effect of the shielding gas on the weld profile GMAW procedures for AHSS GMAW is a popular process for joining AHSS, especially in many production applications where thickness and geometry prevent (resistance spot welding) RSW or in an application requiring filler material. Of all processes discussed regarding bumper welding, GMAW is on the higher order of heat input, meaning there are more weldability concerns as it relates to the joint and base metal. Being of higher heat input, welds of AHSS with GMAW will see a noticeably higher spike in hardness in the near HAZ, and increased softening closer to the base metal. The following section introduces primary welding concerns and offers potential solutions. One example showing, despite the increase alloying content used for Quenched and Partitioned AHSS (Q&P) 980, there is no increased welding defect type or rate compared with mild steel GMAW welds. Figure is the microhardness profile of a 1.6-mm Q&P 980 GMAW weld joint. Both welded seam and HAZ are all less than 500 HV, and there is no obvious softened zone in HAZ. 5-27

107 FIGURE MICROHARDNESS PROFILE OF A 1.6-MM DP 980 GMAW WELD JOINT TRIP 780 lap joint static tensile results for different filler metal and CR conditions are shown in Figure The results are expressed in terms of joint efficiency and the strain at peak load. The data indicates joint efficiencies ranged from about 50 percent to about 98 percent. Strains at peak load ranged from less than 3 percent to nearly 8 percent. Fracture occurred either in the far HAZ or at the weld fusion boundary. Filler metal strength had no discernable effect on the tensile properties. Figure shows static tensile test results of the TRIP 780 butt joints. All the welds failed in the softened region of the far HAZ with joint efficiencies in excess of 89 percent. On average, welds made using higher cooling rate (CR) experienced higher strains during loading than those made using lower CR. As was the case with the lap welds, filler metal strength did not appear to influence the static tensile properties. The abbreviations of high and low CR indicate high and low CR used for each weld. 5-28

108 FIGURE STATIC TENSILE TEST RESULTS OF TRIP 780 LAP AND BUTT JOINTS The static tensile test results of the DP 780 butt welds are shown in Figure All welds failed in the softened region of the far HAZ. As shown, the high CR welds had joint efficiencies in excess of 90 percent. The high CR welds also appear to have slightly greater strains at peak load. FIGURE STATIC TENSILE TEST RESULTS OF DP 780 BUTT JOINTS 5-29

109 Figure (left) shows the TRIP 780 lap joint dynamic tensile results for different filler metal and CR conditions. UTS ranged from 372 to 867 MPa (54 to 126 ksi) and strain at peak load ranged from less than 1 percent to more than 5 percent. The high CR lap joints had lower strengths and strains at peak load. These welds failed along the fusion line presumably due to porosity present at the root. All the low CR lap welds produced with the ER70S-6 wire failed in far HAZ of the bottom sheet. Of the low CR lap joints produced with the ER100S-G wire, two dynamic tensile specimens failed in the softened region of the far HAZ and one failed along the fusion line of the top sheet without the presence of porosity at the weld root. Analysis of Figure (left) indicates filler metal strength did not have a distinguishable effect on the dynamic tensile test results. Figure (right) shows the dynamic tensile test results of the TRIP 780 butt joints. All failed in the softened region of the far HAZ. The UTS of the butt joints ranged from 840 to 896 MPa (122 to 130 ksi) and strain at peak load was generally between 3 and 4 percent. The figure indicates neither filler metal strength nor CR condition had a distinguishable effect on the dynamic tensile test results of the butt joints. FIGURE DYNAMIC TENSILE TEST RESULTS OF TRIP 780 LAP JOINTS AND BUTT JOINTS 5-30

110 The dynamic tensile test results of the DP 780 butt joints are shown in Figure All joints failed in the softened region of the far HAZ. UTS ranged from 841 to 910 MPa (122 to 132 ksi), and strain at peak load ranged from 2.25 percent to less than 4.0 percent. It should be noted similar UTS were obtained for the DP 780 and TRIP 780 butt joints. On average, TRIP 780 butt joints had slightly higher strain at peak load. Neither filler metal strength nor CR condition appears to have a distinguishable effect on the dynamic tensile properties. FIGURE DYNAMIC TENSILE TEST RESULTS OF DP 780 BUTT JOINTS 5-31

111 5.2.3 Resistance welding Resistance welding processes represent a family of industrial welding processes produce the heat required for welding through what is known as joule (J = I2Rt) heating. Much in the way a piece of wire will heat up when current is passed through it, a resistance weld is based on the heating occurring due to the resistance of current passing through the parts being welded. Since steel is not a very good conductor of electricity, it is easily heated by the flow of current and is an ideal metal for resistance welding processes. There are many resistance welding processes, but the most common is Resistance Spot Welding (RSW) (Figure 5.2.9). All resistance welding processes use three primary process variables current, time, and pressure (or force). The automotive industry makes extensive use of resistance welding, but it is also used in a variety of other industry sectors including aerospace, medical, light manufacturing, tubing, appliances and electrical. The RSW process is often used as a model to explain the fundamental concepts behind most resistance welding processes. Figure shows a standard RSW arrangement in which two copper electrodes apply force and pass current through the sheets being welded. If the sheets are steel, the resistance to the flow of current of the sheets will be much higher than the copper electrodes, so the steel will get hot while the electrodes remain relatively cool. But another important critical characteristic exists in most resistance welding processes the contact resistance between the parts (or sheets) being welded. As indicated on the figure, the highest resistance to the flow of current is where the sheets meet ("Resistance" 4). This fact allows a weld nugget to begin forming and grow exactly where it is needed between the sheets. 5-32

112 5-33 FIGURE RSW

113 FIGURE RESISTANCES ASSOCIATED WITH STEEL RESISTANCE SPOT WELDS A typical weld time for RSW of steel is approximately 1/5 to 1/4 of a second. The current required in resistance welding is much higher than arc welding and it is in the range of 8-15 ka. In addition to RSW, three other common resistance welding processes are Resistance Seam (RSEW), Projection (RPW) and Flash Welding (RFW) (Figure ). The RSEW process uses two rolling electrodes to produce a continuous-welded seam between two sheets. It is often the process of choice for welding leak tight seams needed for automotive fuel tanks. RPW relies on geometrical features machined or formed on the part known as projections to create the required weld current density. RFW is very different from the other processes in it relies on a rapid succession of highcurrent-density short current pulses which create what is known as flashing. During flashing, molten metal is violently expelled as the parts are moved together. The flashing action heats the surrounding material which allows a weld to be created when the parts are later brought together with significant pressure. Other important resistance welding processes which are not shown include High-Frequency Resistance Welding (HFRW) (used for producing the seams in welded pipe) and Resistance Upset Welding (RUW). 5-34

114 FIGURE COMMON RESISTANCE WELDING PROCESSES Most welding processes produce welds which provide strong visible evidence of weld quality, so visual examination is often an important approach to verifying the quality of the weld. However, with most resistance welding processes visible examination is not possible due to the "blind" weld location between the sheets or parts being welded. As a result, maintaining weld quality with processes such as RSW is highly dependent on what is known as a lobe curve (Figure ), which is basically a process window for RSW. The lobe curve represents ranges of weld current and time producing a spot weld nugget size of acceptable mechanical properties for the intended application. So, weld quality monitoring with resistance welding processes relies highly on the ability to monitor parameters such as current and time. During production, if a weld is made with parameters outside of the lobe curve, the weld is considered unacceptable. Ultrasonic testing is also often used in the automotive industry for Nondestructive Testing (NDT) of the blind location of RSW. FIGURE RESISTANCE WELDING LOBE CURVE 5-35

115 Resistance spot welding procedures for AHSS In general, for any type of AHSS the resistance spot welding schedules (parameters) should be changed from the typical welding schedules applied to mild steel. These changes may include higher electrode force, adjustments to the weld time and current, use of current pulsation, use of weld and temper parameters, current up-slope and down-slope, and sometimes an increase in the minimum required weld diameter may be necessary. The reasons for these changes in the welding schedules are discussed below. When resistance welded, AHSS require less current than conventional mild steel or HSLA because AHSS have higher electrical resistivity. Therefore, current levels for AHSS are not increased and may even need to be reduced depending on material chemical composition. However, most AHSS grades may require higher electrode forces for equivalent thickness of mild steels because electrode force depends on material strength. If thick mild steel or HSLA steel (of the same thickness) is replaced by an equivalent thickness of AHSS, the same forces may be required during assembly welding. FIGURE SCHEMATIC WELD LOBES FOR AHSS, HSLA, AND MILD STEEL WITH A SHIFT TO LOWER CURRENTS WITH INCREASED STRENGTH 5-36

116 AHSS often have tighter weld windows (welding parameters to give acceptable welds) when compared to mild steels, as shown in Figure The current range (ka) for AHSS of MPa during RSW is shown in Figures and The process window for RSW of AHSS is influenced by the electrode force and welding time used in a major way. The current range increases by an average of 500 A for every additional 500 N of electrode force (Figure ). The current range also increases by an average of 250 A for each additional 40ms of welding time (Figure ). Extra amounts of electrode force and welding time lead to increased current range, allowing for a wider process window. FIGURE RSW WITH AHSS, CURRENT RANGE FOR VARYING ELECTRODE FORCE (Cap Type B 16/6, 6-mm tip diameter, single pulse, 340-ms weld time, 250-ms hold time, plug failures) FIGURE RSW WITH AHSS, CURRENT RANGE FOR VARYING WELD TIME (Cap Type B 16/6, 6-mm tip diameter, single pulse, 3.5-kN electrode force, 250-ms hold time, plug failures) 5-37

117 AHSS has been resistance welded in automotive production lines for the past several years. However, the increased strength and hardness can have adverse effects on the spot weld failure modes observed during peel and chisel testing. The spot welding industry has standardized on an acceptable peel test which pulls a nugget or a full button as shown below in Figure FIGURE EXAMPLE OF A FULL BUTTON NUGGET When resistance spot welding AHSS, full button pulls are less likely due to the increased carbon equivalency when compared to low carbon steels. This increased carbon equivalent is likely to produce hard weld nuggets. This fact is compounded by the higher yield strengths of the material will tend to produce greater stresses concentrating at the edge of the nugget during peel or chisel testing. Conventional modes of peel or chisel testing produce interfacial or partial interfacial failure modes more often. Examples of interfacial and partial interfacial fractures are shown in Figure With AHSS, these types of failure modes may occur even though the weld strengths may be acceptable for the intended application. Full interfacial failures may exhibit high strength, although it may be challenging to differentiate between an interfacial failure and a stuck weld condition (which refers to an unfused bond of unacceptable strength). 5-38

118 FIGURE EXAMPLES OF COMMON FAILURE MODES IN PEEL OR CHISEL TESTING OF AHSS To improve nugget failure modes with AHSS, the hard martensite formed after welding must be softened. One approach to softening is to add a temper cycle at the end of the weld. Figure displays an example schedule graph include pulsing and a tempering cycle following welding. Enough quench time must be included prior to tempering to allow the complete martensitic transformation, and the amount of tempering time and current will correspond to how much softening occurs. Quench time adds cycle time to each weld, so production requirements mandate this time be kept as short as possible. Depending on the carbon equivalence of the steel, other approaches such as reducing the cooling rate may also show positive results. Other approaches also include current pulsing, current sloping, longer weld time and short hold times. 5-39

119 FIGURE EXAMPLE WELD SCHEDULE SHOWING CURRENT PULSING, TEMPERING, AND TWO STAGE FORCE Additional work using Quench and Partition (Q&P) 980 showed less current required than conventional steels because it has higher electrical resistivity. Due to ultra highbase material (BM) strength, it needs higher electrode force than conventional steels which have equivalent thickness. The weld lobe of 1.6-mm Q&P 980 is shown in Figure with the pulsed weld time and force of 5.8 kn. The yellow zone of this figure shows the fracture mode of full button (FBF) when peel tested. FIGURE PULSED CURRENT PROFILE AND WELD LOBE OF 1.6-MM Q&P

120 Weld spot micrograph and microhardness of 1.6-mm Q&P 980 is shown in Figure , in which no weld defects, such as cracks, shrinkage void, pore, no fusion, deep indentation, etc. were found. Hardness testing is typically performed along a diagonal traverse across the weld using a suitable instrument for micro-indentation hardness testing (Vickers or Knoop). FIGURE WELD SPOT MICROGRAPH AND MICROHARDNESS OF 1.6-MM DP 980 In a first step, the spot-welded joint can be subdivided into three zones: weld, HAZ and BM. The weld is covered by the HAZ, where the melting temperature is not reached but high enough to change the microstructure. This region is dominated by inhomogeneous properties due to the different temperature and cooling gradients. Considering the hardness measurements, even a softening in the HAZ compared to the BM can be observed. Finally, the HAZ is surrounded by the BM, which does not show any local changes within the structure. These modifications of microstructure in the HAZ and weld are essential for the load-bearing capacity because the strength and ductility are drastically changed in comparison to the BM. Normally a high hardness is related to high strength and less ductility. In summary, Table provides the AWS C1.1 Spot Welding Parameter Guidelines for steels greater than 700 MPa strength. These general guidelines can be used to approximate which parameters can be used to begin the RSW process of a specific part thickness. From the recommended parameters, changes can be made on a specific stack-up to ensure an acceptable strength and nugget size for a specific application. Additional, more complicated RSW parameter guidelines using a pulsation welding schedule with AC 60 Hz for welding AHSS is included in Table More information regarding parameters and schedules can be found in AWS and RWMA resources. 5-41

121 TABLE SPOT WELDING PARAMETERS FOR LOW-CARBON STEEL >700 MPa (AHSS) TABLE AHSS BARE-TO-BARE, BARE-TO-GALVANIZED, GALVANIZED-TO-GALVANIZED RSW PARAMETERS FOR PULSATING AC 60 Hz 5-42

122 Other considerations in RSW Part fit-up Resistance welding depends on the interfacial resistance between two sheets. Good and consistent fit-up of parts is important to all resistance welding. Part fit-up is even more critical to the welding of AHSS due to increased YS and greater spring back Coating effects A study was undertaken to examine whether differences exist in the RSW behavior of DP 420/800 with a HDGA coating compared to a HDGI coating. The RSW evaluations consisted of determining the welding current ranges for the steels with HDGA and HDGI coatings. The results indicated DP 420/800 showed similar overall welding behavior with HDGA and HDGI coatings. One difference noted between the two coatings was HDGA required lower welding current to form the minimum nugget size. This may not be an advantage in the industry given the current practice of frequent electrode tip dressing. The resistance weldability of coated steels can also cause problems. In many applications, more intricate welding schedules are used to ensure welds meet the size and strength requirements. A study was conducted on 1500 MPa boron hot stamped steel, coated with zinc, to determine the nugget growth and formation mechanisms to properly select parameters for each pulse of a three-pulse welding schedule (Figure ). The first pulse, high current and short weld time, is used to mitigate the effects of the coating on welding and develop contact area at the sheet-to-sheet interface. The second pulse, low current long weld time, is used to grow the weld nugget and minimize internal defects. The third pulse, medium current and long weld time, is used to grow the weldability current range and maximize the nugget diameter. 5-43

123 FIGURE WELD GROWTH MECHANISM OF OPTIMIZED THREE-PULSE WELDING CONDITION Process simulation The advantages of numerical simulations for resistance welding are obvious for saving time and reducing costs in product developments and process optimizations. Modern modeling techniques can predict temperature, microstructure, stress, and hardness distribution in the weld and HAZ after welding. Commercial modeling software is available which considers material type, various current modes, machine characteristics, electrode geometry, etc. An example of process simulation results for spot welding of 0.8-mm DC 06 low-carbon steel to 1.2- mm DP 600 steel is shown in Figure Obviously, this technique can apply to dissimilar thicknesses, material types and geometries. This figure only shows the nugget formation and hardness distribution. 5-44

124 FIGURE SIMULATION RESULTS WITH MICROSTRUCTURES AND HARDNESS DISTRIBUTION FOR SPOT WELDING OF 0.8-MM DC06 LOW-CARBON STEEL TO 1.2-MM DP 600 STEEL Resistance spot welding joint test performance Acceptable weld integrity criteria vary greatly among manufacturers and world regions. Each AHSS user needs to establish their own weld acceptance criteria and the characteristics of AHSS resistance spot welds. AHSS spot weld strength is higher than mild steel for a given button size. It is important to note partial buttons (plugs) or interfacial fractures (IF) do not necessarily characterize a failed spot weld in AHSS. IF fractures may be typical of smaller weld sizes in mild steel or in all weld sizes in AHSS. Reference is made to Specification AWS/ANSI D8.1 Weld Quality Acceptance for additional details (Figures through ). 5-45

125 FIGURE LOAD-BEARING CAPACITY OF SPOT WELDS ON VARIOUS COLD-ROLLED STEEL (Steel type, grade, and any coatings are indicated on the bars) FIGURE REPRESENTATIVE MINIMUM SHEAR TENSION STRENGTH VALUES FOR GROUP 3 STEELS 5-46

126 FIGURE REPRESENTATIVE MINIMUM SHEAR TENSION STRENGTH VALUES FOR GROUP 4 STEELS FIGURE MINIMUM CROSS-TENSION STRENGTH (CTS) VALUES FOR GROUP 2, 3, AND 4 STEELS 5-47

127 The AHSS weld tensile strength is proportional to material tensile properties and is higher than mild steel spot weld strength (Figure ). While testing thick AHSS spot welds (from small button size to expulsion button) the fracture mode during shear-tension testing may change from IF to button pull out or plug. FIGURE TENSILE SHEAR STRENGTH OF SINGLE SPOT WELDS Fatigue strength of spot welds In a comparative study of the spot weld fatigue strength of various steels grades, some of which included grades such as 1.33-mm Fully Stabilized (FS) 300/420 (HDGA), mm DP 340/600 (HDGA), 1.24-mm TRIP 340/600 (HDGA) and 1.41-mm TRIP 340/600 (HDGA), it was concluded base metal microstructure/properties have relatively little influence on spot weld fatigue behavior (Figure ). However, DP 340/600 and TRIP 340/600 steels have slightly better spot weld fatigue performance than conventional low-strength Aluminum-Killed Drawing Quality (AKDQ) steels. Further, it was found fatigue strength of spot welds is mainly controlled by design factors such as sheet thickness and weld diameter. Therefore, if down-gauging with HSS is considered, design changes should be considered necessary to maintain durability of spot-welded assemblies. 5-48

128 FIGURE TENSILE-SHEAR SPOT WELD FATIGUE ENDURANCE CURVES FOR VARIOUS HSS (The data are normalized to account for differences in sheet thickness and weld size. Davidson data in the plot refer to historical data) Embrittlement phenomenon It has been a long-standing challenge to extend the usage of the very high-strength steels in hydrogen-rich environments, given these steels are prone to Hydrogen Cracking, due to their increased mechanical strength. Hydrogen embrittlement (HE) is known to be a premature fracture caused by a small amount of hydrogen atoms concentrated at highly stressed regions inside susceptible high tensile strength materials. (See examples in Figure ) FIGURE EXAMPLES OF HYDROGEN EMBRITTLEMENT (HE) FAILURE IN RSW 5-49

129 The factors controlling the occurrence of HE, susceptible microstructure, stress and the presence of hydrogen, are well known and have been sufficiently quantified to develop procedures to minimize its occurrence mostly in arc welding thicker gage steels. When advanced high-strength steel (AHSS) with tensile strength over 980 MPa are applied in automotive applications, there is a small risk hydrogen embrittlement fracture (sometime called delayed fracture) may occur after welding, while a vehicle is in use. Although there have been no reports that automotive parts made of AHSS have fractured due to hydrogen embrittlement, a risk assessment of delayed fracture for AHSS is considered necessary to ensure the safety of the automotive body and encourage wider use of UHSS sheets. Another common embrittlement phenomenon involves the zinc coating discussed previously. Resistance spot welding is dependent on the interfacial contact resistance between the electrodes and the material. During welding, a metal with a lower melting point such as zinc can penetrate in a liquid state into the grain boundaries of the material. By the end of the welding process, liquid metal embrittlement (LME) can become a problem due to the ductility of the grain boundary being reduced by the impeding tensile stress. (Example can be seen in Figure ) Also, brittle intermetallic compounds, such as Cu5Zn8, are created by the reaction with the Cu electrode and the material at the high temperature, which promotes LME or surface cracking. FIGURE ACTUAL EXAMPLE OF THE LME PHENOMENON IN ZINC COATED AHSS 5-50

130 In summary, LME cracking may occur when a combination of tensile stress, liquid metal and susceptible microstructure exist. Studies are being performed to evaluate whether LME is mitigated by modern automotive RSW processes, where the volume of welds significantly exceeds engineering requirements, or whether the occurrence of LME affects inuse properties at all High frequency welding Fundamentals and principles of HF welding HF welding processes rely on the properties of HF electricity and thermal conduction, which determine the distribution of heat in the workpieces. HF contact welding and highfrequency induction welding are used to weld products made from coil, flat or tubular stock with a constant joint symmetry throughout the length of the weld. Figure illustrates basic joint designs used in HF welding. Figures (A) and (B) are butt seam welds; Figure (C) is a mash seam weld produced with a mandrel, or backside/inside bar. FIGURE BASIC JOINT DESIGNS FOR HF WELDS IN PIPE AND TUBE HF current in metal conductors tends to flow at the surface of the metal at a relatively shallow depth, which becomes shallower as the electrical frequency of the power source is increased. This commonly is called the skin effect. The depth of electrical current penetration into the surface of the conductor also is a function of electrical resistivity and magnetic permeability, the values of which depend on temperature. Thus, the depth of penetration also is a function of the temperature of the material. 5-51

131 FIGURE EFFECT OF FREQUENCY ON DEPTH OF PENETRATION INTO VARIOUS METALS AT SELECTED TEMPERATURES The second important physical effect governing the HF welding process is thermal conduction of the heat generated by the electric currents in the workpiece. Control of the thermal conduction and of the penetration depth provides control of the depth of heating in the metal. Because thermal conduction is a time-dependent process, the depth to which the heat will conduct depends on the welding speed and the length of the electrical current path in the workpiece. If the current path is shortened or the welding speed is increased, the heat generated by the electric current in the workpiece will be more concentrated and intense. However, if the current path is lengthened or the welding speed is reduced, the heat generated by the electric current will be dispersed and less intense. The effect of thermal conduction is especially important when welding metals with high thermal conductivity, such as Cu or Al. It is not possible to weld these materials if the current path is too long or the welding speed is too slow. Changing the electrical frequency of the HF current can compensate for changes in welding speed or the length of the weld path, and the choice of frequency, welding speed and path length can adapt the shape of the HAZ to optimize the properties of the weld metal for a particular application. 5-52

132 HF induction welding procedures for AHSS As discussed earlier, HFIW is the main welding technology for manufacturing cold-formed welded steel tubes. Welded tubes are normally made from flat sheet material by continuous roll forming and the HFIW process. The tubes are widely used for automotive applications, including seat structures, cross members, side-impact structures, bumpers, sub frames, trailing arms and twist beams. A welded tube can be viewed as a sheet of steel having the shape of a closed cross section. Two features distinguish the welded tube from the original sheet material: 1. The work hardening which takes place during the tubeforming process. 2. The properties and metallurgy of the weld seam differ from those of the BM in the tubular cross section. Good weldability is one precondition for successful HF welding. Most DP steels are applicable as feed material for manufacturing of AHSS tubes by continuous roll forming and the HFIW process. The quality and the characteristics of the weld depend on the actual steel sheet characteristics (such as chemistry, microstructure and strength) and the set-up of the tube manufacturing process. Table provides some characteristics of the HF welds in tubes made of DP 280YS/600TS steel. For DP 280YS/600TS the hardness of the weld area exceeds the hardness of the BM (Figure ). There is a limited or no soft zone in the transition from HAZ to BM. The nonexistent soft zone yields a HF weld stronger than the base metal (Table 5.2.3). This is an essential feature in forming applications where the tube walls and weld seam are subject to transverse elongation, such as in radial expansion and in hydroforming. TABLE TRANSVERSE TENSILE TEST DATA FOR HFIW DP 280/600 TUBE 5-53

133 FIGURE WELD HARDNESS OF A HF WELD IN A DP 280/600 TUBE Laser welding One observed potential concern, as it relates to welding AHSS with HF welding, relates to the spring back property of the material. Wheels guide the formation of strip into the open tube, which occurs at the forge rolls. However, in welding of AHSS, even at elevated temperature, the spring back nature often results in rupturing of the weld seam, depending on other parameters. One potential solution is to either add or move rolls closer to the squeeze rolls (weld point), to ensure the weld has fully solidified before exiting the machine. A robust edge forming condition during the roll forming operation is very important for the following welding operation Fundamentals and principles of high energy density welding processes High energy density welding processes are the focused energy needed for welding to an extremely small size area. This allows for very low overall heat input to the workpiece, which results in minimal BM degradation, residual stress and distortion. Welding speeds can be very fast. The two main processes known for extreme energy densities are laser (Figure ) and Electron Beam Welding (EBW). 5-54

134 FIGURE LASER WELDING As shown in Figure , energy densities of focused laser and electron beams can approach and exceed 104 kw/cm2. These energy densities are achieved through a combination of high power and beams focused to an extremely small diameter. Diameters as small as a human hair (0.05 mm) are possible. Plasma Arc Welding (PAW) offers greater energy density than conventional arc welding processes; and is sometimes referred to as the poor man s laser. FIGURE POWER DENSITIES OF VARIOUS WELDING PROCESSES 5-55

135 High energy density processes produce weld profiles of high depth-to-width ratio, as compared to other welding processes (Figure ). As a result, much greater thicknesses can be welded in a single pass, especially with EBW. The figure also illustrates the fact high energy density processes can produce a weld with minimal heating to the surrounding area as compared to the other processes. However, the high depth-to-width ratio weld profile is much less forgiving to imperfect joint fit-up than the profile produced by arc welding processes. FIGURE COMPARISON OF TYPICAL WELD PROFILES Laser and EBW processes are used in a wide variety of industry sectors. Very high weld speeds are possible and the welds are usually aesthetically pleasing. Laser welding is very adaptable to high-speed production so it is common in the automotive sector. When welding with high energy density processes, the laser or EB is focused along the joint line of the workpieces to be welded. The extreme power density of the beam not only melts the material; but also causes evaporation. As the metal atoms evaporate, forces in the opposite direction create a significant localized vapor pressure. This pressure creates a hole, known as a keyhole, by depressing the free surface of the melted metal. The weld solidifies behind the keyhole as it progresses along the joint (Figure ). This method of welding known as keyhole welding is the most common approach to laser and EB and produces the characteristic welds of high depth-to-width ratio. There are some cases where the keyhole mode is not used. This mode is known as conductive mode welding. Conductive mode welds have a weld profile closer to an arc weld Laser welding fundamentals 5-56

136 FIGURE KEYHOLE MODE WELDING The word laser is an acronym for light amplification by stimulated emission of radiation. Lasers produce a special form of light (electromagnetic energy) consisting of single coherent wavelength photons. Light of this form can be focused to extremely small diameters allowing for the creation of the high-energy densities used for welding. The laser beam itself is not useful for welding until it is focused by a focusing lens. Lasers vary in the quality of the beam produced. A highquality beam will diffract less when focused, providing for the creation of a smaller spot size. Reflective lenses are important to lasers as well since they are used in the optical cavity where the beam is generated, as well in the beam delivery systems for some lasers. For these reasons, optics play a major role in laser beam welding. Laser beam welding (Figure ) does not always require additional filler metal and shielding gas is optional. When the beam hits the workpiece, it melts and vaporizes metal atoms, some of which are ionized by the intense beam. This creates what is known as a plume (or plasma) over the weld area that can sometimes interfere with the beam. In these cases, shielding gas may be used to deflect the plume. The choice of laser type depends on cost, the type and thickness of material to be welded and the required speed 5-57

137 FIGURE LASER BEAM WELDING and penetration. Lasers are distinguished by the medium used to generate the laser beam and the wavelength of laser light produced. Although there are many types of lasers, the common lasers for welding include the Nd:YAG, fiber, disk solid-state lasers and the gas-based CO2 laser. The lasing medium in solid-state lasers are crystals (Nd:YAG and disk lasers) or fibers (fiber laser) have material added (doped) will lase when exposed to a source of energy, whereas the lasing medium in the CO2 laser is a gas blend consisting of CO2, He and N2 gas. In all cases, lasing occurs when the atoms/molecules of the medium are excited to a higher energy state through the introduction of additional energy (known as pumping). When this occurs, photons are emitted, which, in turn, excite other atoms/ molecules. These results in a cascade of photons traveling coherent waves of a single wavelength, the two properties laser light is known for. CO2 lasers produce wavelengths of 10.6 µm, while the wavelength of the solid-state lasers is 1.06 µm. CO2 lasers are generally less expensive, but the longer wavelength of light does not allow its beam to be delivered through fiber optic cables which reduces its versatility. Its light is also more reflective, which limits its use with highly reflective metals such as aluminum. The solid-state lasers are generally more compact and require less maintenance than the CO2 laser. They are more 5-58

138 conducive to high-speed production since their beams can be delivered through long lengths of fiber optic cable which can then be attached to a robot. Some of the solid-state lasers such as the fiber laser produce beams of outstanding quality. However, the shorter wavelength of these lasers requires additional safety precautions regarding eye protection. The choices of focus spot size, focus spot location in the joint FIGURE FOCUSING OF THE LASER BEAM and focal length are all important considerations when laser beam welding. Usually, a small focus size is used for cutting and welding, while a larger focus is used for heat treatment or surface modification. As indicated in Figure , the location of the beam focal point can also be varied based on the application. When welding, it is common to locate the focal point somewhere near the center of the joint. Cutting applications, however, benefit from placing the focal point at the bottom of the joint. Weld spatter onto the focusing lens can sometimes be a problem, especially when there are contaminants on the surface of the parts being welded. Approaches to minimizing the spatter problem include choosing a long focal length lens which keeps the lens a safe distance from the weld area, or the use of an air knife to protect the lens. 5-59

139 Laser welding procedures for AHSS Laser-based solutions can offer a high- and cost-effective improvement potential for steel-based joining. The laser joining design has an inherent stiffness increases in direct relation to the laser weld length. Also, at low process time, there is up to a +14 percent torsional stiffness increase without any additional joining technique, shown in Table TABLE STIFFNESS PERFORMANCES COMPARISON FOR SEVERAL JOINING DESIGNS Laser weld shape optimization can help to homogenize performances and increasing the laser weld shape factor leads to a signification reduction of IF fracture risks (Figure ). FIGURE IMPACT OF LASER WELD DESIGN OPTIMIZATION ON FRACTURE TYPE 5-60

140 DP 800* has the advantage of weight reduction and equally good properties when laser welding as the DP 800. The absolute strength of DP 800 is slightly higher, but the ductility for the DP 800HpF is greater, shown in Figure FIGURE ABSOLUTE STRENGTH AND DUCTILITY OF DP 800 AND DP 800 Figure shows a cross-tension test in which both materials fail outside the weld zone. DP 800 fails entirely in the HAZ while DP 800* fails partly in the HAZ and partly in the base metal. FIGURE CROSS-TENSION TESTING OF DP 800 AND DP

141 DP 800 with additional retained austenite and associated bainite. Figure is the microhardness profile of a 1.6-mm Q&P 980 laser weld joint. Microhardness of both welded seam and HAZ are higher than BM, and there is no obvious softened zone in HAZ. FIGURE MICROHARDNESS PROFILE OF A 1.6-MM Q&P 980 LASER WELD JOINT Figure : Erichsen test result for the base metal and weld seam of 1.6-mm Q&P 980, showing good stretchability. 5-62

142 FIGURE ERICHSEN TEST RESULT OF 1.6-MM Q&P 980, LASER WELDED 5-63

143 6. Relevant safety standards in North America and Europe The bumpers on passenger cars sold in the United States must conform to United States National Highway Traffic Safety Administration (NHTSA) 49 C.F.R. Part 581 Bumper Standard (see Section 6.1). The bumpers on passenger cars sold in Canada must conform to Canadian Motor Vehicle Safety Regulations Section 615 of Schedule IV (see Section 6.2). This regulation states a bumper must meet the United States NHTSA Bumper Standard or ECE Regulation 42 as explained in Section 6.2 of this publication. Typically, although not mandatory, the bumpers on minivans sold in the United States and Canada meet the NHTSA requirements for passenger car bumpers. There are no federal regulations in the United States or Canada for bumpers on pickups, full size vans or SUVs. These bumpers are designed to meet OEM internal specifications. The Research Council for Automotive Repairs (RCAR), including its U.S. member, the Insurance Institute for Highway Safety (IIHS), in an effort to reduce the cost of passenger vehicle bumper repairs, have developed a test protocol that simulates a broader range of impacts occurring in actual on-the-road crashes. The IIHS tests, conducted on passenger cars and minivans, are more severe than the NHTSA tests (see Section 6.4). While each RCAR member has its own rating system, the IIHS protocol is not a pass or fail protocol. Rather, it provides a weighted damage estimate that is used to determine the overall rating for a passenger vehicle. Many OEMs select a target overall rating for a vehicle to be sold in the United States and Canada. This target is used when designing the vehicle s bumpers. IIHS has put their bumper test program on hiatus because real-world collision data in the US did not correlate with the test results; however, other RCAR members have shown good correlation in their markets. IIHS will continue to review damage trends. Most passenger vehicles sold in Europe have bumpers that conform to United Nations Economic Commission for Europe ECE Regulation 42 (see Section 6.3). Euro NCAP provides an independent assessment of the safety performance of cars sold in Europe. Pedestrian protection is an integral part of NCAP s overall rating scheme. Of particular significance in bumper design is the leg to bumper impact requirements in the Euro NCAP Pedestrian Protection Test (see Section 5.8.2). In addition, many European bumpers are voluntarily designed to perform well in Research Council for Automotive Repairs (RCAR) tests. RCAR s Low-Speed Offset Insurance Crash Test (see Section 6.6) was developed to prevent unnecessary 6-1

144 damage to the structure of passenger cars in low-speed crashes. This test is now referred to as the RCAR Structural Test. Even if a vehicle performs well in the RCAR Structural Test, it may not exhibit good crash behavior in real world accidents (often due to override or underride). To overcome this possibility, RCAR developed the bumper test to assess how well a vehicle s bumper system protects the vehicle from damage in low-speed impacts: the RCAR Bumper Test (see Section 6.7). 6.1 United States National Highway Traffic Safety Administration (49C.F.R.), Part Bumper Standard Requirements This standard (Reference 6.7) is summarized in Sections through The reader is cautioned that these sections are only a summary. The reader must refer to the actual regulatory document in order to obtain a complete understanding of the standard. The Bumper Standard only applies to passenger cars. 1. A passenger vehicle is subjected to three impact procedures: 2. The pendulum corner impacts - front and rear. 3. The pendulum longitudinal impacts - front and rear. 4. The impacts into a fixed collision barrier - front and rear. Following the three impact procedures, the vehicle shall meet the following damage criteria: 1. Each lamp or reflective device except license plate lamps shall be free of cracks and shall comply with applicable visibility requirements. The aim of each headlamp shall be adjustable to within the beam aim inspection limits. 2. The vehicle s hood, trunk and doors shall operate in the normal manner. 3. The vehicle s fuel and cooling systems shall have no leaks or constricted fluid passages and all sealing devices and caps shall operate in the normal manner. 4. The vehicle s exhaust system shall have no leaks or constrictions. 5. The vehicle s propulsion, suspension, steering and braking systems shall remain in adjustment and shall operate in the normal manner. 6. A pressure vessel used to absorb impact energy in an 6-2

145 6.1.2 Vehicle exterior protection system by the accumulation of gas or hydraulic pressure shall not suffer loss of gas or fluid accompanied by separation of fragments from the vessel. 7. The vehicle shall not touch the test device, except on the impact ridge shown in Figures 6.1 and 6.2, with a force that exceeds 2000 pounds (8.9 kn) on the combined surfaces of Planes A and B (see Figure 6.3) of the test device. 8. The exterior surfaces shall have no separations of surface materials, paint, polymeric coatings or other covering materials from the surface to which they are bonded; and no permanent deviations from their original contours 30 minutes after completion of each pendulum and barrier impact, except where such damage occurs to the bumper face bar, and the components and associated fasteners that directly attach the bumper face bar to the chassis frame. 9. Except as provided in Criterion 8 (above), there shall be no breakage or release of fasteners or joints. 2. The vehicle is at unloaded vehicle weight. 3. Trailer hitches, license plate brackets and headlamp washers are removed. Running lights, fog lamps and equipment mounted on the bumper face bar are removed if they are optional equipment Pendulum corner impacts (See Figure 6.3) 1. Impact speed of 1.5 mph (2.4 km/h). 2. Impact one front corner at a height of 20 inches (500 mm) using Figure 6.1 pendulum. 3. Impact other front corner at a height from 16 to 20 inches (400 to 500 mm) using Figure 6.2 pendulum. 4. Impact one rear corner at a height of 20 inches (500 mm) using Figure 6.1 pendulum. 5. Impact other rear corner at a height from 16 to 20 inches (400 to 500 mm) using Figure 6.2 pendulum. 6. The plane containing the pendulum swing shall have a 60 degree angle with the longitudinal plane of the vehicle. 7. Impacts must be performed at intervals not less than 30 minutes. 8. Effective impacting mass of pendulum equals mass of vehicle. 6-3

146 6.1.4 Pendulum longitudinal impacts (See Figure 6.3) 1. Impact speed of 2.5 mph (4 km/h). 2. Two impacts on front surface, inboard of corner. 3. Impacts on rear surface, inboard of corner. 4. Impact line may be any height from 16 to 20 inches (400 to 500 mm). If height is 20 inches (500 mm), use Figure 6.1 pendulum. If height is between 20 and 16 inches (500 and 400 mm), use Figure 6.2 pendulum. 5. Pendulum Plane A (see Figures 6.1 and 6.2) is perpendicular to the longitudinal plane of the vehicle. 6. For each impact, the impact line must be at least 2 inches (50 mm) in the vertical direction from its position in any prior impact, unless the midpoint of the impact line is more than 12 inches (300 mm) apart laterally from any prior impact. 7. Impacts must be performed at intervals not less than 30 minutes apart. 8. Effective impacting mass of pendulum equals mass of vehicle. FIGURE 6.1 IMPACT PENDULUM (20 Impact Height) (Source: Reference 6.8) 6-4

147 FIGURE 6.2 PENDULUM (20-16 Impact Height) (Source: Reference 6.8) FIGURE 6.3 SAMPLE IMPACT APPARATUS (Source: Transport Canada, Safety and Security) 6-5

148 6.1.5 Impacts into a fixed collision barrier 1. Impact speed of 2.5 mph (4 km/h). 2. Impact into a fixed collision barrier perpendicular to line of travel while travelling longitudinally forward. 3. Impact into a fixed collision barrier perpendicular to line of travel while travelling longitudinally rearward. 6.2 Canadian Motor Vehicle Safety Regulations Section 615 of Schedule IV Requirements This regulation (Reference 6.9) is summarized in Section The reader is cautioned that this section is only a summary. The reader must refer to the actual regulatory document in order to obtain a complete understanding of the regulation. A passenger car shall be equipped with bumpers that conform to either: a) The requirements set out in title 49, part 581 of the United States Regulations or b) The requirements set out in paragraph 6, and the lowspeed impact test procedure set out in Annex 3, except for paragraph 4 of that Annex, of ECE Regulation No United National Economic Commissions for Europe ECE Regulation Requirements This regulation (Reference 6.10) is summarized in Sections through The reader is cautioned that these sections are only a summary. The reader must refer to the actual regulatory document in order to obtain a complete understanding of the regulation. The requirements apply to a vehicle with at least four wheels for the carriage of passengers comprising not more than eight seats in addition to the driver s seat. A passenger vehicle is subjected to two impact procedures: 1. The longitudinal test procedure with an impact device - two impacts at 4 km/h on the front surface and two impacts at 4 km/h on the rear surface. 2. The corner test procedure with an impact device - one impact at 2.5 km/h on a front corner and one impact at 2.5 km/h on a rear corner. 6-6

149 6.3.2 Test vehicle After each impact test, the vehicle shall meet the following requirements: 1. The lighting and signaling devices shall continue to operate correctly and to remain visible. Bulbs may be replaced in the event of filament failure. 2. The hood, trunk lid and doors shall be operable in the normal manner. The side doors shall not open during the impact. 3. The vehicle s fuel and cooling systems shall have neither leaks nor constricted fluid passages, which prevent normal functioning. Sealing devices and caps shall be operable in the normal manner. 4. The vehicle s exhaust system shall not suffer any damage or displacement, which would prevent its normal function. 5. The vehicle s propulsion, suspension (including tires), steering and braking systems shall remain in adjustment and shall operate in a normal manner. 1. The protective devices and the mountings attaching them to the vehicle structure may be repaired or replaced between tests. 2. A vehicle of the same type may be used for each test. 3. Unladen weight means the weight of the vehicle in running order, unoccupied and unladen but complete with fuel, coolant, lubricant, tools and a spare wheel (if provided as standard equipment by the vehicle manufacturer. Laden test weight means the weight of the unladened vehicle, plus the weight of the passengers (75 kg per passenger) distributed as follows: 6-7

150 6.3.3 Impact device 1. The impact device is shown in Figure The impact device may be either secured to a carriage (moving barrier) or form part of a pendulum. 3. The effective mass shall be equal to the mass corresponding to the unladen weight of the vehicle. 4. With Plane A of the impact device vertical, the reference line shall be horizontal 5. The reference line height is 445mm Longitudinal test procedure 1. This procedure consists of four impacts at 4 km/h. 2. Two impacts are on the front surface and two impacts are on the rear surface. 3. On each surface, one impact is made with the vehicle under unladen weight and the other is made with the vehicle under laden weight. 4. The choice of impact location for the first impact on each surface is free. The second should be at least 300mm from the first, provided the impact device does not overhang the corner of the vehicle. 5. Plane A of the impact device shall be vertical and the reference line horizontal at a height of 445mm Corner test procedure 1. This procedure consists of four impacts at 2.5 km/h. 2. Two impacts are on the front surface and two impacts are on the rear surface. 3. On each surface, one impact is at one corner with the vehicle under unladen weight and the second impact is at the other corner with the vehicle under laden weight. 4. The choice of impact location for the first impact on each surface is free. The second should be at least 300mm from the first, provided the impact device does not overhang the corner of the vehicle. 5. Plane A of the impact device shall be vertical and the reference line horizontal at a height of 445mm. 6-8

151 6-9 FIGURE 6.4 IMPACT DEVICE (Source: Reference 6.10)

152 6.4 Insurance Institute for Highway Safety: Bumper Test Protocol (Version VII) Requirements This protocol (Reference 6.11) is summarized in Sections through The reader is cautioned that these sections are only a summary. The reader must refer to the actual protocol document in order to obtain a complete understanding of the protocol Test vehicles Four tests (a front and a rear full-overlap test at 10 km/h and a front and a rear corner test at 5 km/h) are conducted. After each test, a damage estimate is prepared as it would be done in a repair shop. A weighted damage estimate is calculated by adding the front full-overlap damage estimate to the rear full-overlap damage estimate and multiplying the total by two; adding to this amount the front corner damage estimate and the rear corner damage estimate; then dividing the grand total by six to get a weighted average damage estimate. The weighted average damage estimate is used to determine the overall rating for a vehicle. The good/acceptable boundary is $500, the acceptable/marginal boundary is $1,000 and the marginal/poor boundary is $1,500. However, no vehicle can earn a rating of good or acceptable if the vehicle is deemed undrivable or unsafe because of severe headlamp or tail lamp damage, hood buckling, coolant loss or the like. 1. Two vehicles are purchased to conduct the four tests. 2. The front and rear license plate brackets (if provided) and all associated fasteners are removed. Bolt-on trailer hitch reinforcement members that are supplied as optional equipment are removed, but their fasteners are reattached to the vehicle where possible. 6-10

153 6.4.3 Impact barrier Full-overlap impact 1. The Impact Barrier is shown in Figure The bumper barrier is constructed of 12.5 mm steel plate (Figure 6.6) and mounted to a block of reinforced concrete weighing 145,150 kg. 3. A steel backstop is constructed of 12.5 mm steel plate (Figure 6.7). It is mounted to the upper surface of the bumper barrier rearward from the impact face of the bumper barrier. 4. A plastic energy absorber is affixed by nylon push-pin rivets to the impact face of the bumper barrier. 5. An overlying plastic cover is mounted over the plastic energy absorber on the bumper barrier. 6. An overlying plastic cover is mounted over the steel backstop. 1. Two tests - front into barrier and rear into barrier. 2. Impact speed of 10 km/h. 3. The forwarding portion of the bottom edge of the bumper barrier is 457 mm from the floor. 4. At impact, the vehicle centerline is aligned with the bumper barrier centerline. 6-11

154 FIGURE 6.5 IIHS IMPACT BARRIER (Source: Reference 6.4) FIGURE 6.6 STEEL BUMPER BARRIER (Source: Reference 6.4) 6-12

155 FIGURE 6.7 STEEL BACKSTOP (Source: Reference 6.4) FIGURE 6.8 OVERLAP FOR FRONT CORNER TEST (Source; Reference 6.4) 6-13

156 6.4.5 Corner impact 1. Two tests - front corner into barrier and rear corner into barrier. 2. Impact speed of 5 km/h. 3. The forward most portion of the bottom edge of the bumper barrier is 406mm from the floor. 4. At impact, the vehicle overlaps the lateral edge of the barrier by 15 percent of the vehicle s width at the wheel wells (including moldings and sheet metal protrusions) at the corresponding axle - front axle for front corner test (Figure 6.8) and rear axle for rear corner test. 6.5 Research council for automotive repairs (RCAR) low-speed offset crash test (Low-speed structural test) Requirements This test (Reference 6.13) is summarized in Sections through The reader is cautioned that these sections are only a summary. The reader must refer to the actual test document in order to obtain a complete understanding of the test. RCAR states its purpose of this test is to determine a vehicle s damageability and reparability features. Two impacts are conducted. The first is a 15 km/h (9 mph) impact by the front of the test vehicle into a fixed barrier with a 40 percent offset. The second is a 15 km/h (9 mph) impact by a mobile barrier with a 40 percent offset into the rear of the test vehicle. After each impact, the replacement parts required to reinstate the vehicle to its pre-accident condition are recorded. Also, the number of hours required to replace the damaged parts and to repair those items capable of repair, such that the vehicle is reinstated to the pre-accident condition, are recorded. The cost of the replacement parts and hours is estimated. Thus, the results of the crash test indicate the reparability and damageability status of the test vehicle. 6-14

157 6.5.2 Test vehicle Front impact The test procedure applies to people driven passenger vehicles of up to 2.5 times mass. The test vehicle shall be previously undamaged and representative of the series production. The test vehicle for the rear impact may be the same vehicle used for the front impact, provided the damage sustained during the front impact has no effect on the results of the rear impact Rear impact 1. One impact into a non-deformable barrier/former (see Figure 6.5). The former can be adjusted laterally to accommodate various vehicle widths. The former may be secured to a fixed barrier or placed on the ground with arresting devices to restrict its movement. The front face of the former is perpendicular to the direction of travel of the test vehicle. The mass of the barrier/former exceeds twice that of the test vehicle. The steering column side of the vehicle contacts the former. The test vehicle overlaps the former by 40 percent. 2. The test vehicle impact speed is 15 km/h (9 mph). 1. One impact by a mobile barrier into the test vehicle (Figure 6.6). The mobile barrier has a mass of 1000 kg (2205 pounds). 2. The mobile barrier contacts the side of the vehicle opposite to the steering column side. The barrier overlaps the test vehicle by 40 percent. The barrier impact speed is 15 km/h (9mph). 6-15

158 FIGURE 6.9 RCAR FRONT CRASH PROCEDURE (Source: Reference 6.13) 6-16

159 FIGURE 6.10 RCAR REAR CRASH PROCEDURE (Source: Reference 6.13) 6-17

160 6.6 Research Council for Automotive Repairs (RCAR) Bumper Test Requirements This test is summarized in Sections through The reader is cautioned that these sections are only a summary. The reader must refer to the actual test documents (References 6.14 and 6.15) in order to obtain a complete understanding of the test. The RCAR Bumper Test, like the IIHS Bumper Test (6.4), encourages vehicle manufacturers to produce effective bumper systems that feature tall energy absorbing beams and crash boxes, which are fitted at common heights and can effectively protect the vehicle in low speed crashes. To this end, RCAR also publishes a Design Guide (Reference 6.16) to ensure good design practice for reparability and limitation of damage. The RCAR test applies to passenger cars, pickups and SUVs. Bumper beams that have insufficient height will be presumed to fail the test. Also, bumper beams that use the barrier system backstop for energy management will be regarded as unacceptable. Bumper beams are likely to have insufficient height if the relevant bumper engagement is less than 75 mm as shown in Figure For a front bumper, the distance from the floor to the underside of the bumper barrier is 455 mm. For a rear bumper, the distance from the floor to the underside of the bumper barrier is 405 mm. Bumper beams with a relevant engagement less than 75 mm will be tested if the qualifying bumper beam height is 100 mm or more. Bumper beam height is measured in the center of the vehicle, in front of the left siderail and in front of the right siderail. The center of the vehicle bumper height is weighted 50 percent. The left and right siderail bumper heights are each weighted 25 percent. The sum of the three weighted heights is the qualifying bumper beam height. The test involves either the front or rear of a moving car striking a fixed bumper barrier at 10 km/h. The centerline of the car is aligned with the center of the bumper. RCAR does not assign vehicle ratings. It states that results from the RCAR Bumper Test may be used by RCAR members (or the associated test organizations) for rating or consumer information purposes to suit local market conditions. 6-18

161 6.6.2 Bumper barrier 1. The bumper barrier is shown in Figures The rigid bumper barrier is made from steel. It is 100 mm deep and 1500 mm wide. The flat front face has a radius of 3400 mm. The bumper barrier can be mounted at various heights to the unyielding and immovable crash wall. 3. A rigid steel backstop is fixed on top of the barrier. It has the same radius and width as the bumper barrier. 4. An energy absorber is firmly affixed to the face of the bumper barrier. 5. A cover over the energy absorber is wrapped around the bumper barrier and fastened to the top and bottom plane of the barrier. FIGURE 6.11 RELEVANT BUMPER ENGAGEMENT (Source: Reference 6.14) 6-19

162 FIGURE 6.12 BUMPER BARRIER (Source: Reference 6.14) FIGURE 6.13 BUMPER BARRIER WITH BACKSTOP AND ENERGY ABSORBER (Source: Reference 6.15) 6-20