Structural Bonding for Lightweight Construction

Transcription

1 Materials Science Forum Vols (2009) pp Online at : (2009)Trans Tech Publications, Switzerland Online available since 2009/Apr/17 Structural Bonding for Lightweight Construction Martin Hornung 1,a and Michael Hajj 2 Keywords:structural bonding, structural adhesive, structural foam, lightweight construction Abstract The demand for safer, lighter and more fuel efficient vehicles has led to the market entry of light weight construction, with the use of advanced high strength steels, engineering plastic and light weight metals. Maximum weight savings potential will be limited if these are used in isolation. Chemistry based structural adhesives and foams enable the intelligent use of all three weight saving methodologies to overcome the problems of safety, quality and high processing cost. Introduction Historically, conventional steel has been the structural backbone of vehicle construction. To achieve increasing demands for safer and quieter vehicles, often heavy structural steel components are designed to meet the crash and durability criterion. The effect is heavier and less fuel efficient vehicles. What is the value of a one kilogram weight saving? Data from a literature review showed that, for vehicles with internal combustion engine, a 10% curb weight reduction is equal to 4.5 to 6% increase in fuel efficiency. A very detailed analysis of three vehicle classes and two driving cycles based on simulation was presented in the same paper [1]. It was described that an average increase in fuel efficiency of 6.7% could be achieved by 10% curb weight reduction and by re-sizing of the powertrain at the same time. To elaborate, a 140 kilogram body weight reduction for an upper middle class car is equal to about 1,150 fuel cost saving over the lift of a vehicle( kilometer lifetime travel, 1.2 / liter gasoline, average fuel consumption 9.6 liter / 100 kilometer [2]). A further benefit of less fuel consumption is emission reduction of global warming pollutants. This is pivotal in meeting the increasing stringent global emissions targets. Current public perception of alternative fueled vehicles is that they are small, underpowered and operable only over short distances. One way to overcome all these perceptions is by reducing vehicle body weight. But having lighter vehicles the competitiveness of alternative engines will become more favorable. In recent times the competitiveness of OEM s and Governmental demands for safer, lighter and more fuel efficient vehicles has led to the market entry of light weight construction. The three main concepts for vehicle weight reduction are with the use of : 1. Advanced high strength steel (AHSS):Body in white (without closures) weight reduction is possible up to 17 to 25% with the use of AHSS in combination with optimized steel design [3]. Critical applications such as A-pillar and B-pillar reinforcements allowing thinner gauges AHSS to be used whilst maintaining or improving crashworthiness. 2. Engineering plastics {sheet molded composites (SMC) or carbon fibre reinforced polymer (CFRP): Weight saving of up to 30% are achievable when compared to equivalent steel design, in application of underbody shields, exterior body panels, spare wheel wells and semi-structural components [4].

2 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher.(trans Tech Publications Ltd, Switzerland, (ID: /04/09, 08:27:55))

3 50 Light Metal Technology Light weight metals such as aluminum and magnesium. Body in white (without closures) weight reduction is possible between 21 and 40% for aluminum using recent methods of design [3]. The potential weight saving listed above are achieved while using each methodology exclusively. The methodologies however are not mutually exclusive, therefore maximum weight savings potential will be possible only through intelligent combination of all three concepts. However, whether one concept or combinations of all three concepts are used in the design of a car body, maximum weight saving will be limited by safety / quality requirements (energy absorption and stiffness of body structure, risk of galvanic corrosion due to dissimilar metals) and the need of process cost control (stamping operations of AHSS, joining of dissimilar substrates with the difference in thermal expansion). The main target for this paper is to demonstrate how the use of Henkel s chemistry based structural materials, such as structural adhesive and structural foam can, on the one hand, contribute to further weight savings, and, on the other hand, maintain today s vehicle safety / quality level and support process cost control. Fundamentals - Mechanical Stiffness, durability and energy absorption of vehicle components are limited by loss their section integrity under quasi-static or dynamic load. Spot weld points can crack or local cracks and local buckling deformation can occur. These failure modes can be avoided / improved by reducing stress maxima, e.g. at spot weld points (Figure 1A) and by broadening the deformation area of car body components, e.g. B-pillar side impact (Figure 1B) Figure1. A)Increased energy absorption of lap shear test specimen by combination of spot welding and adhesive bonding [5] and B) elimination of local bucking due to broadening of the deformation area [6]. Chemistry based structural material have been developed by Henkel to increase or maintain stiffness,

4 durability and energy absorption of vehicle components by keeping the same weight or even reduce weight. Henkel s structural solutions can be separated into two basic categories: structural adhesives and structural foams. Structural adhesives must be able to form and sustain a strong bond between the adherends in various environmental conditions over a long period of time. While for pure stiffening applications the modulus of the adhesive is most important mechanical parameter a combination of high modulus and high flexibility is a key for adhesives applied to improve vehicle component behavior under high strain rates.

5 Materials Science Forum Vols Figure 2. Differences in mechanical properties for Stiffening (only) Adhesives and for Impact Resistant Adhesives. The concept of different mechanical properties for Stiffening Adhesives and for Impact Resistant Adhesives is visualized in Figure 2. Figure 3. Typical curve for compression strength of structural foam commercialized by Henkel. Both parameters are important for optimum distribution of stress, optimum energy transfer between outer panel and carrier and broadening of the deformation area. Structural foams are usually not applied as pumpable adhesives but as preformed parts. The preformed parts may be fashioned using structural foam alone, or a structural carrier, made of metal

6 or plastic, may be used in conjunction with the foam. The design of these parts depends on the inner shape of the cavity into which they are inserted and the resulting body loads they are intended to augment. When exposed to high temperatures (150 to 200 C over min 15 min), such as those in an automotive E-coat oven, the structural foam starts to expand forming a strong bond between the outer skins of the cavity or between the outer skin of the cavity and the carrier of the foam. In case a static or dynamic load is applied, energy is transferred between the outer skins or the outer skin and the carrier maximizing reinforcement contribution. Key parameters for structural foams are compression strength (see Figure 3) and adhesive strength.

7 52 Light Metals Technology 2009 Galvanic corrosion is an issue for joint dissimilar metals in presence of an electrolyte (like salty water). Electrons can flow to the metal nobler than the other and accelerate corrosion of the vehicle part made of the less noble metal. Cured adhesives usually have excellent isolating properties and prevent flow of electrons. Therefore, joining of dissimilar metals by the use of structural adhesives is a tool to avoid galvanic corrosion and to improve the mechanical properties of the metal structure at the same time. Experimental Data - How They Work Together The three main design tools currently utilized, for vehicle body weight reduction, are the application of AHSS, engineering plastics and light weight metals. All three concepts have been studied in combination with Henkel s chemistry based structural materials: either as idealized box beams, as case studies with real parts or as commercialized solutions within customer projects. The studies have been partially carried out as physical tests and as simulation, respectively. Steel / Advanced High Strength Steel and Henkel s Structural Solutions Steel and structural adhesives The combination of spot welding and adhesive bonding (weld-bonding), has been shown in previous publications, to increase the torsional stiffness of hat sections by 20%. A reduction in number of spot welds of 50 % was possible without losing the beneficial effect of the adhesive [7]. In addition, the positive benefit of weld-bonding on the fatigue durability of lap shear test specimen has been tested (Henkel internal report). Recent box beam testing has provided additional insight into the benefits of using structural adhesives. The study evaluated hat section box beams in 3 point bending, and compared spot welding only to bonding only. Additionally, two different structural adhesives were compared; one adhesive was designed only for stiffness while the other was developed to provide improved impact performance. Figure 4 shows that under 45 deg impact, the energy absorption of the box beams increased significantly in case an impact resistant structural adhesive has been used for joining the steel. For the same impact energy, the displacement of the beam bonded using impact resistant structural adhesive has been reduced significantly. The weight increase of the box beam due to the application of the adhesive was 20 g or 0.5 % of the spot welded box beam without adhesive.

8 Figure 4. Energy absorption and displacement of steel box beams under 45 deg impact depending on joining method: spot welding versus bonding with stiffening adhesive (no spot welding applied) versus bonding with impact resistant adhesive (no spot welding applied).

9 Materials Science Forum Vols The reason for the improved result is the increased energy absorption of the steel panels by elimination of local weld flange separation (see Figure 5). Figure 5. Weld flange separation and energy absorption at 130 mm deflection for box beam joined by spot welding only and joined by bonding using an impact resistant adhesive (no spot welding applied). Steel and structural foam Three point bending tests of idealized box beams reinforced with structural foam have been carried out. The flanges have been joined using spot welding. In 45 deg impact tests, box beams reinforcement by structural foam showed much higher energy absorption than spot welded only box beams. For the same impact energy, the displacement of the beam reinforced using structural foam has been reduced significantly about 45 % (see Figure 6). The weight increase of the box beam due to the application of the structural foam was 200 g or 5 % of the spot welded only box beam. The box beams reinforced with structural foam also showed local weld flange separation (equivalent to result for spot welding only box beam shown in Figure 5). As demonstrated above, local weld flange separation is limiting the energy absorption capabilities of the test section. Consequently, by additional application of impact resistant adhesive in the form of weld bonding, a further increase in energy absorption of the structural foam reinforced box beam is possible.

10 Figure 6. Energy absorption and displacement of steel box beams under 45 deg impact: spot welding only versus reinforcement with structural foam plus spot welding.

11 54 Light Metals Technology 2009 The functionality of structural foam to increase energy absorption has been explained by simulation and physical testing [6]. In the example described, the energy is transferred between the outer skin and the reinforcement panel of a B-pillar. This energy transfer is maximizing the energy absorption of the reinforcement panel and is preventing local buckling deformation (see Figure 7). The solution described has been applied on a vehicle with about 6 Kg/car weight reduction and equal crash performance compared to the steel only solution. Figure 7. Increase of energy absorption of reinforcement panel and eliminated local buckling by reinforcement using structural foam due to energy transfer between outer skin and reinforcement panel [6]. Engineering Plastics and Henkel s Structural Solutions Using Henkel s structural solutions, engineering plastics can be used in conjunction with structural foam as an alternative to steel reinforcements. A simulation study has bas carried out to show the potential weight savings by using structural foam and nylon carriers (Figure 8). Multiple steel parts have been replaced by three parts with structural foam on nylon carriers, one steel part was upgraded in quality, and another steel part was significantly downgraded in quality. By these means, a weight saving of 12.5 Kg/car was achieved. The performance regarding 40 % front offset crash, IIHS side impact crash, IIHS side pole crash and roof crush has been maintained within 5 % of the reference steel solution. In addition to reduced weight, the solution developed in the study also showed reduced investment costs and improved processing. The three part per side solution reduced the steel part count, eliminated the need for many stamping dies and removed over 200 spot welds from the assembly. The parts were designed to be pinned into place within the assembly, without additional fixtures, a process that could be done either manually or robotically. The added flexibility allows the part to be fastened to the inner or outer panel of the section at various stages of assembly, even at a tier supplier.

12 Materials Science Forum Vols Figure 8. Henkel case study: weight reduction and process simplification achieved due to the use of structural foam on nylon carrier. Light Weight Metals and Henkel s Structural Solutions Light Weight Metals and Henkel s Structural Solutions In modern light weight construction vehicles, alternative materials are used to replace steel to reduce car body weight. To study the effect of light weight metals like aluminum and magnesium on energy absorption under dynamic load, the steel reinforcement of box beams has been replaced and dynamic 45 deg impact 3 point bending tests have been carried out. The energy absorption and the weight of box beams made by using 1.4 mm 590 MPa steel, 2.0 mm aluminum and 2.0 mm magnesium as reinforcement panel, respectively, has been compared (Figure 9). The box beams have been assembled by using impact resistant structural adhesive only. No additional joining method has been applied.

13 Figure 9. The energy absorption and the weight of box beams made by using 1.4 mm 590 MPa steel, 2.0 mm aluminum and 2.0 mm magnesium as reinforcement panel. In case of aluminum as reinforcement panel, a weight reduction of about 18.5 % per box beam goes along with a 10.7 % drop in energy absorption.

14 56 Light Metals Technology 2009 Conclusions The information summarized in this paper highlights the benefits of using chemistry based structural materials, like structural adhesive and structural foam as strategic tool in modern car body construction. Basing on a mass optimized vehicle using AHSS, further mass elimination is possible by replacement or down gauging of steel elements while vehicle safety / quality level can be maintained after optimization. Synergistic effects and therefore increased potential in weight reduction can be achieved by combination of structural foams with structural adhesives. Process cost can be reduced, e. g. by simplified stamping processes due to down gauging of AHSS elements or downgrading of steel quality. Application of chemistry based structural materials offer the opportunity of joining dissimilar materials. Therefore, e.g. steel reinforcements / foam carriers can be replaced with plastic, aluminum or magnesium. In case of joining dissimilar metals the risk of galvanic corrosion can be minimized due to the electrically isolating character of cured adhesives. Therefore, it can be concluded, that the use of chemistry based structural materials enables innovative design and engineering scenarios which are needed to meet the future requirements in vehicle manufacturing. References [1] Wallentowitz, H., Wohlecker, R., Johannaber, M. and Espig, M. Determination of weight elasticity of fuel economy for conventional ICE vehicles, hybrid vehicles and fuel cell vehicles, Forschungsgesellschaft Kraftfahrwesen mbh Aachen, Body department, June 2007 [2] From average value of 623 data for Volkswagen Passat type vehicles with 66 to 220 KW [3] Wallentowitz, H., Wohlecker, R., and Henn, R. Communication module mass reduction, Forschungsgesellschaft Kraftfahrwesen mbh Aachen, November 2006 [4] From Innovation Driving Automotive SMC, Jan 2007 [5] International Iron and Steel Institute: Bonding of high strength steel [6] Ishida, K., Fukahori, M., Hanakawa, K., Tanaka, H., and Matsuda, K. Development of a technique to strengthen body frame with structural foam, No , SAE 2001 World Congress [7] Hornung, M., Doba, T., Agarwal, R., Butler, M., and Lammerschop, O. Journal of the Adhesion Society of Japan, 44, 258, 2008

15 Materials Science Forum Vols Light Metals Technology 2009 doi: / Structural Bonding for Lightweight Construction doi: / References [1] Wallentowitz, H., Wohlecker, R., Johannaber, M. and Espig, M. Determination of weight elasticity of fuel economy for conventional ICE vehicles, hybrid vehicles and fuel cell vehicles, Forschungsgesellschaft Kraftfahrwesen mbh Aachen, Body department, June 2007 [2] From average value of 623 data for Volkswagen Passat type vehicles with 66 to 220 KW [3] Wallentowitz, H., Wohlecker, R., and Henn, R. Communication module mass reduction, Forschungsgesellschaft Kraftfahrwesen mbh Aachen, November 2006 [4] From Innovation Driving Automotive SMC, Jan 2007 [5] International Iron and Steel Institute: Bonding of high strength steel [6] Ishida, K., Fukahori, M., Hanakawa, K., Tanaka, H., and Matsuda, K. Development of a technique to strengthen body frame with structural foam, No , SAE 2001 World Congress [7] Hornung, M., Doba, T., Agarwal, R., Butler, M., and Lammerschop, O. Journal of the Adhesion Society of Japan, 44, 258, 2008