Formability and Testing Methods

Transcription

1 Formability and Testing Methods October 15th, 2014 Hyunok Kim, EWI Forming Center Taylan Altan, OSU - Center for Precision Forming (CPF) 1

2 F.A.Q. for Materials to Engineers Which material properties or parameters help us to select the material? How can I obtain the material properties? Can we form the material at room temperature? If not, what is the preferable temperature to heat the material to make it more ductile? What should we consider for selecting a material in post manufacturing operations (e.g. heat treating, welding, joining, machining, painting, etc.)? How much cost savings can be obtained by changing the material? 2

3 Formability vs. Ductility Formability is the ability of a material to undergo the desired shape changes without necking failures. Ductility is the ability of a material to deform plastically without fracturing. 3

4 Toughness vs. Strength Toughness indicate the energy required to crack a material. Strength indicates the resistance of a material to failure under the applied stress (tensile, compressive, or shear). Increasing strength usually leads to decreased toughness. Tempered steel is tougher but weaker than quenched steel. [Courtesy of ] 4

5 Basic Deformation Modes S L A de dl l e l 1 dl l 0 l b l 1 l l 0 Engineering stress Engineering strain Engineering shear strain a 5

6 True Stress, Strain and Strain Rate True Stress (based on instantaneous dimensions) L A True Strain (based on instantaneous dimensions) d ln dl l e 1 l 1 l 0 dl l ln l l 1 0 6

7 Basic Mechanics of Materials Volume Constancy ε x ε y ε z 0 [Courtesy of Altan and Tekkaya, 2012]] True Strain Rate (instantaneous) d dt dl / dt l V l 7

8 Deformation Work and Heat Energy Specific energy for deformation u ideal 1 0 d Specific energies in metal forming u total u ideal When u Forming efficiency: n n 1 K u K d Y 1 friction redundant Temperature increase of deforming material: u ideal total Where ρ = density and C = specific heat of the material. u u 1 K ( n1) ( n 1) 0 Avg. flow stress T u C total 8

9 How to Define the Behavior of Material Elastic behavior of a metal: (Hook s law) E Plastic behavior of a metal is defined by flow stress. Flow stress is an instantaneous yield stress and is a function of: σ = fun(t, ε, ε, Microstructure) Material related factors: compositions, microstructure, grain size, heat treatment, and prior history of plastic deformation Process related factors: temperatures and strain rates Plastic deformation is caused microscopically either by the slip or twinning of atomic layers of a material. 9

10 Sheet Formability Tests Uniaxial Tensile Test Bending Test Draw Bend Test Olsen Cup Test & Erichen Cup Test Swift Cup Test Limiting Dome Height Test Viscous Pressure Bulge Test Fukui Conical Cup Test Conical Cup Wrinkling Test Yoshida Buckling Test 10

11 Tensile Testing A uniform slender test specimen is stretched along the longitudinal direction. z t o w o y Sheet Metal Tensile Test Specimen Tensile Test Machine 11

12 Standard Tensile Test Standardized tests: ASTM-E646 (K, n), E517 (r), and JIS - No.5 Example data from tensile test: Modulus of Elasticity (E) Yield Strength (YS) Ultimate Tensile Strength (TS) Total Elongation (e t ) Anisotropy: r-value (width / thickness strain ratio) Uniform elongation limit: n-value (work hardening exponent) ASTM-E8 Standard Tensile Geometry JIS No.5 Standard Tensile Geometry 12

13 Engineering Stress-Strain Tensile strength (ultimate tensile strength), S u [Courtesy to Atlas of Stress-Strain Curves] q : The reduction in area 13

14 Yield Point Behavior of Steels [Courtesy to Atlas of Stress-Strain Curves] 14

15 Stress Converting to True Stress-Strain True Invalid data portion F Eq.1 UTS UTS Engineering e = Eq.2 F Yield Eq.3 Strain Eq.3 and Eq.4 are valid only to the onset of necking! Eq.4 15

16 How to determine strain hardening exponent, n-value [Courtesy to Atlas of Stress-Strain Curves] The higher the value of n, the greater the strain to which a material can be stretched before necking starts. 16

17 Types of Stress-Strain Curves Hollomon s model (Power-law model) Ludwik s model (Shifted power-law Plastic, linear strain hardening Perfectly plastic model) [Courtesy to Hosford and Caddell, 2007] K = strength coefficient n = strain-hardening (work-hardening) coefficient m = strain rate sensitivity exponent K n m Fields and Backofen model 17

18 Engineering Stress (psi) Example Eng. Stress-Strain Zero Rolling Direction Comparison of Materials Curves High Strength Steel Bake Hard Steel AKDQ Bake Hard Steel High Strength Steel Aluminum 6111 AKDQ Steel Aluminum [Courtesy of Altan and Tekkaya, 2012] Engineering Strain 18

19 Effects of n-value on Tensile Tests Comparison of Final lengths of the Specimens with Different n values (for Elongation) [Courtesy of OSU-CPF] Magnitudes of elongation at the onset of necking (a) Specimen before deformation b) n =0.2 c) n = 0.4 d) n =0.6 19

20 Stress Effects of Strain Rate and Temperatures on Flow Stress K n m Strain 20

21 Strain-rate ranges [Courtesy of Atlas of Stress-Strain Curves] 21

22 Bauschinger Effect An example of the Bauschinger effect and hysteresis loop in tensioncompression-tension loading [Courtesy of Atlas of Stress-Strain Curves] 22

23 Tensile Test Lankford Coefficient (Anisotropy value) w 0 r d w d t w ln t ln o o w t f f 23

24 Anisotropy (ASTM E 517) Normal Anisotropy r r r90 r 4 Normal anisotropy (r) Rolling Direction r 0 o r 90 o r 45 o Planar Anisotropy (Earing Tendency) r r 0 r 90 2r 45 2 Effects of Anisotropy on Earing in Drawing 24

25 Correlation between anisotropy and earing height [Courtesy: Hosford and Caddell, 1993] 25

26 Formability in Bending - Wedge Bend Testing Provides quantitative measure of bendability Crack determined by load drop. Final outside bend angle is measure of bending performance. [Courtesy to J. Dykeman, 2013] 26

27 Olsen Cup / Erichsen Cup Tests The cup height at the fracture point is a measure of the stretching ability. The maximum load point is measured during the tests. It is hard to obtain repeatable data, because the friction affects the results. ASTM E 643 Olsen Cup Test (Taylor 1988) 27

28 The Swift Cup / LDR Test In the Swift cup test, in order to determine LDR (Limiting Draw Ratio= max. blank diameter / punch diameter), blanks of different diameters are drawn until one fails. Testing can be shortened by having a good estimation that the LDR's of most materials fall within a range of 2 ±0.2. Usually 6 to 10 blanks with D/d intervals of 0.02 give an acceptable LDR. The Swift cup test (Dieter, 1984) 28

29 Sheet Formability Limiting Dome Height (LDH) Test 29 [Grote, 2009]

30 Determination of Forming Limit Diagram (FLD) using LDH Test Forming Limit Diagram (FLD) (Courtesy of Kalpakjian,1997) FLD tested stainless steel samples 30

31 Viscous Pressure Bulge Test Viscous Pressure Bulge (VPB) test can provide the larger range of plastic flow stress data for the use of FE simulations. Laser Sensor Sheet Viscous medium Pressure transducer 254 mm Stationary Punch After Forming 254 mm Page 31 31

32 Video of Bulge Test Click here to run a video 32

33 Hydraulic (Viscous Pressure) Bulge Test (Material-A) (Material-B) For the same dome height, sample-1 has a better formability than sample-2 To evaluate the formability under biaxial stretch conditions Useful for quality control of incoming sheet coil or blanks in production The higher the bulge at fracture, the better is the formability of the sheet. 33

34 Summary of Formability and Testing Methods Formability is a material property and indicates the ability of the metal to deform without failure. Often formability is related to uniform or total elongation in tensile test. Better indication of formability is obtained from the biaxial bulge test (the bulged height at fracture). Formability is important to design the tooling and understand the effects of process parameters. 34

35 Questions & Contacts Taylan Altan Hyunok Kim