Tensile Testing for Sheet Metal Formability. Engineering Quality Solutions, Inc. / 4M Partners, LLC PMA Die Design & Simulation Technology

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1 Tensile Testing for Sheet Metal Formability Daniel J. Schaeffler, Ph.D. President, Engineering Quality Solutions, Inc., Chief Content Officer, 4M Partners, LLC, May

2 YTE splits uniform elongation formability strain tensile properties stress N-value dogbone TYE yield strain hardening failure cracking necking 2

3 Hardness Resistance to Permanent Indentation Quick and Easy Test Procedure Dependent Poor Indicator of Formability 3

4 Hardness Mohs hardness Mineral Chemical formula Absolute hardness 1 Talc Mg 3 Si 4 O 10 (OH) Gypsum CaSO 4 2H 2 O 3 3 Calcite CaCO Fluorite CaF Apatite Ca 5 (PO 4 ) 3 (OH,Cl,F ) 48 6 Orthoclase Feldspar KAlSi 3 O Quartz SiO Topaz Al 2 SiO 4 (OH,F ) Corundum Al 2 O Diamond C

5 Schematic of Rockwell Testing resistance to indentation HRB: 1/16 diameter ball Rockwell B Hardness = d mm Depth from minor load Depth from major load d=difference HRB 80 = 0.10 mm HRB 65 = 0.13 mm HRB 50 = 0.16 mm (human hair 0.10mm) One Rockwell point = 2 microns = inch 5

6 Anvil Effect Indentation depth <10% of sample thickness If not, then risk of shiny spot (burnish) on other surface Zhang, Dhaigude, and Wang, The Anvil Effect in the Spherical Indentation Testing on Sheet Metals, Procedia Manufacturing Volume 1, 2015, Pages , doi: /j.promfg

7 Tensile Testing 7

8 Definitions Strength (stress) = Resistance to Deformation = Force / Area Strain = % Displacement = Change in length / Original length 8

9 Tensile Testing Engineering Stress (ksi or MPa) Tensile Strength Yield Strength Total Elongation ܨ ܣ =ݏ 0.2% Engineering Strain (%) 9

10 Elongation: Total vs Uniform Uniform Elongation necking begins deformation from beginning of forming through necking post-necking localized elongation Engineering Stress (ksi or MPa) Total Elongation deformation from beginning of forming through splitting Engineering Strain (%) 10

11 Necking and Fracture 11

12 Tensile Testing Engineering Stress/Strain vs True Stress/Strain ܨ ܣ = =ݏ = ݏݏ ݎݐ ݎ ܧ ݎܨ ݎܣ ݐ ݏݏݎܥ ݐ ܫ ܨ ܣ = σ =ݏݏ ݎݐ ݑݎ = ݎܨ ݎܣ ݐ ݏݏݎܥݏݑ ݐ ݐݏ ܫ ܮ ܮ = = ݎݐ ݎ ܧ ܮ ܮ = ܮ = ݐ ܮ ܥ ݐ ܮ ݑ ܩ ݐ ܫ = ln = ݎݐ ݑݎ ܮ ܮ = ݐ ܮ ܥݏݑ ݐ ݐݏ ܫ ݐ ܮ ݑ ܩ ݐ ܫ σ = s (1+e) ϵ = ln (1+e) 12

13 Engineering vs True σ = s (1+e) ϵ = ln (1+e) 13

14 ASTM Tensile Bar 14

15 Calculating Elongation 2 inches 2.68 inch 2.3 inch 15

16 Tensile Bar Specification 16

17 Effect of Gauge Length 17

18 n-value means Better Ability to Distribute Strains 10% 10% > 55% 18

19 N-Value 19

20 N-Value 20

21 Data Points from Tensile Testing 21

22 Holloman Power Law: true stress-strain σ = Kεn 22

23 Strain Hardening Exponent = N-Value Holloman Power Law: σ = Kεn ln σ Slope = n lnσ = lnk + n(ln ε) n the slope of the ln σ - ln ε plot ln ε 23

24 σ = Kεn lnσ = lnk + n(ln ε) 24

25 σ = Kεn lnσ = lnk + n(ln ε) K=743MPa, n=0.176 Strain = 20% Slope = n-value = Strain = 10% 25

26 Physical Meaning of N-value Calculated as slope of the ln σ - ln ε plot Higher slope = higher n-value The larger the YS and TS gap, the better the formability. 26

27 Typical Stress-Strain Curves YS TS U-EL T-El N-Value MPa MPa % % (10%-20% or UEL) DP 350/ HSLA 350/

28 With both having similar YS, EL, and n-value Why does DP form better than HSLA? YS TS U-EL T-El N-Value MPa MPa % % (10%-20% or UEL) DP 350/ HSLA 350/

29 With both having similar YS, EL, and n-value Why does DP form better than HSLA? YS TS U-EL T-El N-Value MPa MPa % % (10%-20% or UEL) DP 350/ HSLA 350/

30 Typical Stress-Strain Curves YS TS U-EL T-El N-Value MPa MPa % % (10%-20% or UEL) DP 350/ HSLA 350/ N-Value (4%-6%)

31 Simulation Input? Which n-value to use?? 4% to 6% 6% to 12% 10% to 20% 10% to Uniform Elongation? 31

32 Plastic Strain Ratio Lankford Coefficient Plastic Anisotropy r-value Ratio of true width strain to true thickness strain in uniform elongation region Higher r-value indicates better resistance to thinning ௪ ௧ 32

33 Plastic Strain Ratio Lankford Coefficient Plastic Anisotropy r-value Ratio of true width strain to true thickness strain in uniform elongation region Low r-value width strain is small compared with thickness strain Material with low r-value: longer and thinner after straining without much width change 33

34 Plastic Strain Ratio Lankford Coefficient Plastic Anisotropy r-value Ratio of true width strain to true thickness strain in uniform elongation region High r-value width strain is large compared with thickness strain Material with high r-value: longer and more narrow after straining without much thickness change 34

35 Worldwide Specifications EN/SEW High Strength Steel - Focus on Yield Strength - YS steps: 40 MPa - Test Direction: Transverse - Tensile Test Length: 80mm (DIN) SAE/ASTM High Strength Steel - Focus on Yield Strength - YS steps: not consistent JFS/JIS High Strength Steel - Test Direction: Longitudinal - Focus on Tensile Strength - Tensile Test Length: 50mm (ASTM) - YS steps: not consistent - Test Direction: Transverse EMS.ME High Strength Steel - Tensile Test Length: 50mm (JIS) - Focus on Yield Strength - YS steps: 40 MPa - Test Direction: Transverse - Tensile Test Length: 50mm (ASTM) 35

36 Strength Conversion psi = pounds per square inch ksi = kilo (thousand) pounds per square inch MPa = mega Pascal = 1,000,000 Pascal GPa = giga Pascal =1,000 MPa 1 ksi = MPa 1 ksi 7 MPa HSLA 50XF = HSLA 350/450 36

37 Tensile Strength vs Elongation 80 Total Elongation (%) Mild AA5XXX BH AA6XXX-T4 20 AA6XXX-T6 Mg AA7XXX-T6 200 MPa 30 ksi 500 MPa 75 ksi MS 800 MPa 115 ksi 1100 MPa 160 ksi 1400 MPa 200 ksi 1700 MPa 245 ksi 2000 MPa 290 ksi Tensile Strength 37

38 Specific Strength vs Elongation Effect of Density 80 Total Elongation (%) Mild BH MS 10 0 Mg Tensile Strength / Density = Specific Tensile Strength (MPa/(kg/m3) 38

39 TWIP: Twinning Induced Plasticity X-IP : Xtremely formable + Xtremely high strength steels with Induced Plasticity 39

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