Manufacturing (Bending-Unbending-Stretching) Effects on AHSS Fracture Strain

Transcription

1 Manufacturing (Bending-Unbending-Stretching) Effects on AHSS Fracture Strain Hong Zhu ArcelorMittal Global R & D - East Chicago

2 Crush Distance (mm) Background Tensile Strength (MPa) HSLA350 DP590/HSLA350 DP590 HFT590 DP965 DP980-SF M1310 FEA Simulation Component fracture occurs during crash Energy absorption predictions for AHSS components are affected by component fracture From SAE , B. Yan et al. 2

3 Background (cont d) Fracture criteria needed for fracture prediction Fracture behavior of AHSS is also affected by prestrain during manufacture, such as stamping Understanding the manufacturing effect on AHSS fracture is important Current research activities in that area: Non-linear strain paths project, funded by DOE / ASP, is focusing on in-plane pre-strain modes. MIT industrial fracture consortium (2 nd phase) is investigating some pre-strain modes (e.g., punching) However, the most common mode, bending-unbendingstretching, is not covered by either of them. 3

4 Purpose of Present Work To fill the knowledge gaps concerning manufacturing effect on AHSS fracture Investigate the bending-unbending-stretching effects on AHSS fracture. 4

5 Experiment Hat-section channels were stamped (drawing mode) Side wall undergoing bending-unbendingstretch Flange as-received condition Specimens were cut from the side walls and flanges Subsize tensile Mini three-point bending Notched tensile machined Notched tensile punched L direction 5

6 AHSS Grades Tested Material Coating Microstructure YS (MPa ) TS (MPa) UE (%) TE (%) DP780 Uncoated F+M TRIP780 GA F+B+RA Nominal thickness: 1.5 mm 6

7 Channel Draw: Determining Process Conditions Objective: Minimize side wall curl and avoid fracture Variables: die radii, punch radii and binder pressure Final process conditions: Strip size: 508 mm x 100 mm Draw distance: mm Channel width: 80 mm Punch radius: 2t Die radius: 2t Binder pressure: kn for DP kn for TRIP780 Drawing directions: L 7

8 Manufacturing Strain Estimation Outer fiber strain history Punch side Tension through bending Compression through unbending Tension through stretching Die Punch Stretch Die side Compression through bending Tension through unbending Tension through stretching Effective plastic strain due to bendingunbending: = 2 * t/r ~ 0.4 eff. pl Stretch was measured by thinning strain (from ultrasonic thickness gauge measurement) 8

9 Manufacturing Strain Material Locations Thickness Thinning / Stretch Strain eff. pl Due to B-U DP780 Uncoated TRIP780 GA flange side wall ~ 0.4 flange side wall ~ 0.4 Average of all channel parts 9

10 Subsize Tensile Test: Specimen Specimen configuration: 25 mm gage length, 6 mm width (ASTM-E8/E8M -09) 3 replicates each 10

11 Subsize Tensile Test: Stress-Strain Curves DP780 TRIP780 DP780 TRIP780 Wall Flange 11

12 Subsize Tensile Test: Stress-Strain Curves (2) The true stress/strain curves were shifted by the amount of stretch strain 12

13 Summary of Subsize Tensile Test Results Grades Locations Thinning /Stretch Prestrain eff. pl due to B-U YS (MPa) TS (MPa) UE TE Fract. Strain DP780 Uncoated TRIP780 GA flange side wall ~ flange side wall ~ There is no effect of bending/unbending on fracture strain The fracture is affected by stretch strength 13

14 Mini Three-Point Bending Test Specimen: 50 mm x 40 mm Bending direction: Outer fiber on punch side Bending-unbending-stretchingbending Testing setup: Punch blade radius: 1*t Roller diameter: 30mm Gap between rollers: 3*t Test stopped at 5% load drop Microcracks were observed Fracture strain measured by 2mm grids 14

15 Mini Three-Point Bending Test: F-D curves TRIP780 DP780 Flange Wall 15

16 Mini Three-Point Bending Test: Fracture Strain Grades DP780 Uncoated TRIP780 GA Locations Thinning Strain Prestrain eff. pl Due to B-U Surface Fracture Strain flange side wall ~ flange side wall ~ Bending-unbending effect: not significant Stretching effect: significant Fracture is controlled by total strain 16

17 Machined Notch Tensile Test Specimen configuration Notch diameter: 20 mm Effective width at the notch: 20 mm Machined Fracture strain was measured by DIC system (15 fps) 2 mm gauge length 1 mm away from hole edge 17

18 Machined Tensile Test: F-D Curves Peak load similar due to thinning Nominal stress increases due to hardening Displacement decreases after prestrain 18

19 Machined Notch Tensile Test: Fracture Strain Grades DP780 Uncoated TRIP780 GA Locations Thinning / Stretch Prestrain eff. pl due to B-U Fracture Strain flange side wall ~ flange side wall ~ Bending-unbending effect: not sensitive Stretching effect: significant 19

20 Punched Notch Tensile Test Specimen configuration Notch diameter: 20 mm Width of notch center: 20 mm Punched with a clearance of 0.1 t Fracture strain was measured by DIC (15 fps) Gage length: 2mm Measured 1.0 mm away from punch edge 20

21 Punched Notch Tensile Test: F-D Curves Peak load increase after prestrain not significant due to thinning Nominal stress increases after prestrain Displacement decreases after prestrain 21

22 Punched Notch Tensile Test: Fracture Strain Grades DP780 Uncoated TRIP780 GA Locations Thinning / Stretch Pre Strain eff. pl Due to B-U Fracture Strain flange side wall ~ flange side wall ~ Bending-unbending effect: not sensitive Stretching effect: sensitivity is reduced by punch damage 22

23 Summary: Average Fracture Strain Uniaxial Bending Machined Punched Pre Stretch Fracture Strain Pre Stretch Fracture Strain Pre Stretch Fracture Strain Pre Stretch Fractur e Strain DP780 Uncoated F SW TRIP780 GA F SW

24 Discussion: Fracture Strain Bending fracture strain is larger than in-plane test due to strain gradient The effect of in-plane prestrain is significant for uniaxial tension, bending and machined notch specimens. However, the effect is reduced for punched notched specimen due to punch damage. Effective Plastic Strain is not a valid index for fracture of AHSS when bending-unbending is involved, Total Strain may be a better parameter 24

25 Conclusions Manufacturing effects (bending-unbending-stretching) on AHSS fracture were investigated for DP780 Uncoated and TRIP780 GA and the following conclusions were drawn: Bending-unbending does not have significant effect on fracture behavior Effective plastic strain is not a good index for fracture under bending-unbending mode Total strain is a more appropriate parameter to characterize fracture For punched edges, prestrain effect is significantly reduced by punch-induced damages Manufacturing effects must be considered in AHSS fracture prediction of crash events 25

26 Future Work Needed Testing: develop industrial standard testing procedures to characterize manufacturing effect FEA simulations: re-evaluate and develop simulation methodologies (e.g., material models and fracture criteria) for chain simulations to incorporate manufacturing effects into structure analysis (e.g. crash simulation) 26

27 Acknowledgments Dr. L. Greve at Volkswagen AG Mr. S. Malcolm at Honda R & D Americas Drs. B. Yan and Blake Zuidema at ArcelorMittal Global R & D Mr. G. Girman and Mr. G. Volk at ArcelorMittal Global R & D 27

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