Prediction and Reduction of Edge Cracking in Forming Advanced High Strength Steels

• Difficult to reliably evaluate edge cracking as well as to accurately predict.

Hyunok Kim, EWI-FC
Anoop Samant, KTH Parts Ind.

DESIGN OF THE NEW HOLE
EXPANSION TESTING METHOD

• A new hole expansion ratio (HER) testing method was developed to evaluate edge cracking with AHSS.
• Testing method includes both punching and hole expansion tests.

• Determination of the more effective hole expansion testing procedure and tooling design that are:
− Sensitive to the hole-edge quality and material properties
− Easy to measure or capture edge cracking
− Capable to obtain consistent test results for the HER.

• Three key variables—force, die stroke, and HER—are compared when maximum thinning reached 6% for DP980.

A 75-mm hole gives a more distinguishable change of the hole diameter and punch stroke than a 10-mm hole.

• At least five samples were tested in each condition.
• Hole diameter: 75 mm (≈ 3 in.).

• New hole expansion testing gave up to 11.4% (8.5 mm) difference between DP980 and TRIP780 samples (with the punched hole edge and a dome punch).
• The ISO standard HER test (with 10-mm hole size) gave only 0.8% (0.8 mm) difference between DP980 and TRIP780 samples (with the punched hole edge conditions).

Hyunok Kim, EWI-FC
Jim Dykeman, Honda R&D

HALF SPECIMEN DOME TEST (HSDT)

• High impact of cutting method on test results.

• The HSDT results between Honda and EWI showed good correlations.
• EWI used a 6-in. diameter dome punch, and Honda used a 4-in. diameter dome punch.

• EWI evaluated three failure criteria to predict edge cracking that was obtained from the hole expansion and half-dome stretching tests:

1. Maximum thinning
2. Maximum principal strain
3. Modified FLD for edge cracking.
• These failure criteria were applied to FE simulations to predict the edge cracking.

Sheared edges gave about 50% reduction in the maximum thinning compared to the water-jet edges.

Anoop Samant, Kevin Casanova, KTH Parts Ind.

Jianhui Shang and Hyunok Kim, EWI-FC

REDUCTION OF EDGE CRACKING WITH
INDUSTRIAL 

Simulation results for TRIP780 with 60 Degree Bend Block and 70-mm Flange

• As the flange length increased from 20 to 70 mm, failures occurred, and the failure type changed from edge cracking to ductile cracking.

• A 3-in. diameter HER testing showed about 10⨯ distinguishable results of different materials and trimming methods compared to the ISO standard HER testing method (i.e. about 10% vs. less than 1% HER).

• HSDT is a simple testing method to evaluate edge cracking with less effort in preparing the samples and easily comparing the maximum die stroke at the onset of the crack initiation compared to the HER test.

• The maximum thinning-based failure criteria is recommended for use in industrial stamping simulations.

• The maximum thinning-based failure criteria reasonably predicted the edge cracking with TRIP780 samples with the KTH die.

• TRIP780 showed the largest change in edge cracking for different trimming conditions (i.e. water-jet vs. shearing).

• TRIP780 and DP980 are susceptible to edge cracking. #GDIS | #SteelMatters  34  Acknowledgements

• This project was conducted with support from the Ohio Development of Services Agency (ODSA) on Advanced Manufacturing Program (Grant No. TECG20141159) on “Implementation of Advanced Formability Testing Methods for Lightweight Vehicle Structures.”