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The risk of skid lines for Class A panels has to be assessed before releasing the die development for hard tooling. Criteria are needed to predict skid lines in the formability evaluation stage to avoid expensive changes to tooling and process for resolving skid line issue in production. In this study, criteria using three different measured parameters were developed and validated. A draw-stretch-draw (DSD) test procedure was developed to generate skid lines on lab samples for the physical evaluation. This was done using tooling with various die entry radii and different draw beads. The skid line severity of lab samples was rated by specialists in the inspection of automotive outer panel surface quality. The skid line rating was correlated with geometric measurements of the lab samples after the DSD test. The sensitivity of the appearance of skid lines to tooling and process parameter variations was identified.

This study introduces a new practical calibration approach of ductile fracture models by performing square punch tests on metal sheets. During square punch tests, ductile fracture occurs at either the corner of die or punch radius when applying different clamping loads and lubrication conditions. At the corner of die radius, in-plane pure shear is induced at the intersection between the side-walls and the flange by combined tension and compression. On the other hand, the material at the corner of the punch radius is under combined bending and biaxial tension. The material studied in this paper is advanced high strength steel (AHSS) DP780 from ArcelorMittal. Isotropic J2 plasticity model with mixed Swift-Voce hardening rule is calibrated from uniaxial tensile tests.

To aid in understanding die wear when stamping AHSS, a study to characterize the contact pressure distribution in drawbeads during stamping had been undertaken. As direct measurement of contact pressure for a drawbead is not feasible during metal flow, a combination of experimental and Finite Element (FE) simulation techniques were used to determine the contact pressure distributions and the maximum contact pressure for a number of different conditions. Testing was conducted using the Drawbead Simulator (DBS) for two different bead configurations. The materials in this investigation were 0.7mm and 0.8mm EG BH210 and EG DP500. Static Implicit FE analyses were conducted with ABAQUS Standard using 2D plane strain continuum elements. A combined hardening model in conjunction with strain rate effects was used to describe material behavior as it flows through the drawbeads.