Competing failure modes in hole expansion testing of AHSS sheets

To identify limit edge strains of AHSS sheets, a multitude of parameter identification tests can be conducted. Among these, hole expansion tests (HET) using different punch geometries are most common. Herein, sheets of two AHSS grades were tested in hole expansion tests using a conical and flat punch. Different edge conditions were tested, including sheared edges with varying cutting clearances as well as worn cutting tools, pre-pierced holes and milled holes. Competing failure mechanisms could be observed for these two AHSS grades. For DP800, hole expansion ratio (HER) values of conical and flat HET were almost identical for different cutting clearances, indicating the edge crack susceptibility of this grade regardless of the exact strain gradient in the hole vicinity due to different punch geometries. HET for CP1000 revealed failure at considerably lower HER values for the flat punch than for conical punch, and the flat punch HERs were independent of the edge condition. However, closer inspection of the specimens during the test reveal that all flat HET samples of CP1000 fail from necking in the bulk material, rendering these tests invalid in terms of edge limit strain identification. In such cases, the forming limit curve is sufficient to describe failure.