Local ductility in steel sheet, towards a practical and robust measurement method

Local ductility has become a useful and significant mechanical property of sheet metal, particularly for advanced-high-strength-steels (AHSS) [1, 2]. The widely accepted hole expansion test (HET) [3] has two problems: It mixes up the effects of shear cutting with the intrinsic material ductility, and the variance of test results is large. As a complementary approach, a workgroup consisting of universities, steel producers, automotive suppliers and OEMs have developed VDA-guideline 238-110 [4] for industrial application of the TTS (True Thickness Strain at fracture at minimum thickness) and TFS (True Fracture Strain from area reduction), to quantify local ductility based on the postmortem microscope analysis of the fracture surface of tensile test samples. In 18 laboratories, more than 1440 tensile tests have been performed on a sample batch of 3 AHSS grades (CR440Y780T-DH, CR780Y980T-CP, and HR660Y760T-CP). Using ASTM, JIS and ISO tensile specimens, each laboratory determined the conventional mechanical properties as well as TTS and TFS. For comparison HET were also performed. The measurement results were analyzed statistically in terms of gauge repeatability and reproducibility (GRR). Measurement variance for TTS is comparable to elongation at breakage and significantly lower than HET. TTS results clearly show superior local ductility of the -CP steels over -DH. An analysis of fracture morphology shows differences between the hot-rolled and cold-rolled -CP grades and a tendency for the -DH steel to fail at the edge. For -CP steels, TTS and TFS results are indistinguishable, for the -DH grade, the TFS value is slightly higher than TTS, likely a side-effect of higher global ductility.