Experimental investigation of key assumptions in analytical failure criteria for sheet metal forming

Tearing concerns in sheet metal forming can be predicted based on the strain and stress in the material using analytical models, e.g., the Marciniak Kucyzinski (M-K) model and the Derov et al. model respectively. An assumption to these models is that a thin area of concentrated deformation exists which is referred to as the defect region. Other key assumptions for the models are related to when the material is predicted to fail. For the M-K model, the failure is related to the incremental strain ratio inside and outside the defect region. Similarly, for the Derov et al. model, the failure is related to a critical stress concentration factor, i.e., the ratio of the effective stress inside and outside the defect region. In order to investigate these key assumptions, Marciniak tests with coupled Digital Imaging Correlation (DIC) to measure the strain in the material as well as the size of the defect region were conducted on 1018 steel with eight specimen geometries, which varied the strain path from uniaxial to balanced biaxial. The results show that the parameters investigated to predict failure (i.e., the incremental strain ratio and critical stress concentration factor) were not constant for the various strain paths for both analytical models considered