Tearing failure of ultra-thin sheet-metal involving size effect in blanking process: Analysis based on modified GTN model

Abstract

In this account, the ductile tearing behavior of a 0.08 mm thick ultra-thin martensitic stainless steel sheet was probed during the blanking process. The experiments suggested that void growth was suppressed around the narrow region due to the relatively low-stress triaxiality. The typical failure phenomena were exhibited in the form of tearing, which may imply that the conventional Gurson–Tvergaard–Needleman (GTN) model failed to predict such shearing domination failure. Therefore, a modified GTN model based on Lode parameter was tested to describe the failure mechanism. Furthermore, to reflect the remarkable material strengthening behavior at the micrometer scale, the Mechanism-based Strain Gradient (MSG) plasticity was implemented in the User MATerial (UMAT) subroutine of ABAQUS, and a finite element model of three-dimensional blanking processing was then built. The cohesive elements were inserted into the finite element mesh so that the tearing process could be visualized. The numerical results generated by the proposed model were compared with the experimental observations, as well as data from conventional plasticity model. The analysis revealed that shear damage rather than microvoids was the primary cause of tearing failure. The effects of strain gradient on distributions of stress level, void volume fraction and shear damage evolution were examined. It was concluded that size effect played a significant role in inducing the tearing failure, and the modified GTN model was able to capture shear damage evolution inside the shear region.