Study on Metal Sheet Ductile Fracture using Square Punch Test 2018-01-0808

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. The four-parameter modified Mohr-Coulomb (MMC4) fracture locus [1] was converted to all-strain-based modified Mohr-Coulomb (eMMC) fracture locus and fracture forming limit diagram (FFLD) in the space of principal strains under plane stress condition. The eMMC and FFLD [2] can be calibrated directly from measured fracture strains at die and punch radius of the AHSS sheets in square punch tests, as well as from uniaxial tensile tests. For analyzing possible close to shear-induced fracture by different combinations of tension and compression at the die radius, the DP780 sheets were arranged at 45° with respect to the orientation of the square punch. The square punch tests were simulated in Abaqus/Explicit. The finite element results show that the force-displacement response of the punch, the fracture initiation and crack propagation at the critical locations in square punch tests were correctly predicted. The load path of the mid-surface at the die radius was found close to proportional loading of in-plane shear.