Ductile Damage in Tension and Bending for DP980 Steel Sheets

Abstract

The mechanical behavior of DP980 steel sheets of 1.7 mm thickness has been investigated with both tensile and bending tests. Free bending tests were performed on square samples of 60mm side. The bending tool has a sharp radius of around 0.4 mm and the sample simply lies on two rollers. Scanning electron micrography observations were performed in order to check the occurrence of cracks, that indicate the onset of rupture in bending. Moreover, X-ray microtomography observations were performed on smooth and notched tensile specimen, with a specific small-size geometry, and bending specimen. Maximum void volume fractions of 1.5 10-3 were recorded and the influence of the triaxiality ratio was investigated, by changing the notch radius. In the case of bending, samples were cut in the bent area and void volume fraction distribution was analyzed along the sheet thickness. Material parameters for Gurson-Tvergaard-Needleman (GTN) model, associated with isotropic hardening and von Mises yield criterion, were identified from the tensile tests. Inverse identification was performed over the different sample geometries, showing that GTN model can not capture the triaxiality ratio influence. Finite element simulations of the bending test were then carried out, in order to compare experimental and predicted void volume fractions in the sheet thickness.