Damage initiation and fracture loci for advanced high strength steel sheets taking into account anisotropic behaviour

Abstract

In this work, ductile failure loci for advanced high strength (AHS) steel sheet grade 780 and 1000 were determined by using a combined approach between experiments and FE simulations. Effects of anisotropic behaviour of the steels were considered. Tensile tests of sheet specimens with various geometries taken from the rolling, transverse and diagonal direction were performed. During the tests, the direct current potential drop (DCPD) method and digital image correlation (DIC) technique were applied for identifying damage initiation on the micro–scale and fracture occurrence of the steels, respectively, under different states of stress. Subsequently, FE simulations of the tensile tests were carried out and stress triaxialities, equivalent plastic strains and Lode angles were evaluated for the corresponding critical areas. Hereby, the von Mises, Hill’s 48 and Yld2000–2d yield criteria coupled with the Swift hardening law were applied. The threshold values within the triaxiality range of 0–0.577 obtained from different samples were plotted as the 3D failure loci of the examined steels. The predicted damage initiation states were also verified by SEM analysis. Then, influences of different material orientations and yield functions on the shape alteration of the determined failure locus were studied. To investigate formability of the steel sheets the damage initiation and fracture loci were transformed to strain based forming limit curves (FLCs). Additional limiting strains for the shear stress region were here incorporated. Finally, a non–symmetric rectangular cup test was conducted for the investigated steels until fracture. Then, plastic strain distributions, strain paths of the critical areas and achieved drawing depths were evaluated by the FLCs. It was found that the proposed FLCs could fairly predict fracture incidence of the formed parts under shear deformation. Moreover, damage initiations were predicted at about 70% of the final drawing depth.