In-situ three-dimensional investigation on micro ductile fracture mechanism of mild steel

Abstract

In this paper, in-situ micro-computed tomography (micro-CT) experiments were performed on mild steel samples under monotonic tension using X-ray microscopy. A three-dimensional (3D) visualization model was reconstructed and the characteristics of void evolution under different strain states, such as void distribution, void shape, void growth and void coalescence, were quantitatively analyzed from a 3D perspective. It is found that void growth mainly depends on the stress state, while void coalescence is controlled by the stress state and intervoid spacing. The critical intervoid spacing for void coalescence occurring at the post-necking stage was given. Two stages of void coalescence, local coalescence and global coalescence, were found during void evolution. Local void coalescence was first observed at a strain of 10.02%, well before necking, but has a negligible effect on the overall void evolution. Global void coalescence was developed after the formation of a strain localization band dominated by large coalesced voids in the necking region. The propagation of fine cracks between voids along the strain localization band leads to the formation of macroscopic ductile cracks. Overall, the micro ductile fracture of mild steel is triggered by the combined effects of multiple damage mechanisms rather than independent competition.