Influence of shear stress on ductile failure initiation: a micromechanical analysis of strain localization

Shear stress is recognized as a critical factor influencing the ductile failure process. In the present work, we investigate ductile failure initiation under shear-dominated loading by identifying the onset of strain localization, which is predicted using a rigorous bifurcation analysis. Micromechanical computations are performed using cubic voided unit cells (UCs) subjected to fully periodic boundary conditions. Proportional stressing is imposed to keep constant stress triaxiality (T) and Lode parameter (L) throughout the deformation history of UCs. In the proportional stressing, two loading configurations are realized to comparatively study the effect of shear stress component for a given pair of T and L: normal stresses (without shear) versus normal stresses combined with one shear stress component. The effects of initial porosity and void configuration (pattern of voids arrangement) are also systematically analyzed. The results show that the influence of shear stress is strongly modulated by the amount of the initial porosity. While shear stress promotes strain localization at low initial porosity, it conversely delays it at high initial porosity within a specific range of Lode parameters. This porosity-dependent trend reversal is rationalized by a transition from a ligament-thinning-dominated failure mode to an intervoid-shearing-dominated failure mode as the stress state varies. Furthermore, for a given overall initial porosity, the void configuration significantly affects the critical strain locus, the extent of which depends on the imposed triaxiality. These findings provide valuable micromechanical insights for refining predictive modeling in engineering applications.