As a result of heterogeneities associated with their two-phase microstructure, dual-phase (DP) steels reveal various damage mechanisms, leading to the nucleation of voids, microcracks, and other defects at all stages of deformation. It has been reported that the evolution of the damage and fracture is highly dependent on the applied boundary conditions. Here, we identify the micromechanics of damage in DP steels under different stress states and investigate its correlation with the microstructural factors, such as volume fractions and mechanical phase contrast (strength difference between phases). To model the responses, numerical simulations with RVE analyses are carried out, where the crystallography of both phases, as well as the substructure of martensite, are taken into account. We incorporate a triaxiality and Lode angle-sensitive damage model to predict the local damage and subsequent global fracture. Through material and numerical assumptions, the effect of triaxiality and Lode angle is studied independently of each other. \\
The results show that the critical fracture strain in DP steels exhibits a strong dependence on both the stress triaxiality and the Lode parameter. The ductile fracture initiation locus is extracted for DP steels with different volume fractions. The fracture locus is a non-symmetric convex function of the Lode angle parameter, and a monotonically decreasing function of the stress triaxiality. Under an imposed zero triaxiality, the lowest fracture strain is observed for a shear-dominated stress state, where the Lode angle is zero. An increase in martensite volume fraction leads to more plastic strain and damage in ferrite regardless of each stress state. Higher stress-strain partitioning and hence significant damage accumulation is observed for higher mechanical phase contrast. The results of this study provide more accurate insight into the distinct effect of stress state and microstructure related parameters.