Micromechanical modeling of damage mechanisms in dual-phase steel under different stress states

In this paper, the fracture behavior and micro-damage evolution in DP600 and DP980 steels were studied using experimental and numerical methods. First, four specimens with different loading conditions were tested to investigate the influence of the stress state on the fracture behavior and in-situ tensile tests were carried out in order to evaluate damage evolution in the two steels. Afterwards, 3D RVEs based on random martensite phase distribution were generated for both materials and a VUMAT subroutine was utilized to include the Modified Mohr-Coulomb (MMC) damage model in the ferrite phase and predict the macroscopic fracture strain under complex loading conditions. Finally, damage mechanism in the RVE was compared to the in-situ test. It was observed that damage initiation mechanism in DP steels is dependent on the size of ferrite phases. In DP steels with large ferrite phases, strain localization in the middle of the phase caused damage initiation, whereas for steels with smaller ferrite grains, such as DP980, strain localization in the boundary of two phases is the dominant damage initiation mechanism. Furthermore, damage occurred by formation of voids, initiation of micro-cracks near the voids, and propagation and coalescence of these micro-cracks. Also, the response surface methodology can be used to calibrate parameters of the MMC damage model and the resulting FE model can accurately predict the stress-strain curve and fracture strain for all considered loading conditions, except for the shear loading condition. Finally, the proposed micromechanical FE model can be used to predict the same damage mechanisms as the in-situ tensile test.