Damage mechanism analysis of a high-strength dual-phase steel sheet with optimized fracture samples for various stress states and loading rates

In this study, it is aimed to investigate the damage mechanisms of a DP1000 steel with very fine-grain microstructure and further study the effect of the stress states and loading rates on the damage mechanism. To ensure clear and systematic conclusions are drawn about the effect of stress state on the damage mechanism, careful study is performed on achieving a constant stress state of the fracture initiation spot during the entire deformation history by designing a fracture specimen family for various stress states. The specimens feature an identical dog-bone specimen layout with only differences at the detailed notches and cuts to ease the experimental and analysis effort. After testing the optimized fracture specimens from quasi-static to intermediate loading rates, the scanning electron microscopy is employed to analyze the damage mechanism of the investigated material. Both dimple and shear fracture modes are observed in DP1000 and the underlying damage mechanisms are ferrite-martensite interphase debonding and martensite cracking. By systemic analysis, it is found that the stress state and loading rate have distinct influences on triggering the dominant fracture mode and the underlying damage mechanism. Furthermore, the damage mechanism of DP1000 is compared with DP600 steel and significantly different behavior is observed for two types of dual-phase steels.