Observation of multi-scale damage evolution in transformation-induced plasticity steel under bending condition

Transformation-induced plasticity (TRIP) steel is a representative first-generation multi-phase advanced high strength steel (AHSS) having superior mechanical properties mainly attributed to phase transformation during plastic deformation. AHSS, such as TRIP steel, exhibits an excellent strength-ductility combination compared to conventional carbon steels and high strength and low alloy grades, but is relatively inferior in formability. In particular, in multi-phase AHSS, microscopic damage occurs by strain partitioning at the heterogeneous phase interfaces, propagates, and leads to bulk-scale cracks. In this study, the damage behavior of TRIP steel was investigated in multi-scale by combining macro-scale digital image correlation (DIC) and micro-scale DIC. We analyzed the initiation region and propagation path of cracks in TRIP steel and demonstrated that microscopic damage can occur even if no macroscopic damage or load drop is found. Therefore, it is essential to analyze the damage and cracks in multiple scales when forming the developed material for user safety.