An experimental evaluation on rate sensitivity of fracture toughness in sheet specimens made of SUS304 metastable austenitic stainless steel

Utilizing transformation-induced plasticity (TRIP) steel with strain-induced martensitic transformation (SIMT) on automotive steel plates and developing products which have exceptional energy absorption capabilities can be used to reduce weight. For this purpose, it is important to clarify the fracture toughness of TRIP steels and its strain rate dependency to ensure the use of impact-absorbing members over a wide range of deformation rates. However, inconsistent rate dependent trends were observed in the previous studies, possibly as a result of the transition of the fracture mode. Therefore, the effect of fracture modes and strain rate on the fracture toughness values needs to be examined sufficiently. In the present study, the fracture toughness of the specimens made of SUS304 metastable austenitic stainless steel, which is a kind of TRIP steel, is evaluated by tensile tests at various strain rates. Direct current potential difference and digital image correlation methods are introduced to detect the time of fracture initiation during deformation, especially at high deformation rates by using the developed impact tensile tests based on the split Hopkinson pressure bar. The fracture surfaces of the fractured specimens are observed by using an electron scanning electron microscopy to determine not only the fracture modes and their rate dependent effects but also the reinforcement mechanism of SIMT in mixed modes. The results demonstrate that fracture toughness shows negative strain rate dependency, and mixedmode fracture can be found near the crack in the case of the impact tensile test.