Constitutive modeling of metastable austenitic stainless steel at cryogenic temperatures: A secant method approach considering stress triaxiality

Metastable austenitic stainless steel is a primary candidate material for large-scale liquefied hydrogen storage tanks essential for achieving carbon neutrality. At cryogenic temperatures, this material undergoes strain-induced martensitic transformation, a phenomenon known to be highly dependent on the stress field (specifically, stress triaxiality). However, conventional elastoplastic finite element method (FEM) analyses often fail to fully incorporate this dependency, potentially compromising safety evaluations. This study presents a novel FEM analysis technique that integrates the secant method—a homogenization scheme—to model the constitutive behavior of the evolving dual-phase steel. To calibrate and validate the model, in situ neutron diffraction experiments were conducted during tensile deformation at 20 K, determining the stress–strain relationships and volume fractions for each phase. The proposed method demonstrates superior accuracy compared to previous methods, particularly in predicting the localized stress field near crack tips, thereby enabling more rigorous fracture mechanics assessments. Finally, the proposed model was applied to simulate fracture toughness tests. The results demonstrated that while the global relationship between the J-integral and crack tip opening displacement (CTOD) showed minimal deviation from conventional analysis, the local stress field was significantly elevated due to transformation-induced hardening driven by high stress triaxiality at the crack tip.