Refinement MRP-135 constitutive equation model for tensile properties of austenitic stainless steel

This study focuses on analyzing and enhancing MRP-135 constitutive models, which are crucial for assessing the tensile properties of irradiated stainless steels in the reactor vessel internals of pressurized water reactors. The newly developed model, named M1, incorporates parameters from MRP-135, while minimizing modifications to ensure high compatibility with existing models. A significant improvement in M1 is the resolution of the inversion phenomenon observed between the yield strength and ultimate tensile strength at low temperatures in Type 316 stainless steel, a challenge previously identified in MRP-135 Rev. 2. M1 integrates the temperature effects from the earlier version, MRP-135 Rev. 1, to address this issue effectively. Additionally, for Type 316, M1 refined the equations to represent the cold work factor more accurately, enabling precise interpolation of strength variations across different levels of cold work from solution-annealed to cold-worked conditions. A comparative analysis of the M1 and the MRP-135 models against actual irradiated tensile test data for Type 304 and 316 stainless steel demonstrated the superior accuracy of M1, as indicated by a lower root-mean-square deviation. This study highlights the importance of continuous improvements in constitutive models, with future work aimed at further refining these models based on comprehensive irradiation test data.