A unified constitutive modelling for predicting low cycle behaviour and fatigue life of Eurofer97 under coupled temperature and neutron irradiation effects

Reduced Activation Ferritic/Martensitic (RAFM) steel, like Eurofer97, is a reference structural material for fusion reactors (ITER, DEMO), where extreme conditions (irradiation, high temperatures, complex loads) cause Low Cycle Fatigue (LCF). Accurate LCF prediction is vital for safety but challenged by coupled temperature, irradiation, and specimen size effects, often isolated in current models. A unified model integrating these within a Continuum Damage Mechanics (CDM) model is needed. This study presents a validated unified viscoplastic damage model, implemented in ABAQUS as a user-defined material subroutine (UMAT), integrating an improved CDM approach into a modified Chaboche model. It includes temperature dependent parameters for kinematic and isotropic hardening, a specimen size correction, and a temperature and irradiation dose dependent irradiation hardening model, all validated against extensive Eurofer97 LCF data (Room Temperature (RT)–450 °C, various strains, sizes, irradiated dose). The model accurately simulated Eurofer97's cyclic behaviour (two stage softening) and predicted LCF reduction from size effects. Crucially, it captured the complex temperature dependent effect of neutron irradiation at 16.3 displacements per atom (dpa) on LCF lifetime (e.g., extension at 250 °C, negligible at 450 °C), matching defect mechanisms. LCF lifetime predictions for combined effects were generally within a factor of 1.5–2.0 of experimental data, with interpolated parameters useful for uncalibrated temperatures. This research offers a validated tool for predicting Eurofer97's LCF under combined influences, aiding reactor design, safety, and advancing constitutive modelling.