Experimental characterization and constitutive modelling of the ratcheting behaviour of EUROFER97/3 steel at high temperatures

EUROFER97/3 is the current reference reduced-activation ferriticmartensitic (RAF/M) steel for European fusion reactor design and features improved chemical purity and δ-ferrite control compared with earlier EUROFER97 batches. In fusion reactors, structural components are subjected to complex cyclic thermomechanical loading with non-zero mean stresses arising from start-up and shut-down transients, temperature gradients, and plasma-induced fluctuations. Under such conditions, ratcheting may occur, leading to progressive strain accumulation and excessive deformation that can reduce component lifetime. Existing experimental data and constitutive descriptions for earlier EUROFER97 batches are therefore not directly applicable. In this study, the ratcheting behaviour of EUROFER97/3 is investigated under stress-controlled cyclic loading at 450 °C and 550 °C. Uniaxial tensioncompression tests are performed over a range of peak stresses, stress ratios (R), and stress rates. The results show that ratcheting strain accumulates along the direction of mean stress and increases with increasing peak stress and more negative value of R, with a maximum near R = −0.98. ... mehrAt 550 °C, EUROFER97/3 exhibits a weaker ratcheting behaviour to stress rate compared with EUROFER97/2. The experimental data are used to calibrate a viscoplastic constitutive model based on a modified Armstrong–Frederick kinematic hardening framework. A Sobol’ global sensitivity analysis is further conducted to quantify the relative influence of loading parameters on ratcheting response. This work provides new experimental data and a validated modelling framework for predicting the ratcheting behaviour of EUROFER97/3 under fusion-relevant conditions.