A refined energy-based multiaxial fatigue rule up to crack initiation: a case study for an in-vessel component subjected to complex combined loads

Predicting crack initiation is crucial for ensuring the structural integrity of fusion reactor components, which operate under severe multiaxial thermomechanical loading. This study applies a multiaxial fatigue rule (MFR) to the EUROFER97 Shielding Liner cylindrical support of the DEMO divertor, adapting a procedure from the literature to the component’s specific geometry and modelling requirements. The baseline MFR approach was first evaluated against fatigue life requirements, showing conservative predictions. To improve accuracy under plastic deformation, a modified formulation based on the total accumulated strain energy density amplitude was introduced, replacing the original LMS energy parameter that underestimates damage in low-cycle fatigue regimes. The modified approach provided less conservative predictions, though further experimental validation and broader application are needed to establish robustness. In addition, a preliminary investigation on design margins revealed that applying constant margins at the strain level leads to non-uniform margins in the energy density space, raising the question of the most consistent stage for margin application. Overall, this work presents a promising energy-based methodology for multiaxial fatigue life prediction in fusion components and outlines key aspects for future refinement toward reliable fatigue design protocols.