Microstructure-informed crystal plasticity for HCF initiation life of HSS Q690

Abstract

High-strength steel (HSS) Q690 offers a superior strength-to-weight ratio for lightweight structural applications but remains susceptible to high-cycle fatigue (HCF) failure. Conventional design codes significantly underestimate its fatigue life, and the scarcity of test data hinders the development of reliable S–N curves. Meanwhile, full-scale fatigue testing is time-consuming and costly, necessitating efficient alternatives. This study proposed a novel microstructure-informed framework for predicting the HCF crack initiation life of HSS Q690, integrating electron backscatter diffraction (EBSD), crystal plasticity (CP) modelling, and a physically motivated fatigue indicator parameter (FIP). A new CP model was developed, incorporating two Armstrong-Frederick backstress terms and two-stage isotropic hardening, to uniquely capture the pronounced cyclic softening of HSS Q690, which is not addressed by existing CP models. EBSD-informed 3D representative volume element (RVE) models were used to simulate slip-band plasticity, while ensemble simulations quantified the scatter arising from microstructural variability. The framework was calibrated using stress relaxation and cyclic tests and validated against 39 HCF experiments under varying mean stress levels. The predicted fatigue lives and life distributions showed close agreement with experiments, demonstrating that this framework provides not only a cost-effective alternative to conventional fatigue testing but also new mechanistic insight into the fatigue behaviour and life scatter of HSS Q690.