High-cycle fatigue of AISI 304 stainless steel under multiaxial non-proportional bending–torsion loading: damage behavior and stress-parameter-based life evaluation

• Filling the data gap under multiaxial high-cycle fatigue loading. • Revealing distinct fatigue failure mechanisms under uniaxial and multiaxial loading. • Proposing an effective life evaluation method for multiaxial high-cycle fatigue. High-cycle fatigue (HCF) tests were performed using AISI 304 stainless steel under bending (BT) and multiaxial combined bending–torsion (CT) loading using a self-designed compact rotating-weight fatigue testing machine. Fifteen tests covering a wide stress amplitude range were conducted, complemented by in-situ surface crack monitoring and post-experimental microstructure observation. The multiaxial non-proportional loading produced a pronounced debit in fatigue life relative to uniaxial loading and promoted multiple crack initiation sites. Microstructural analyses revealed enhanced local lattice misorientation and more heterogeneous intragranular orientation development under non-proportional loading, while no deformation induced martensitic transformation was detected. To improve life evaluation in the HCF regime, a stress-parameter-based life evaluation method was developed by extending the Itoh–Sakane method through the introduction of an additional material parameter that quantifies non-proportional sensitivity in the elastic term. The proposed approach predicted both BT and CT lives within a factor-of-two scatter band, outperforming Basquin evaluation and the original Itoh–Sakane method.