Experimental and crystal plasticity modelling studies have been carried out to investigate nonproportionality and stress state effects in fatigue in a 316 stainless steel and nickel-based
superalloy RR1000 which have substantial effects on fatigue life. Stored energy density has
provided a reasonably consistent and unifying explanation for the experimental observations
of fatigue life in axial, torsional, in-phase proportional tension and torsion, and nonproportional loading regimes. A single fatigue quantity (the critical stored energy density,
equating to new surface energy) has been shown to provide good qualitative and reasonable
quantitative prediction of the experimental observations of the complex loading, providing a
mechanistic explanation for the fatigue behaviour. For the case where significant densities of
GNDs develop (for the fine-grained nickel), the latter is found to differentiate the proportional
and non-proportional fatigue lives and its contribution to the local stored energy is crucial for
capturing the correct fatigue lives under the differing loadings.