Role of geometrically necessary dislocation density in multiaxial and non-proportional fatigue crack nucleation

Experimental and crystal plasticity modelling studies have been carried out to investigate non-proportionality 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 non-proportional 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.