Multiaxial fatigue life response depending on proportionality grade between normal and shear strains/stresses and material ductility

Abstract Current experiences show that a non-proportional loading of ductile materials such as wrought steels, wrought aluminium or wrought magnesium alloys, not welded or welded, causes a significant fatigue life reduction under an out-of-phase shear strain or shear stress superimposed on a normal strain or normal stress compared with proportional in-phase loading. But when ductility, here characterised by tensile elongation, is reduced by a heat treatment or by another manufacturing technology such as casting or sintering, the afore-mentioned life reduction is compensated or even inversed, i.e. longer fatigue life results compared with proportional loading. However, some materials reveal even despite good elongation values a neutral behaviour or a prolongation of life. Responsible for these observations are not only the overall material ductilities or matrix-ductilities, but also the interaction between matrix and microstructural features such as manufacturing-dependent nodules, pores, internal defects like microporosities. They cause by local constraints a hindrance of deformations despite good overall ductilities. This complex life behaviour under non-proportional loading cannot always be estimated. Therefore, not only in strength hypotheses, but also in experimental proofs of multiaxial loaded parts, especially safety-critical components or structures, with real or service-like signals, emphasis must be placed on retaining non-proportionalities between loads and stresses/strains, respectively.