A stress invariant based criterion to estimate fatigue damage under multiaxial loading

This paper summarises an attempt to propose a novel stress invariant based approach suitable for estimating fatigue damage in the presence of complex multiaxial fatigue loadings. In more detail, according to the devised method, fatigue damage is evaluated by studying the components of the deviator path projected along the axes of a convenient frame of reference, whose orientation univocally depends on the profile of the applied loading path. The rainflow counting method used in conjunction with the so-called modified Wöhler curve method is subsequently employed to estimate the number of cycles to failure in the medium-cycle fatigue regime. The accuracy of the proposed approach was systematically checked by means of several experimental data taken from the literature and generated by testing different metallic materials under both in-phase and out-of-phase biaxial fatigue loading. This exercise allowed us to prove that the systematic application of the proposed approach resulted in highly accurate predictions and it held true independently of degree of multiaxiality of the stress field damaging the fatigue process zone.