A simple and efficient reformulation of the classical manson-coffin curve to predict lifetime under multiaxial fatigue loading. Part I: plain materials.

This paper summarises an attempt to devise an engineering method suitable for predicting fatigue lifetime of metallic materials subjected to both proportional and non-proportional multiaxial cyclic loading. The proposed approach takes as a starting point the assumption that the plane experiencing the maximum shear strain amplitude (the so-called “critical plane”) is coincident with the micro/meso-crack initiation plane. In order to correctly account for the presence of both non-zero mean stresses and non-zero out-of-phase angles, the degree of multiaxiality/non-proportionality of the stress state damaging crack initiation sites is suggested here to be evaluated in terms of the ratio between maximum normal stress and shear stress amplitude relative to the critical plane. Such a ratio is used then to define non-conventional Manson-Coffin curves, whose calibration is done through two strain-life curves generated under fully-reversed uniaxial and under fully-reversed torsional fatigue loading, respectively. The accuracy and reliability of our approach was systematically checked by using approximately 350 experimental data taken from the technical literature and generated by testing 13 different materials under both in-phase and out-of-phase loading. Moreover, the accuracy of our criterion in estimating lifetime in the presence of non-zero mean stresses was also investigated. Such an extensive validation exercise allowed us to prove that the fatigue life estimation technique formalised in the present paper is a reliable tool capable of correctly evaluating fatigue damage in engineering materials subjected to multiaxial cyclic loading paths.