A multiaxial fatigue life criterion for non-symmetrical and non-proportional elasto-plastic deformation

A new low-cycle multiaxial fatigue life prediction methodology based on the concept of an effective shear strain is proposed. This effective shear strain is derived by averaging the total shear strains acting on all planes passing through a material point. The proposed model, which is formulated as a generalised equivalent strain, takes into account the effect of non-symmetrical loading cycles. The main advantage of the model relies on the small number of material parameters to be identified. The axial cyclic stress-strain curve, the basic strain-life curve (Manson-Coffin) and an additional life curve obtained under zero to tension strain controlled axial fatigue tests are sufficient to allow application of the proposed criterion in all loading conditions. The experimentally observed fatigue lives of proportional and non-proportional multiaxial strain controlled low-cycle fatigue tests from un-notched tubular specimens, have been compared with the predicted lives of the proposed approach showing in all cases a good agreement.