Multiaxial fatigue: An overview and some approximation models for life estimation

A brief overview of some important issues in multiaxial fatigue and life estimation is presented. These include damage mechanisms and damage quantification parameters, material constitutive response and non-proportional hardening, cycle counting and damage accumulation in variable amplitude loading, and mixed-mode crack growth. It is shown that capturing the correct damage mechanism is essential to develop a proper damage quantification parameter for robust multiaxial fatigue life estimation. Additional cyclic hardening of some materials under non-proportional multiaxial loading and its dependence on the load path as well as material microstructure is also discussed. It is argued that critical plane damage models with both stress and strain terms are most appropriate since they can reflect the material constitutive response under non-proportional loading. Importance of a proper cycle counting method to identify cycles in a variable amplitude load history, and capability of the linear cumulative damage rule to sum damage from the counted cycles are also discussed. As mixed-mode crack growth can constitute a significant portion of the total fatigue life, analysis of crack growth rates and correlations under combined stresses is presented. Several models as well as some simple approximations in capturing the aforementioned effects in multiaxial fatigue life estimations are also described. The approximation models include an estimation model for obtaining material non-proportional cyclic hardening coefficient, and a simple multiaxial fatigue life estimation model for steels based on hardness as the only required material property.