This paper presents a unified approach to fatigue damage in metals based on the theory of damage mechanics. The theory that takes into account the gradual material degradation or deterioration under load is ideally suited for characterizing material behaviors progressively damaged under fatigue loading. This is because fatigue damage is caused by material degradation resulting from the initiation, growth, and coalescence of microcracks/voids in real-life materials under repeated/cyclic loading. In addition, a unified fracture criterion based on the damage mechanics theory has been developed to predict the threshold conditions of macrocrack initiation and propagation as well as damage evolution in a material element with or without the presence of a macrocrack. This type of unified approach would not otherwise be possible using the conventional fatigue design methodology based on either the S-N diagram or the concept of fracture mechanics.
The proposed fatigue damage model is based on the thermodynamic theory of irreversible processes with internal state variables. With the introduction of a new damage effect tensor, the necessary constitutive equations of elasticity and plasticity coupled with damage are developed. The constitutive equations derived enable the formulation of a fatigue damage dissipative potential function and a fatigue damage criterion. The criterion is designed to subdivide the overall damage into two domains, namely, “fatigue damage” and “plastic damage.” The fatigue damage evolution equation is subsequently developed based on the hypothesis that the overall damage is induced by the summation of “fatigue” and “plastic” damages. The model has been applied to predict successfully the fatigue life of smooth, notched, and center-cracked specimens under a wide range of loading cycles.