An Energetic Consideration of Fatigue Mechanism of Metal under Mean Stress

Abstract

It was observed that the propagation rate of fatigue crack is faster under the presence of tensile mean stress. The purpose of the present study is to give a theoretical basis of the observed characteristics of fatigue crack. At first, the energies of a crack are evaluated, the elastic energy, the surface energy, the interaction energy with external stress field and the accumulated strain energy with stress cycles. Then, the energetic conditions of crack growth are discussed. It is suggested that the process of crack propagation is devided into nearly three stages, according to the size of crack, that is, the initiation, the growth of dislocation-type crack and the propagation of semi-macroscopic crack. It follows that, the mean stress have influence on every stages, which is approximately expressed in the form pV, where p and V are the hydrostatic component of stress and the volume of crack, respectively. Various other characteristics of fatigue crack are also discussed based on the energetic consideration.