Cyclic slip irreversibility and fatigue life: A microstructure-based analysis

The evolution of fatigue damage is intimately related to different types of more or less pronounced irreversible cyclic slip, associated with gradual permanent microstructural and topological changes which ultimately can cause fatigue failure. Only in a few cases has it been possible to assess quantitatively the cyclic slip irreversibility. Based on a limited amount of available experimental data amenable to analysis, an attempt is made in this work to explore whether there exists an explicit relationship between the cyclic slip irreversibility and fatigue life. The existence of such a relationship is suggested by a microstructure-based reformulation of the Coffin–Manson fatigue life law, which involves the cyclic slip irreversibility. The analysis of the experimental data in two adequately documented cases indicates that the cyclic slip irreversibility is related very satisfactorily to fatigue life via a power law which contains two constants. These two constants can be expressed in terms of the fatigue ductility coefficient and the fatigue ductility exponent of the Coffin–Manson fatigue life law.