Generalization of the Coffin Equations With Respect to the Effect of Large Mean Plastic Strain

Abstract

Introduction of a correction factor accounting for large mean plastic strain to the kinetic law for the rate of damage per cycle is proposed. Integration of the corrected kinetic damage law under the assumption of a stable hysteresis loop (constant strain amplitude, constant mean plastic strain) leads to a generalized Coffin-Manson formula. The formula is useful for calculation of the fatigue life of structures after transition of transient ratchetting to the plastic shakedown state. Integration of the damage law is performed also for the case of superimposed cyclic and monotonic strain. The results of calculations are compared with the experimental results reported by Coffin (1970) and described by the author using a quasilinear criterion. An improved fatigue failure criterion for this case is presented and a good convergence with the results of tests for Nickel A samples is shown.