FATIGUE CRACK-GROWTH AND CLOSURE AT HIGH CYCLIC STRAINS

The concept of a cyclic J integral DELTA J is briefly reviewed and equations are given for estimating its value. Fatigue crack growth rate data for various test specimens of an alloy steel are analyzed using DELTA J with the aid of rough crack closure measurements. Without the plasticity correction of DELTA J, there is an extreme lack of correlation for the cases of cracks growing under reversed plastic strains. Including a correction for crack closure together with DELTA J gives the best correlation of growth rates among various geometries, degrees of plasticity and R ratios of 0 and minus 1. Crack closure is decreased by increasing plasticity, with the crack being open during even most of the compressive portion of completely reversed loading at high cyclic strains.