3630 - THE INFLUENCE OF BIAXIAL STRESS ON THE FATIGUE BEHAVIOR OF DEFECT-CONTAINING STEELS

A modified linear-elastic method of analysis is proposed for the determination of fatigue strengths and fatigue lifetimes of biaxially loaded components containing defects. The method assumes that the fatigue strength and the fatigue lifetime are both controlled by fatigue crack propagation considerations, and that the fatigue cracks propagate as Mode I cracks. The modifications made to the standard method of linear analysis are as follows: 1. A modification to include consideration of elastic-plastic behavior. 2. A modification to deal with the inability of the standard approach to predict fatigue crack growth behavior in the small crack regime. 3. A modification to deal with the development of crack closure in the wake of a newly-formed crack. A number of material constants are involved in the analysis. These include the effective fatigue crack growth threshold, ΔKeffth; the crack opening level for a macroscopic crack, Kopmax; and k, a constant which determines the rate of crack closure development with crack advance. Experiments were carried out under combined cyclic loading conditions employing specimens of two steels which differed in their mechanical properties and which contained simulated surface defects of various sizes to check on the accuracy of the predictions. It was found that both the predicted fatigue strengths as well as the predicted fatigue lives agreed well with the experimental results.