Fatigue Fracture Stages of Metals and Alloys and Stage-to-Stage Transition Criteria

Abstract

Basic mechanisms of scattered and localized fatigue damage of metals and alloys are analyzed. The methods of evaluating the criteria of scattered-to-localized fatigue damage transition are proposed and substantiated based on the analysis of short and long crack propagation behavior. One of the methods is based on the analysis of short fatigue crack growth kinetics defined by their size or growth rate against the number of load cycles. Crack sizes and a number of cycles, corresponding to a more intensive increase in the crack growth rate, are taken to be the transition criteria. With the scattered-to-localized fatigue damage transition at the stresses above the endurance limit, the form and parameters of the equation describing the crack size against the number of load cycles are changed. Another method is built upon the analysis of variation of the short crack propagation rate against the stress intensity factor. In this case, the stress intensity factor span and a corresponding crack size at which the stress intensity factor starts varying at a higher rate are taken as the transition criteria. The above methods are used to assess crack sizes and a number of load cycles, corresponding to the scattered-to-localized fatigue damage transition, for carbon, alloy, austenitic steels and an aluminum alloy accounting for stress levels and properties of examined materials. Fatigue crack sizes on the transition at the stresses above the endurance limit are established to decrease with stresses and remain smaller than the crack sizes at the endurance limit. Main crack sizes are shown to correlate with the endurance limit of examined materials, decreasing with its increase.