Derivation of Effective Strain-Life Data, Crack Closure Parameters and Effective Crack Growth Data from Smooth Specimen Fatigue Tests

Small crack growth from notches under variable amplitude loading requires that crack opening stress be followed on a cycle by cycle basis and taken into account in making fatigue life predictions. The use of constant amplitude fatigue life data that ignores changes in crack opening stress due to high stress overloads in variable amplitude fatigue leads to non-conservative fatigue life predictions. Similarly fatigue life predictions based on small crack growth calculations for cracks growing from flaws in notches are non-conservative when constant amplitude crack growth data are used. These non-conservative predictions have, in both cases, been shown to be due to severe reductions in fatigue crack closure arising from large (overload or underload) cycles in a typical service load history. The work in this paper is concerned with the derivation of crack closure constants for use in two models; the effective strain-life fatigue model that can be applied to predict fatigue lives for smooth and blunt notched specimens under constant and variable amplitude loading and the effective fatigue crack growth model that can be used to predict fatigue lives of short cracks growing out of notches or flaws under constant and variable amplitude loading. Fatigue life calculations based on equations using crack opening stress are shown to give accurate fatigue life predictions for smooth and notched specimens subjected to variable amplitude service load histories. The influence of metal hardness on crack opening stress behaviour is deduced by comparing data for the three metal hardness levels used in this investigation.