Abstract:It is well known that the mean stress or stress ratio of fatigue loadings has a strong effect on the shape of S-N curves. An understanding of the relationships among S-N curves corresponding to different mean stresses or stress ratios would be very useful in engineering applications. In this study, based on continuum damage mechanics, a mathematical expression of an S-N curve is deduced from a new damage evolution law. This mathematical expression can well represent the whole S-N curve, not only the linear part in bi-logarithmic diagrams, but also the transitional part near the fatigue limit. The effect of mean stress on an S-N curve is represented by two state parameters. The relationships between these state parameters and the mean stress are proposed and examined. By using these relationships, the concepts of equivalent symmetric amplitude and equivalent symmetric cycles are introduced. We have found that all S-N curves under non-symmetric states can be rearranged into the same curve as that of symmetric fatigue by adopting these equivalent parameters.
Considering interacted damage accumulations induced by corrosion, cyclic fatigue, and stress corrosion cracking, a phenomenological corrosion fatigue (CF) life evaluation formula has been proposed. A concept of nominal CF limit has been introduced, which indicates whether cyclic damage accumulation is active or not. It is found that cyclic fatigue damage accumulation enhanced by corrosion is the dominative damage under relatively high stress amplitudes (compared to the nominal limit), whereas, under low stress amplitudes, corrosion damage accumulation would become the dominative one. Stress amplitudes below the nominal fatigue limit would lead to CF too due to corrosion damage accumulation, which can finally make the effective amplitude larger than nominal limit. Once the parameters included in formula have been determined by uniaxial tests, CF life under any complicated stress state could be estimated in a unified way.
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