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.

Key words: Damage mechanics, S-N curve, Evolution law, Equivalent method, Fatigue limit

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