Influence of an overload applied within compressive base-line loading on crack propagation retardation in AM60B magnesium alloy

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Application of a tensile overload within an otherwise constant amplitude cyclic loading scenario is known to retard the resulting crack propagation in metals, thus resulting in longer fatigue life. The magnitude of retardation and crack length increment that would be affected by such an overload would depend on the overload ratio and the nature of baseline loading. However, the influence of an overload within a baseline loading with negative stress ratio on crack propagation has not been extensively investigated. Consequently, the influence of overload (with various overload ratios), applied within various baseline loadings consist of both positive and negative stress ratios, is considered in our experimental investigation. It is shown that the retardation response of AM60B magnesium alloy varies when the stress ratio of the baseline loading changes. Wheeler model's parameters are evaluated separately for positive and negative stress ratios. For positive stress ratio, the affected zone was modified based on the baseline loading's stress ratios.The experimental results reveals that retardation trend in crack propagation would be affected significantly by negative stress ratios; therefore, Wheeler's model would have to be modified to account for this important issue. Moreover, the influence of loading sequence (i.e., when the overload is followed by a compressive underload) on retardation of crack propagation is also investigated.Finally, surface roughness of the fractured specimens is carefully examined by a profilometer and its variation with respect to stress ratio is reported in this paper.