Study on evaluation method of distribution of fatigue lives of defect-sensitive materials

Fatigue tests under different R ratios were conducted using an extruded Mg alloy. In order to estimate the maximum inclusion area within the dangerous section of the specimen, the extreme-value statistics proposed by Gumbel was utilized. An initial crack length can be estimated by using the distribution of the extreme-value of the inclusion areas with an assumption that inclusion is equivalent to the crack. To analyze the small fatigue crack propagation behavior, a modified linear elastic fracture mechanics parameter, M proposed by McEvily et al was used. The relationship between the crack propagation rate da/dN and M parameter was found to be appropriate in predicting the fatigue lives under different R ratios. Moreover, the distribution of fatigue lives can also be predicted with Monte-Carlo simulation using the distribution of the extreme-value distribution of the inclusion areas and M parameter. The estimated results were in good agreement with the experimental ones.