Abstract
In this study, fatigue tests under different R ratios were conducted on the AZ61 Mg alloy to investigate its fatigue lifetimes and fatigue crack growth (FCG) behavior. The fracture surface of the failed specimens was investigated using a scanning electron microscope to study the size of the intermetallic compounds from which the pioneer fatigue crack initiated and led to the final failure of the specimen. To determine the maximum size of the intermetallic compounds existing within the cross section of the specimen at higher risk, Gumbel’s extreme-value statistics were utilized. In the present study, the intermetallic compounds contained within the specimen were assumed to be the initial cracks existing in the material before the fatigue tests. A modified linear elastic fracture-mechanics parameter, M, proposed by McEvily et al., was used to analyze the short FCG behavior under different stress ratios, R. The relation between the rate of FCG and M parameter was found to be useful and appropriate for predicting the fatigue lifetimes under different R ratios. Moreover, the probabilistic stress-fatigue life (P-S-N) curve of the material under different R ratios could be predicted with this method, which utilizes both the FCG law and a statistical distribution of sizes of the most dangerous intermetallic compounds. The evaluated results were in good agreement with the experimental ones. This correspondence indicates that the estimation method proposed in the present study is effective for evaluation of the probabilistic stress-fatigue life (P-S-N) curve of the material under different R ratios.
Highlights
Magnesium alloys are attractive materials from the viewpoints of lightness, dimensional stability, cutting performance, specific strength, and recyclability
The intermetallic compounds contained within the specimen were assumed to be the initial cracks existing in the material before the fatigue tests
The probabilistic stress-fatigue life (P-S-N) curve of the material under different R ratios could be predicted with this method, which utilizes both the fatigue crack growth (FCG) law and a statistical distribution of sizes of the most dangerous intermetallic compounds
Summary
Magnesium alloys are attractive materials from the viewpoints of lightness, dimensional stability, cutting performance, specific strength, and recyclability. For these reasons, they are used in various industrial products, such as car components and structural materials. It is well known that defects and intermetallic compounds are contained in Mg alloys when they are processed. It is considered that these defects and intermetallic compounds strongly influence the statistical distribution of the fatigue lifetimes and fatigue crack growth (FCG) behavior. Several studies on the fatigue of extruded Mg alloys [1] [2] [3] [4] [5] have been performed. There are few studies on the distribution of the fatigue lifetimes of the extruded Mg alloys
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