Distributions of pore size and fatigue weak link strength in an A713 sand cast aluminum alloy

The population and size of porosities in an A713 sand cast aluminum alloy were statistically measured and the pore size distribution could be quantified using a general extreme value (GEV) distribution function. The stress-number of cycles to failure (S-N) curve of the alloy was characterized using four-point bend fatigue testing at f=20. Hz, R=0.1, and in ambient air. The fatigue strength of the alloy was measured to be 94.5. MPa. Crack populations on the surfaces of the specimens fatigued at different stress levels were counted, and plotted against the applied maximum stress. It was found that the relationship between crack population and stress was a Weibull distribution function. By taking the derivative of the Weibull function, the density and strength distribution of fatigue weak links were derived. The strength distribution of fatigue weak links was profoundly different from the pore size distribution, indicating that the strength distribution of fatigue weak links, instead of the simple pore size distribution, should be used to evaluate the fatigue properties of this alloy.