Influence of casting defects on fatigue properties of SiC particulate dispersed aluminum alloy composite castings

Castings of an aluminum alloy composite containing 30 vol% of SiC particles (MMC castings) and of Al-9 mass% Si alloy were prepared and subjected to three-point bending fatigue tests. The MMC and Al-Si alloy castings contained 2.4 and 0.9 vol% of cavities, respectively, and the size of these cavities followed a power-law distribution at larger cavity sizes. The fatigue lifetime of the MMC castings was shorter than that of the Al-Si alloy castings. When a notch was formed in the bottom (tension-side surface) of each specimen, the fatigue lifetime of the Al-Si alloy castings was markedly reduced, whereas there was no change in that of the MMC castings. The size of the fatigue fracture region was almost the same for flat specimens and for notched specimens. After fatigue testing, many fine cracks were observed on the bottom of the specimen; when the length of these cracks was related to σaDd1/2, where σaD is the stress amplitude at a distance D from the fracture position on the specimen bottom and d is the size of the cavity from which the crack initiated, it was found to be distributed in the range between σa maxd1/2 and σa mind1/2, where σa max is the maximum stress amplitude in the measured area and σa min is the minimum stress amplitude. Numerical calculations were carried out for a model in which fine cracks initiate and propagate from casting defects, coalesce to form a major crack, and then the major crack grows to a critical size that results in final failure. The fatigue lifetime calculated by taking the coalescence of cracks into account agreed well with the measured fracture lifetime for MMC and Al-Si alloy castings.