Crack Initiation and Propagation Behaviors in Rotating Bending Fatigue of Al-Si-Mg Cast Aluminum Alloy Containing Artificial Defects and Applicability of the √area Parameter Model

Natural defects in cast aluminum alloys, such as gas and shrinkage porosities, significantly reduce the fatigue strength. It is essential to understand how these defects relate to fatigue limits to develop a practical fatigue strength prediction model. This study investigated the fatigue crack behaviors in a cast aluminum alloy A356-T6 by performing rotating bending fatigue tests using specimens containing an identical artificial defect of 287 μm in √area. The obtained S-N data exhibited large scatter, and the √area parameter model gave a non-conservative prediction for the fatigue limit. The fracture surface was macroscopically normal to the loading direction but microscopically composed of many smaller facets with different inclinations. The size and orientation variations of the grains greater than the artificial defect are supposed to be related to creating these facets and causing the scatter in fatigue strength. Unsatisfactory fatigue limit prediction based on the √area parameter model may be attributed to the misestimation of effective defect size and the cracks microscopically extending in a mixed-mode manner.