Prediction of Fatigue Performance in Aluminum Shape Castings Containing Defects
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Prediction of Fatigue Performance in Aluminum Shape Castings Containing Defects Q.G. WANG and P.E. JONES Fatigue properties of cast aluminum components are controlled by maximum defect size in the material. The larger the maximum defect size, the lower the fatigue strength and life. In the presence of casting defects, crack initiation can be ignored and fatigue life is mainly spent in crack propagation. Therefore, fatigue life of aluminum castings can be predicted by long or short crack growth models. The main problem is defining a starting defect size from readily available data, such as twodimensional (2-D) pore size measurements on metallographic sections. In this article, an extremevalue statistics (EVS) method was used to estimate the maximum defect size in 319 castings from conventional metallographic data. The maximum defect size predicted by EVS agrees quite well with the initiation defect sizes measured from fracture surfaces, and the predicted fatigue life is in reasonable agreement with the experimental data. I.
INTRODUCTION
SHAPE cast aluminum components have been widely used in fatigue critical structural applications, such as engine blocks, cylinder heads, and chassis and suspension components, to improve automotive fuel economy. Methods to accurately predict fatigue properties of these castings early in the component and manufacturing process design cycle are needed. There are two design philosophies for structural components—infinite life and damage tolerant design. Infinite life design does not allow crack initiation and propagation under service loading, while damage tolerant design assumes the presence of casting imperfections and permits crack propagation. For aluminum shape castings, the presence of casting defects and discontinuities is almost inevitable. Furthermore, below the gigacycle life regime, there is no apparent fatigue endurance limit for cast aluminum alloys. Therefore, the damage tolerant design approach is appropriate for fatigue loaded aluminum shape castings. Fatigue properties of aluminum shape castings are sensitive to the defect size.[1–5] In recent years, the maximum defect size has been recognized as the most important parameter in determining the fatigue properties of aluminum shape castings. The larger the maximum defect size, the lower the fatigue strength. In the presence of imperfections, fatigue strength is little affected by chemical composition, heat treatment, or solidification time, as reflected by dendrite arm spacing and the sizes of eutectic silicon and intermetallic particles.[6–8] In damage tolerant design, the crack propagation life is estimated from the crack propagation rate and initial crack size. For a given initial crack (defect) size, the fatigue life Q.G. WANG and P.E. JONES, Senior Project Engineers, are with the Materials Engineering Technical Specialists and Materials Engineering Lab, GM Powertrain, General Motors Corporation, Pontiac, MI 483402920. Contact e-mail: [email protected] This article is based on a presentation made in the symposium ‘‘Simula
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