An A356 cast aluminum alloy is being used for automotive chassis components. The microstructure of this alloy is complex and varies over spatial length scales; often, the progression and synergistic cooperation of damage at various length scales eventually leads to final fatigue failure of the component. In this paper, a methodology is discussed in which finite element analyses are performed at different spatial length scales to characterize each of a range of mechanisms that are known to contribute to fatigue failure of A356. At each scale, appropriate continuum constitutive relations are used for the constituents. The microscale, which ranges from 1-40 #mu#m, focuses on assemblies of interdendritic silicon particles and microporosity associated with local shrinkage or particle fracture/debonding. The next scale is that of dendrite cell size, D_(cs) typically on the order of 40-100 #mu#m; gas porosity at this scale is also fairly common. The next is a mesoscale comprised of significant numbers of dendrite cells, but with shrinkage and gas pores with dimensions that may reach 150-500 #mu#m or more. A fatigue damage formulation is introduced that represents four different regimes: small crack incubation period, microstructurally small crack growth, physically small crack growth, and long crack growth. Mechanisms of crack incubation and growth at various microstructure scales differ according to their relative balance of incubation, small and long crack propagation. In some cases, certain regimes may be bypassed completely. Finally, an example methodology is outlined to assess the fatigue failure of a structural component (e.g., cm-m).
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