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The fatigue strength of permanent mould and sandcast aluminium is strongly influenced by the cooling rate during casting. Slower solidification yields larger secondary dendrite arm spacing with a coarser eutectic and greater defetcs i.e. porosities and intermetallic β (FeSiAl5). The negativ effect of porosity is greater than that of the amount/size of the β-platelets. The defects affect both the crack nucleation and the propagation phases. For very large defects, the initiation phase is reduced and a large portion of the life is spent as crack propagation. The secondary dendrite arm spacing (DAS) represents a good over all indication of the fatigue strength of the material, which is proportional to (DAS)−1/2. This relationship can be used to estimate the fatigue strength at various locations in a cast aluminium component.