Porosity is well known to be a potent initiator of fatigue cracks in cast aluminum alloys. This article addresses the observed scatter in fatigue life of a cast A356-T6 aluminum-silicon alloy due to the presence of porosity. Specimens containing a controlled amount of porosity were prepared by employing a wedge-shaped casting mold and adjusting the degassing process during casting. High-cycle fatigue tests were conducted under fixed stress conditions on a series of specimens with controlled microstructures (especially, the secondary dendrite-arm spacing), and the degree of scatter in the results was assessed. Stochastically, such scatter was found to be adequately characterized by a three-parameter Weibull distribution function. Large pores at or close to the specimen surface were found to be responsible for crack initiation in all fatigue-test specimens, and the resultant fatigue life was related to the initiating pore size through a relationship based on the rate of small-fatigue-crack propagation. With respect to the probabilities for the pores of various sizes and locations to initiate a fatigue crack, a statistical model was developed to establish the relationship between the porosity population and the resultant scatter in fatigue life. The modeling predictions are in agreement with the experimental results. Moreover, Monte-Carlo simulation based on this model demonstrated that the average pore size, pore density, and standard deviation of the pore sizes, together with the specimen size and geometry, are all of consequence regarding scatter in fatigue life.

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A B S T R A C T Fatigue strength optimization of cast aluminium alloys requires an understanding of the role of micropores resulting from the casting process. High cycle fatigue tests conducted on cast A356-T6 show that the pore size and proximity to the specimen surface significantly influence fatigue crack initiation. This is supported by finite element analyses (both elastic and elastic-plastic) which demonstrate that high stress/strain concentration is induced by pores which are both large and near to the specimen surface. A new pore-sensitive model based on a modified stress-life approach has been developed which correlates fatigue life with the size of the failure-dominant pore. The model prediction is in good agreement with experimental data.A p = projected area of pore α = scaling factor of pore effect b = Basquin exponent d 0 = diameter of specimen cross-section d e = equivalent diameter of pore ε/ε ∞ = normalized maximum principal strain ε local , ε ∞ = local strain, far-field nominal strain k 0 , k 1 , k 2 = stress/strain concentration parameters k ε = strain concentration factor k t , k σ = stress concentration factor in elastic case, in elastic-plastic case k g = geometric mean values of k σ and k ε λ 2 = secondary dendrite arm spacing N f = fatigue life s = proximity of pore to the specimen surface σ /σ ∞ = normalized maximum principal stress σ local , σ ∞ = local stress, far-field nominal stress σ a , σ ∞ a = stress amplitude, far-field nominal stress amplitude σ f = fatigue strength coefficient I N T R O D U C T I O NMicrostructure and defect population are important factors which can strongly influence the fatigue properties of a material. The roles of casting porosity, silicon particles and secondary dendrite arm spacing (SDAS) in cast alu-* This paper is dedicated to the memory of Professor H. M. Flower. minium alloys must be understood in order to optimize the fatigue performance of these alloys. Because of the potential weight reduction and consequent improvement in fuel economy of vehicles, cast aluminium alloys are being used to replace traditional cast iron and forged steel components in the automotive and aerospace industries. However, the relatively low fatigue resistance of these cast alloys is known to be an obstacle

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