Scatter in fatigue life due to effects of porosity in cast A356-T6 aluminum-silicon alloys |
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Authors: | J Z Yi Y X Gao P D Lee H M Flower T C Lindley |
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Affiliation: | (1) the Department of Materials, Imperial College London, SW7 2BP London, United Kingdom |
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Abstract: | 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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