Effect of microporosity and loading condition on fatigue life of A356 casting alloys

Abstract

This study quantitatively describes the mutual contributions of microporosity and loading conditions (i.e. strain rate and stress amplitude) on the fatigue properties of low-pressure die-cast A356 alloy, in terms of the defect susceptibility coefficient of fatigue life to microporosity variation under various loading conditions. A fatigue test is conducted in high-cycle axial fatigue mode with a variation of strain rate, applying different loading frequencies (0.3–30 Hz) in a stress ratio of R = - 1.0. Computational topography (CT) analysis and scanning-electron microscope (SEM) fractographic observations were performed to evaluate the size and distribution of micro-voids. The overall dependence of fatigue life on microporosity variation can be linearly described as an exponential form of the defect susceptibility coefficient of fatigue life to the microporosity variation, with the maximum fatigue life achievable in a defect-free condition. The contribution of microporosity to fatigue life becomes insensitive as the strain rate increases at given stress amplitudes, whereas it becomes more dominant with the increase of stress amplitude at a given strain rate. Even though the existence of micro-voids inside a material fundamentally influences major stress concentrations and the origins of fatigue-crack propagation, its practical contribution to fatigue life by the variation of strain rate and stress amplitude is intimately related to the stress concentration by position at the near-circumference of the specimen, rather than the nominal variation of load-carrying capacity by the size and area fraction of micro-voids.