Effect of Stress Ratio on Fatigue Life of A356 Aluminium Casting Alloys

The present study aimed to investigate the dependence of fatigue properties of A356 casting alloy on the variation of stress ratio (R-value) in terms of the mutual contribution of strain components and porosity to fatigue life. The fatigue test was carried out in axial high-cycle fatigue mode through the variation of R-value by the addition of compressive load to a fixed tensile stress amplitude (R = 0.0, − 0.5, − 1.0) and by the variation of tensile and compressive loads in a fixed total stress range (R = − 0.58 ~ − 1.25), respectively. The decrease in R-value by the addition of a compressive load at a fixed tensile stress amplitude wholly reduces the slope of the S-N curve (fatigue strength exponent), because the nominal level of fatigue life becomes increasingly sensitive with the porosity variation as R-value decreases; fundamentally, the increase in total stress range induces a remarkable increase in the plastic strain as well as the elastic strain. Meanwhile, the decrease of R-value by the variation of tensile/compressive load to a fixed total stress range induces a different aspect on the dependence of fatigue life on the porosity variation: the nominal value of defect susceptibility coefficient remarkably increases with a wide range and the decrease of maximum fatigue life is restricted to a small range, by a relatively small variation of both strain components. However, as the R-value decreases in both loading conditions, the propagation path of a fatigue crack gradually transitions from a cracking mode, in which the cracking damage of eutectic Si particles dominates, to a mixed mode of matrix penetration by local plastic deformation and cracking damage of eutectic Si particles, which mainly accumulate by compressive loading conditions.