Effect of strain rate on fatigue property of A356 aluminium casting alloys containing pre-existing micro-voids

Abstract

This study aims to investigate the dependence of fatigue life on strain rate variations by loading frequency, and simultaneously, to evaluate the mutual relation between the strain rate and microporosity on the fatigue property in terms of the defect susceptibility of fatigue life to microporosity variation. The test specimen was prepared from the rim section of an automotive wheel fabricated by low-pressure die-casting on an A356 alloy. The high-cycle fatigue test was carried out in a strain rate range corresponding to the frequency range of 0.3–30 Hz at a stress ratio of R = −1. The strain rate increased to a two-order interval from 10−2 to 100 s−1 as the loading frequency increased from 0.3 Hz to 30 Hz. The nominal fatigue strength coefficient and exponent decreased from 754 MPa and 0.18 to 293 MPa and 0.09, respectively, with increasing strain rate. The fatigue life in low-stress amplitude slightly increased with the strain rate, while it remarkably decreased with the strain rate as the stress amplitude increased. This is because the elastic/plastic strain component ratio (Δεe/Δεp) decreased upon increasing the plastic strain component, accompanied by increasing strain rate. The plastic strain component was remarkably increased with the strain rate and stress amplitude, and decreased by the strain-hardening effect on the lapse of fatigue cycle, even though the elastic strain component remained at a constant level, depending mainly upon the stress amplitude. Additionally, the contribution of microporosity to the variation of fatigue life was underestimated as the strain rate increased, and became more sensitive with the variation of the elastic/plastic strain component in the fatigue cycle as the stress amplitude increased. The main propagation path of fatigue cracks is changed from a mixed mode of the cracking failure by the damage evolution of eutectic Si particle and the matrix penetration by shear deformation of matrix region, to a cracking failure mode where the damage evolution of eutectic Si particles dominated as the strain rate increased.