Effect of strain rate on elastic-plastic deformation in high cycle fatigue properties

Abstract

The aim of the present study was to investigate the contribution of the strain rate to the transition of elastic-plastic deformation behavior and the difference between the fatigue strength coefficient and monotonic tensile strength in terms of the modified Basquin’s relation, which considers microporosity variation. The transition phenomena of elastic-plastic deformation were evaluated through comparison of the overall contour of the hysteresis loops measured in high cycle fatigue tests of a low-pressure die-cast A356 alloy. The increase of the alternating stress amplitude at a given excitation frequency causes a variation in the strain rate per unit time interval that depends fundamentally on the excitation frequency. In addition, the transition of elastic-plastic deformation behavior is induced by variation of strain rate in high cycle fatigue test, i.e., typically, a variation of the elastic modulus and extension of the elastic deformation region, compared with monotonic deformation under a very slow strain rate. The dependence of the elastic modulus on the strain rate due to the variation of the stress amplitude can be described in an exponential form of strain rate. The modified Basquin’s equation which includes the contribution of the strain rate and microporosity to the fatigue strength coefficient and fatigue life was re-established, including the dependence of the elastic modulus and transition of elastic-plastic deformation on the variation of the strain rate.