Cyclic behaviour of materials

Abstract

The effect of strain rate on the cyclic behaviour of two materials is presented. For the material that cyclically hardens (En 32B) a decrease in strain rate decreases the maximum cyclic hardening of the material, whilst for a material that cyclically softens (En 25) a decrease in the rate of deformation increases the maximum cyclic softening. It is concluded that the effect of strain rate on the cyclic stress-strain curve should be more closely studied than the effect of frequency since the frequency may be constant whilst the straining rate may vary considerably in plastically deformed zones. For those zones that suffer low strain rates the inclusion of time-dependent deformation processes can maximize the degree of softening or minimize the degree of hardening, thereby creating a greater localization of the plastic strain which should increase strain-concentration factors. Finally it is argued that it is better to predict fatigue behaviour from an incremental-step high-strain fatigue test on a single specimen than from static-test data. For the same reasons correlations between static tests and fatigue tests should be discouraged, especially for those materials that exhibit marked cyclic softening.