Fatigue damage formation and microstructure of ultrafine grained copper under two-step loading

Abstract

In order to study the effect of stress change on the surface damage formation of ultrafine grained copper, two step fatigue stress tests were conducted. The microstructural evolution and crack growth direction of ultrafine-grained copper fatigued under constant stressing depends on the magnitude of the applied stress amplitude. The growth direction occurred perpendicular to the loading axis at low stresses, and changed to 45o to the loading axis at high stresses. In the case of high-to-low block stressing, the 45o inclined growth direction under high stress changed to the perpendicular direction under subsequent low stress. In low-to-high block stressing, although the growth direction perpendicular to the loading axis at low stresses was still retained at subsequent high stress, the degree of zigzag behavior in the crack growth drastically increased after the stress change. The sizes and topographies of evolved microstructures, which have a great effect on the crack path formation, depended on stress histories (order, cycle number and magnitude of the applied stress). The formation behavior of the fatigue damage under high- and low-stress amplitudes and the effect of pre-stressing on crack growth direction are discussed from the viewpoints of the microstructural evolution under different stress histories.