Fatigue-induced grain coarsening and crack growth behavior in ultrafine-grained copper under different loading histories

The crack growth direction in ultrafine-grained copper under constant stressing depends on the applied stress amplitude. The growth direction is perpendicular to the loading axis at low stresses and inclines at 45° to the loading axis at high stresses. To clarify physical causes of such different behaviors at high and low stress amplitudes, two-step fatigue stress tests are conducted. The formation mechanisms of crack paths at high and low stress amplitudes and effect of pre-stressing on subsequent crack growth direction are discussed considering the morphological features of surface-damage formed under different cyclic-stress histories.