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.

► At high-stress, the crack path was 45° inclined direction to the loading axis (LA).
► At low-stress, the crack grew perpendicular to the LA.
► At low-to-high stress, the crack path direction was perpendicular to the LA.
► At high-to-low stress, a 45° inclined path turned into a path perpendicular to the LA.
► The grain coarsening had a great influence on crack growth direction/mechanism.