Slip structure and Crack Growth during Fatigue in Aluminum and Aluminum-1% Magnesium

The surface slip structure produced by fatigue deformation in aluminum and aluminum- 1 magnesium alloy was observed. Slip bands, rumples, and "mat" type and linear features were all observed in pure aluminum. The rumples and "mat" type and linear structures were explained in terms of a model involving intersecting slip. The fatigue cracks, nucleated at a notch, were found to propagate either along the grain boundaries or the slip bands, depending upon the orientation of the grains, the overall direction of propagation being nearly perpendicular to the stress axis. A peculiarity was noted concerning the propagation of a branch-off crack in the direction of the applied stress. The crack propagation in the cold worked alloy was found to be mainly transcrystalline and involved heavy distortion ahead of the crack tip