Fatigue damage mechanisms in fiber-reinforced metal matrix composites

An experimental investigation of fatigue damage mechanisms in a fiber reinforced metal matrix composite with weak interfaces has been conducted. In the presence of notches, single mode I matrix cracks initially grow with minimal fiber failure. The tractions exerted by the intact fibers shield the crack tip from the applied stress and reduce the rate of crack growth relative to that in the unreinforced matrix alloy. In some instances, further growth is accompanied by fiber failure and a concomitant loss in crack tip shielding. The magnitude of the interface sliding stress inferred from the comparisons between experiment and model predictions is found to be in broad agreement with values measured using alternate techniques. In contrast, the transverse fatigue properties are found to be inferior to those of the monolithic matrix alloy, a consequence of the poor fatigue resistance of the fiber/matrix interface.; In addition, multiple matrix cracking during fatigue of unnotched specimens is examined. Saturation levels of crack density increase with applied stress amplitude. The saturation crack densities are broadly consistent with predictions of a model which incorporates the reduction in crack driving force due to interaction with adjacent cracks. In comparison with pristine material, the presence of the cracks results in additional strain during loading. The additional strain is composed of an elastic component and a fiber sliding component, both of which increase with crack density. The residual strength of the cracked material is on the order of the fiber bundle strength in the composite, independent of crack density.; The sliding resistance of the fiber/matrix interface is found to decrease with cycling. Measurements of changes in the cyclic traction law exponent indicate that the degradation does not occur uniformly along the fiber sliding length. Examination of the coatings of cycled fibers confirms the existence of a wear mechanism, the magnitude of the damage decreasing with distance from the crack plane. The observed changes in the traction law exponent are consistent with a model which incorporates variations in sliding stress along the slip length.