Fatigue crack propagation resistance in homogeneous and graded alumina–epoxy composites

Fatigue crack propagation was investigated in homogeneous and graded alumina–epoxy composite specimens, produced via a multi-step infiltration technique. Crack-extension toughening was observed in homogeneous composite specimens under monotonic loading and attributed to the development of a bridging zone behind the crack-tip. A similar increase in crack propagation resistance was observed under cyclic loading. This was quantified, in an approximate manner, using an adjustment based on the Paris-law relation. Cracks in graded composite specimens underwent deflection and significant variations in crack-propagation resistance were observed as the crack traversed the graded region. The adjustment procedure provided a useful approximate method for characterising these variations, which resulted from crack-extension effects and the spatial variation of both intrinsic and extrinsic crack-growth resistance.