Fatigue Failure Mechanisms in a Unidirectionally Reinforced Composite Material

The fatigue failure mechanisms in a unidirectional glass fiber-epoxy material were studied. Specimens were made by filament winding the glass fiber roving and vacuum impregnating with a low viscosity room temperature curing resin. The fatigue response could be divided into three distinct life ranges. In the first region (< 200 reversals), there was a small dependence of the fatigue life on cycling. Behavior here was believed to be dependent upon the fiber mean strength and strength distribution. In the second region (200 to 10⁶ reversals), the logarithm of the applied stress decreased almost linearly with the logarithm of the number of cycles. Fatigue failure in this region occurred by the growth of matrix microcracks, which lead to preferential fiber failure, and was followed by interfacial shear failure. In the third region (> 10⁶ reversals), the applied stress was below the microcrack initiation stress and none of the few specimens tested failed. Unlike behavior in the first two regions, where defects were formed in the first cycle and subsequently propagated, most of the cycles in this region were used in crack nucleation.