Abstract
The objective of this study was to investigate the influence of polyester toughness on lon gitudinal fatigue behavior of glass-fiber reinforced composites. The toughness of the polyester was varied by adding a flexible isophthalic polyester resin as a modifier to a standard polyester resin. Such blends with different toughness properties were used to fab ricate the composite laminates. The laminates were tested at four different tensile stress levels under a load controlled mode. The results show that at a high fatigue stress, the rate of increase in fatigue life with respect to decrease in fatigue stress is nearly equal for the composites studied. However, in a stress range between 35 to 45 percent of the ultimate strength, the rate of increase in fatigue life with decrease in fatigue stress improves with use of the ductile polyester resin in the composite. Scanning electron microscopy indicates that the composite with tough polyester does not develop microcracking until 80 percent of fatigue life is reached. On the other hand, extensive microcracking was observed in the specimens with a polyester having high ductility and very low strength. In general, the final failure of the laminate was a combination of debonding and fiber breaks. The study shows that the toughness of the polyester helps in delaying the initiation of microdamage and offers a reliable index for measuring contribution of the matrix in FRP under fatigue loading.
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