Combined impact and fatigue of glass fiber reinforced composites

The low velocity impact and tension–tension fatigue behavior of glass fiber reinforced polyester resin composites have been investigated. Two fiber geometries, namely [±45°]4 and [0/90°]2s stitch bonded glass fibers were studied. The results revealed that even low energy impact could seriously impair the tensile properties of [±45°]4 composites. For the [0/90°]2s glass fiber composites, a critical impact energy was found. Below this energy level the tensile properties were hardly affected by the impact but above it the tensile properties reduced with increasing energy. Low velocity impact also reduced the fatigue lives of the composites and this reduction could be related to the degradation in tensile strength. By normalizing the fatigue stress against the post-impact residual tensile strength of the composite, it was found that for each fiber geometry a single S–N curve could be drawn, which encompassed both undamaged and impacted samples. This implies that fatigue lifetimes of impact damaged composites can be predicted from measurement of the residual tensile strength of impacted specimens and the S–N curve of undamaged samples.