Reduction in interlaminar shear strength by discrete and distributed voids

Discrete voids of different sizes were simulated by embedding PTFE monofilaments, tubes and strips at the mid-plane of unidirectional glass fibre and carbon-fibre/epoxy plates. Short-beam shear tests were carried out to determine the effect of the defects on interlaminar shear strength. For those specimens where failure initiated from the defect, linear elastic fracture mechanics was not able to predict the reduction in strength. However, excellent correlation was obtained with a previous finite element analysis which used non-linear springs to model the interfaces between plies. In other specimens failure initiated above and below the defect. For these failures, the main factor appears to be the increase in stress due to the reduction in net cross-section. Failure of specimens with high levels of distributed voidage was also consistent with failure being controlled mainly by the reduction in net section. It is suggested that the commonly observed decrease in interlaminar shear strength with voidage is due to a combination of the reduction of cross-sectional area due to distributed voidage and initiation of failure from larger discrete voids.