REDUCTION IN INTERLAMINAR SHEAR-STRENGTH BY DISCRETE AND DISTRIBUTED VOIDS

Discrete voids of different sizes were simulated by embedding PTFE mon ofilaments, tubes and strips at the mid-plane of unidirectional glass fibre and carbon-fibre/epoxy plates. Short-beam shear tests were carri ed out to determine the effect of the defects on interlaminar shear st rength. For those specimens where failure initiated from the defect, l inear elastic fracture mechanics was not able to predict the reduction in strength. However, excellent correlation was obtained with a previ ous 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 b e the increase in stress due to the reduction in net cross-section. Fa ilure of specimens with high levels of distributed voidage was also co nsistent with failure being controlled mainly by the reduction in net section. It is suggested that the commonly observed decrease in interl aminar shear strength with voidage is due to a combination of the redu ction of cross-sectional area due to distributed voidage and initiatio n of failure from larger discrete voids.