A study of damage development in a weft knitted fabric reinforced composite. Part 1: Experiments using model sandwich laminates

Coupons consisting of single layers of Milano weft knitted glass fabric reinforced epoxy resin have been tested with the knitted fabric oriented at various angles to the loading direction, alone and sandwiched between outer layers of unidirectional glass/epoxy reinforcement. The sandwich coupons enable the initiation of damage to be observed directly. Tensile tests have shown that the first damage occurs as microdebonding between the loop cross-over points in the knitted fabric structure. Matrix cracking damage develops from these initiation sites and the pattern of cracking is intimately related to the fabric architecture and the fabric orientation with respect to the loading direction. Cyclic tests of sandwich specimens, and a representation of the results in terms of cumulative strain, indicate that some of the acoustic emission activity during loading and unloading of the specimens is likely to be associated with the debonding and pulling-out of knitted fabric tows across the fracture surfaces of the matrix cracks.