Size effect on compressive strength of T300/924C carbon fibre-epoxy laminates

The effect of specimen gauge section (length × width) was investigated on the compressive behaviour of a T300/924C [45/-45/0/90]3s carbon fibre-epoxy laminate. A modified Imperial College compression test fixture was used together with an antibuckling device to test 3 mm thick specimens with a 30 × 30, 50 × 50, 70 × 70, and 90 × 90 mm gauge length by width section. In all cases failure was sudden and occurred mainly within the gauge length. Post-failure examination suggests that 0° fibre microbuckling is the critical damage mechanism that causes final failure. This is a matrix dominated failure mode and its triggering depends very much on initial fibre waviness. It is suggested that manufacturing plays a significant role in determining the compressive strength and may be more important as the section thickness of the composite increases. Additionally, compressive tests on specimens with an open hole are performed. The local stress concentration arising from the hole dominates the strength of the laminate rather than the stresses in the bulk of the material. It is observed that the remote failure stress decreases with increasing hole size and specimen width but is generally well above the value one might predict from the elastic stress concentration factor. This suggests that the material is not ideally brittle and some stress relief occurs around the hole. X-ray radiography reveals that damage in the form of fibre microbuckling and delamination initiates at the edge of the hole at ∼ 80% of the failure load and extends stably under increasing load before becoming unstable at a critical length of 2-3 mm (depending on specimen geometry). This damage growth and failure are analysed by a linear cohesive zone model. Using the independently measured laminate parameters of compressive unnotched strength and in plane fracture toughness the model predicts successfully the notched strength as a function of hole size and width. © 2002 IoM Communications Ltd.