Rate dependence of mode I fracture behaviour in carbon-fibre/epoxy composite laminates

The rate dependence of mode I interlaminar fracture behaviour in unidirectional carbon-fibre/epoxy composite laminates has been investigated over a wide range of loading rates from quasi-static (displacement rate, δ = 0.01-500 mm min^-1) to impact (δ - 5-20 m sec^-1) at room temperature. Impact fracture tests were performed by the WIF (wedge-insert-fracture) method with a SHPB (split Hopkinson pressure bar) system for accurate measurement of impact fracture toughness, while quasi-static fracture tests were performed by the DCB (double-cantilever-beam) method with a screw-driven testing machine. In the present composite laminates, the fracture toughness decreased stepwise with increasing loading rate showing a distinct rate-sensitive transition region and two rate-insensitive regions above and below. As a consequence of this stepwise characteristic, the crack growth behaviour varied with loading rate: in and below this transition region, the crack grew unstably accompanied by high-speed propagation and arrest; but above the transition region, the crack grew stably and continuously. This trend was well explained by a simple model incorporating the rate dependence of fracture toughness and the contribution of kinetic energy in the specimen during unstable crack propagation.