Tomoaki Kurokawa

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

Abstract 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 mm see−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.

Stress rise at the impact end of a circular bar induced by its radial inertia.

Stress wave induced by the longitudinal collision of an elasto-plastic bar with either a rigid wall or an elastic bar was analysed numerically. The constitutive equation of the elasto-plastic bar was assumed to be rate-independent.A steep rise in axial stress at the impact end occurred immediately after collision and the peak value of the average axial stress within the cross section of the impact end was found to be equal to the predicted value as calculated using the stress-strain relationship in a uni-axial strain state. This is because, during a short period after impact, the immediate area of the impact end is in a state of uni-axial strain caused by its radial inertia.This high stress level dropped and relaxed as the dilatational wave propagated from the cylindrical surface to the central axis. This stress rise was apparent only in the space equivalent to one measurement of the bars diameter. Beyond that point there was no particular rise in stress.The stress level oscillated and asymptotically approached to a constant value, which is calculated by Karmans theory in a uni-axial stress state.