Microbuckle propagation in carbon fibre-epoxy composites

Abstract

Observations of microbuckle propagation in uni-directional carbon fibre-epoxy material are described. The fibres buckle either in the plane of the specimen or out-of-plane, depending on the constraints on the free surface. Large scale bridging models of in-plane and out-of-plane microbuckles are reported. The in-plane and out-of-plane microbuckles are modelled as mode II and mode I cracks, respectively. Sliding behind the microbuckle tip is resisted by a constant shear stress of 90 MPa for the in-plane microbuckle, and by a constant normal stress of 220 MPa for the out-of-plane microbuckle. For both the in-plane and out-of-plane microbuckles a microbuckle tip toughness in the range 10–17 KJ/m 2 is inferred from the experiments. The observed relative displacements across an out-of-plane microbuckle agree with theoretical values using the mode I bridging model. Micrographs of the propagating microbuckle tip show that the details of the failure mechanism are similar for both in-plane and out-of-plane microbuckling. Both develop kink bands with a width of between 25 and 70 μm and with a propagation angle β of between 25° and 30°. A process zone extends about 250 μm ahead of the kink band tip, wherein the fibres buckle and break. Fibres in this region become almost straight again on unloading. When the deduced large scale bridging model of microbuckling failure for unidirectional material is applied to failure at a sharpened slit in multi-directional laminates, reasonable agreement is found between the theoretical and the observed compressive fracture toughnesses.