MICROBUCKLE PROPAGATION IN CARBON FIBER-EPOXY COMPOSITES

Observations of microbuckle propagation in uni-directional carbon fibr e-epoxy material are described. The fibres buckle either in the plane of the specimen or out-of-plane, depending on the constraints on the f ree 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 behin d 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) k J/m2 is inferred from the experiments. The observed relative displacem ents across an out-of-plane microbuckle agree with theoretical values using the mode I bridging model. Micrographs of the propagating microb uckle tip show that the details of the failure mechanism are similar f or both in-plane and out-of-plane microbuckling. Both develop kink ban ds with a width of between 25 and 70 mum and with a propagation angle beta of between 25-degrees and 30-degrees. A process zone extends abou t 250 mum ahead of the kink band tip, wherein the fibres buckle and br eak. Fibres in this region become almost straight again on unloading. When the deduced large scale bridging model of microbuckling failure f or unidirectional material is applied to failure at a sharpened slit i n multi-directional laminates, reasonable agreement is found between t he theoretical and the observed compressive fracture toughnesses.