Loading rate dependency on mode I interlaminar fracture toughness of unidirectional and woven carbon fibre epoxy composites

Abstract The loading rate effect on mode I interlaminar behaviour of unidirectional and woven composites has been analysed from quasi-static to dynamic loading rates. Fibre bridging, which can overestimate the measured fracture energies and conceal the strain rate effect, has been avoided by manufacturing woven and unidirectional (but with a small amount of transversal E-glass) carbon fibre epoxy composites. Double Cantilever Beam tests have been performed from 8.3 × 10 −5  m/s to 0.19 m/s loading rates, measuring crack length and specimen aperture by a high speed camera. Results over the analysed loading rates have shown an interlaminar fracture energy release rate reduction of 19% for woven reinforcement, and 31% for unidirectional. The main G IC reduction on the unidirectional composite (a 24% reduction) is given when increasing testing loading rate from quasi-static to a dynamic (approximately 0.05 m/s). This reduction has been associated to a change in the crack growth behaviour from quasi-static to dynamic loading rates, which in fractographic analysis is found as less fibre/matrix interface failure and larger matrix brittle cracking. A rate parameter called crack rate has been defined from the time derivative of the crack length, which is measured experimentally. This rate parameter allows the comparative study of G IC values obtained at different loading rates and crack lengths.