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

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 GIC 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 GIC values obtained at different loading rates and crack lengths.