Experimental characterization of the interlaminar fracture toughness of a woven and a unidirectional carbon/epoxy composite

An experimental program has been carried out at the University of Pisa, in collaboration with Leonardo Helicopter Division, to investigate the differences in interlaminar fracture toughness properties of a graphite/epoxy composite material, available in two forms: unidirectional tape and five harness satin fabric. To this end, tests have been carried out in mode I, mode II and mixed I+II mode on specimens manufactured with the two material systems by Leonardo HD, following their industrial standards. The results show a considerably higher toughness of the fabric, as a consequence of the peculiar features at the delamination interface and of the other local mechanisms, capable of absorbing energy, that are present only in fabric composites. Some fractographic observations confirm these mechanisms. Finally, numerical analyses have been carried out, modelling with Finite Element the various tests, to complement and evaluate the data reduction methods used to derive the toughness values.