THE FRACTURE SURFACES OF HYBRID FIBER COMPOSITES

A study has been made of the fracture surfaces of unidirectional hybrid glass/carbon/epoxy laminates after environmental stress corrosion (ESC) by dilute sulphuric acid. Labelled isotope studies showed that the acid does not diffuse through the uncracked epoxy resin matrix, and must therefore travel through microcracks. Photomicrographs show how the carbon fibre plies can act as primary crack stoppers. Secondary cracks continue into the glass plies, often at a short distance along the carbon fibres from the primary crack. The carbon plies fail to prevent the acid from migrating further into the laminate, and from eventually reaching and attacking the inner glass plies. The sequence of events by which ESC cracks fracture the glass fibres and propagate through glass plies is discussed. It is proposed that some of the damage observed in glass fibres occurs after fibre fracture rather than before.