Hygrothermal degradation and fracture process of advanced fibre-reinforced plastics

Abstract

An investigation has been carried out on the effects of water absorption on the mechanical properties and fatigue strength of unidirectionally reinforced carbon-epoxy (CFRP) and aramid-epoxy (AFRP). T-1/347 normal-type CFRP absorbed more water than MM-1/982X heat-resistant-type CFRP. After a long immersion of about 7 months, however, MM-1/982X composites absorbed water rapidly and the fatigue strength was considerably decreased. In T-1/347 wet specimens preconditioned in water for 2 months a high amount of water absorption degraded the matrix and decreased the strength of the fibre-matrix interface, thereby worsening the mechanical properties. For MM-1/982X composites a small amount of water absorption moderately decreased the interfacial strength and increased the ductility of the matrix, thereby improving the mechanical properties. However, the fatigue lives of wet specimens of both CFRPs were decreased in water. Water absorption lowered the yield strength of the epoxy resin, thereby changing the shape of the stress-strain curves of AFRP. This had no effect on the tensile strength of the composites. Water absorption increased the fatigue strength of both Dupont's Kevlar-49 and Teijin's Technora AFRP. The water absorption increased the ductility of the matrix and enabled local realignment of the fibres. The Technora composite had a stronger interface than the Kevlar-49 composite. Water absorption lowered the interfacial strength of both AFRPs and changed the failure mechanisms. In these tests the synthesized evaluation of acoustic emission (AE) signals using several AE parameters was carried out and the fracture mechanisms were discussed in terms of the observations of internal damage by a scanning acoustic microscope and of fracture surfaces by a scanning electron microscope.