INFLUENCE OF WATER-UPTAKE ON INTERLAMINAR FRACTURE PROPERTIES OF CARBON-FIBER-REINFORCED POLYMER COMPOSITES

Composite materials in practical use can be subjected to a wide variet y of different loading conditions. The most important conditions are m echanical stresses and environmental attacks. An issue of major concer n in the utilization of composites is associated with the occurrence o f delaminations or interlaminar cracks, which may be related to manufa cturing defects or are induced in service by low-velocity impacts. The main environmental attacks are temperature, humidity, radiation, and chemical exposure. Three materials were investigated; two thermosettin g matrices (unmodified and toughness-modified epoxy, EP and EP(mod)) a nd one thermoplastic matrix (semicrystalline polyetheretherketone, PEE K), all reinforced with unidirectional continuous carbon fibres. Sampl es of these materials were exposed to water in baths of different temp eratures; they were taken for mechanical testing after various time pe riods. As a result of absorbed moisture, G(IC)-values increased with m oisture content of the samples, whereas G(IIC)-values decreased. By me ans of scanning electron microscopy, fracture surfaces were examined. Evidence was found that the increase of G(IC)-values was due to a grea ter ductility of the matrix (as a result of the moisture absorbed) and hence more energy-consumptive fibre-bridging. On the other hand, inte rface failure, as well as a loss of shear strength of the epoxy with i ncreasing amount of moisture absorbed, were responsible for the decrea se in the G(IIC)-values. The thermoplastic matrix system (CF/PEEK) exh ibited no influence Of moisture on the Mode I property, but a decrease of the values for Mode II.