The effects of thermal and hygrothermal ageing on the failure mechanisms of graphite-fabric epoxy composites subjected to flexural loading

Abstract Failure mechanisms of graphite-fabric reinforced epoxy composites loaded in flexure have been studied, following ageing in dry, wet and hot environments. Experimental results based on mechanical loading to failure, dynamic mechanical analysis, electron microscopy and infrared spectra indicated that in the case of high temperature ageing in dry conditions the failure mechanism is characterized by shear at short loading spans and by tension at long spans. This failure mode changes to shear delamination after long duration ageing. Wet environments up to 50°C hardly affected the mechanical properties and failure mechanisms compared to those at ambient conditions. However, a reduction in the glass transition temperature was noticeable. Ageing in boiling water was harmful to the 120°C curing composite which was based on a dicyandiamide cured epoxy. Leaching of partially reacted components of the dicyandiamide epoxy system resulted in void formation, a considerable drop in mechanical properties and a change in the failure mechanisms. Enhanced hygrothermal stability of the 120°C dicyandiamide cured system was achieved by curing or post curing at 175°C.