The static and fatigue properties of advanced epoxy-based composites reinforced with carbon, glass or aromatic polyamide (Kevlar-49) fibres have been measured for a range of different loading and environmental conditions. Cross-plied laminates were tested in tension in the 0/90 and +/-45° orientations and also under flexural loading. The laminates were similar, except for the type of fibre. The effects of environmental exposure were assessed by preconditioning test specimens to equilibrium by either drying at 60°C, storage at 65%RH at ambient temperature or boiling in water. Moisture absorption was through the resin alone for CFRP and GRP and by additional absorption by the fibres for KFRP. Fatigue testing revealed that the tensile performance in the 0/90 orientation is strongly dependent on the level of cyclic strain. 0/90 CFRP has excellent fatigue and environmental resistance but GRP exhibits a steep fatigue curve and the static and low cycle fatigue strengths are both reduced by boiling. The fatigue strength of 0/90 KFRP is reduced by drying, more so than by boiling, and in all conditions the stress/log-life curves are characterised by a downward curvature or 'knee'. Tensile preloads do not significantly affect the residual fatigue properties or the equilibrium levels of moisture uptake, although extensive damage involving cracking in both longitudinal and transverse plies may lead to increased absorption rates. A tendency for Kevlar fibres to split or 'defibrillate' plays an important role in most failures of KFRP. It limits the shear strength and causes flexural failures to occur at the compression surface at low stress levels. 0/90 CFRP also fails at the compression surface in flexure but GRP fails at the tensile surface, the environmental fatigue performance resembling that under axial tensile loading. The +/-45° tensile and low cycle fatigue strengths are sensitive to the effects of conditioning, all laminates exhibiting optimum performance after conditioning at 65%RH, although generally these effects become insignificant at long lives.
|Date of Award||1985|