Compressive behaviour of CFRP laminates exposed in hot-wet environments.

This thesis describes an experimental study of the compressive failure of T800/924C carbon fibre-epoxy laminates exposed in hot-wet environments. The specimens were immersed in boiling water so that the moisture equilibrium level was reached in a period of few weeks. The moisture level and moisture diffusion through the thickness of the laminate were measured experimentally and compared with theory (Fick's Law). Uniaxial compression tests were carried out in a Celanese test rig and the failure mechanisms were studied under various environmental conditions. Fracture characteristics were identified using optical and scanning electron microscopy. The critical failure mechanisms observed were in-plane and out-of- plane fibre microbuckling. At test temperatures higher than 50°C the failure mode switched from in-plane to out-of-plane microbuckling. As the temperature increased the shear strength and stiffness of the resin were considerably reduced. This decreased the amount of side support for the fibres and reduced the strain level at which fibre microbuclding occurred. Finally, recent theoretical models were employed to predict the compressive stress-strain response and strength of unidirectional laminates. It was found that although the theoretical models do not exactly predict the compressive strength of the laminate, they are sufficiently accurate for the cases examined.