Effects of moisture absorption on the different modes of carbon/epoxy composites delamination

Abstract

It is essential that moisture attack be considered when characterising interlaminar fracture toughness. However, limited work has been reported on the correlation between interlaminar fracture toughness of mixed mode I/II ratios and moisture content. Therefore, the present study aims to characterise the delamination of carbon/epoxy composites, immersed in distilled water at constant 70 °C. The specimens are tested at various water absorption levels using mode I, mode II and mixed-mode I/II loading configurations. The results revealed that mode I fracture toughness is insensitive to the moisture attack. However, approximately 50% drop in mode II and mixed-mode I/II fracture toughness are observed at high moisture content levels. Fibre bridging is also found in all wet specimens with an increment of up to 60–100% in fracture energy being observed for the tested water absorption levels. Upon water exposure, fractured surfaces are also found to possess a more porous matrix with a more significant fibre/matrix interface debonding. Through the study, a new three-dimensional criterion is proposed to characterise the variation of fracture toughness with both mode ratio and moisture content. The criterion is expected to allow for a better fundamental understanding with respect to the extent of moisture attack towards delamination of composite materials.