Effect of curing overheating on interlaminar shear strength and its modelling in thick FRP laminates

During the curing process, the exothermic crosslinking reaction inside epoxy resin-based composites causes overheating and, eventually, a thermal degradation of the matrix. Thick sectioned epoxy composites require an optimized curing cycle in order to minimize the temperature raise inside the composite. However, the recommended cycle for thin laminates is often wrongly used also for thick sectioned composites, resulting in an overheating. In this work, the mechanical behaviour of laminates having different thicknesses was investigated. Specimens were cured according to a curing cycle optimal for thin laminates which causes overheating in thicker laminates. The curing temperature histories at several positions inside the laminates were monitored and recorded and, following a new methodological approach, a series of comparative interlaminar shear tests was performed. Experimental results show a decrease of the interlaminar shear strength in thicker laminates. A simple model to account for the detrimental effect of the exothermic peak on the interlaminar shear strength is proposed.