Analytical criterion to prevent thermal overshoot during dynamic curing of thick composite laminates

Abstract

Local overheating during curing of thermosetting resins is likely to occur for thick laminates or during fast curing. Overheating may lead to heterogeneous mechanical properties along the laminate thickness or even to an uncontrolled reaction. To avoid overheating, most thermoset resin manufacturers recommend a “safe” cure cycle. However, these cure cycles can be improved to shorten cure times in thin laminates and may not be good enough to avoid overheating in thick laminates. In this paper, we propose a new analytical model to determine the critical thickness above which thermal runaway occurs when the laminate is heated at a constant rate up to a constant temperature. The model considers different thermal boundaries between the mould and the laminate, i.e., from a perfect thermal contact to a contact of infinite resistance. The analytical model was corroborated through the numerical integration of the equations governing it and experimental data from the curing process of a thick laminate composed of the commercial VTC401 epoxy resin and M55J carbon fiber system. Model predictions indicate that, under the manufacturer's recommended cure cycle, which includes an initial heating rate of 2 K/min, thermal runaway occurs in laminates thicker than 12.4 mm, aligning with experimental observations. A 20-mm-thick laminate, exceeding this threshold, was cured using a reduced heating rate of 0.3 K/min based on our criteria, successfully preventing overheating. The maximum temperature gradient recorded experimentally remained below 1 °C, confirming the model's prediction of uniform thermalization.