Effect of ageing temperature and fibre orientation on hygrothermal degradation of aerospace-grade carbon fibre epoxy laminates

Abstract

The long-term durability of carbon fibre-reinforced polymers (CFRPs) in aerospace applications is challenged by hygrothermal ageing, which can degrade structural performance. While accelerated ageing protocols are widely used, it remains unclear whether conditioning temperature and humidity directly affect mechanical degradation or simply influence the rate of moisture uptake. This study investigates the effects of both fibre orientation and hygrothermal conditioning regime on the moisture uptake and compressive behaviour of aerospace-grade CFRP laminates. Three layups ([0/90]8s, [45/-45]8s, and [0/90/45/-45]4s) were conditioned at 60°C and 80°C under 90% and 100% relative humidity. Microstructural damage, including void growth, interfacial degradation, and microcracking, was confirmed via X-ray CT and SEM. DMA indicated matrix plasticisation with a 13% reduction in Tg. The key finding is that compressive strength and modulus are governed solely by the final moisture content—not by the conditioning temperature or humidity. Lower temperatures required longer exposure to reach saturation, whereas higher temperatures accelerated moisture uptake without altering mechanical response. This insight enables accelerated conditioning protocols to simulate long-term exposure reliably. Aged specimens showed up to 35% strength loss when tested at 80°C and over 90% at 200°C. Modulus declined more gradually, indicating lower sensitivity to defect-driven damage. CT scans revealed a shift from fibre kinking to matrix-dominated failure at elevated temperatures. Fibre architecture influenced degradation severity: [0/90/45/-45]4s retained more strength, while [45/-45]8s showed pronounced modulus loss due to reliance on matrix shear stiffness. These results offer new insight into ageing protocols and highlight moisture content as the dominant factor governing compressive performance in hygrothermally aged CFRPs, guiding future durability assessments of CFRPs.