Effects of hygrothermal history on the structural performance of aerospace composite materials: Preliminary experiments and mass diffusion models

Experimental research was conducted to characterise the effects of hygrothermal history on the structural performance of Hexcel™ IM7/8552 composite laminates. The laminates were first conditioned with a series of absorption/desorption cycles at accelerated hot/wet condition (70 °C in water). Gravimetric techniques have been used to measure the weight variation and consequently record the moisture content and the material saturation. After various cycles of absorption and desorption, changes to key structural properties such as stiffness, glass transition temperature (T_g), and tensile strength of the materials were evaluated via Differential Scanning Calorimetry (DSC), resonant vibration testing and tensile testing. Scanning Electron Microscopy (SEM) was also conducted to inspect for physical damage to the composite. Gravimetric test results have clearly shown Fickian diffusivity and saturation to increase with each hygrothermal cycle. The effective diffusivity was also found to increase. This suggests that a potential permanent change in the material can occur in a composite laminate when subjected to hygrothermal history, whilst stiffness and tensile strength are virtually unaffected. Besides experimental work, Finite Element modelling was performed to study the complex moisture diffusion in thick laminates containing ply-drops. The model is complementary to the experiments as it allows the analyst to understand the "preferred paths" of moisture diffusion and to understand which parts of the structure are likely to saturate first.