Mechanical degradation of composite structures subjected to environmental effects.

Polymeric materials have inherent advantages thanks to the mechanical properties that they lend to a structure enhancing its useful life in factors of safety, reliability and aesthetics. Nevertheless, the durability may be affected by other considerations including environmental attack resulting in unexpected failures and maintenance costs, making it therefore essential to accurately predict the overall performance of these structures. This study was designed to evaluate the joint strength of an adhesively bonded composite Single Lap Joint (SLJ), exposed to a hostile environment i.e. cycles of temperature and moisture, mechanical damage and fatigue. The aged joints under hygrothermal cycles were tested under static and dynamic loads. A combined experimental-numerical Cohesive Zone Model (CZM) was calibrated to predict the joint strength degradation, and damage propagation. The composite SLJ of T800/M21 bonded with FM94 was subjected to hygrothermal cycles in an environmental chamber (maximum 70 °C and minimum - 20 °C), at maximum 85 % Relative Humidity (RH). The results showed that the strength degraded consequent to the increasing number of cycles. The strength reduced by 42 % under static load after 714 cycles in comparison to unaged joints. The fatigue life was evaluated at 30%, 40% and 45% ultimate static load to a maximum of one million cycles, resulting in a continuous fatigue life reduction with the increase in the number of aging cycles. A characterisation of the moisture diffusion parameters was performed on adhesive (FM94) and composite laminate (T800/M21) subjected to hygrothermal cycles. A displacement-diffusion analysis was conducted to determine the effect of moisture on the elasticity of the adhesive. The displacement-diffusion model results and shear lap test results were employed to establish the degradation parameters of the CZM, thus predicting the degradation of the joint with an accuracy of 13 % at 714 hygrothermal cycles.