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.PhD in Manufacturin

Effect of hygrothermal cycles on mechanical performance of composite adhesively bonded joints

This paper numerically and experimentally studied mechanical performance of composite adhesively bonded single-lap joints in the presence of hygrothermal cycles, under static tensile loading. Joint performance was predicted by the development of a coupled experimental-numerical approach based on cohesive zone modelling. Composite adherends of aerospace grade carbon fibre-reinforced Hexply® M21/T800 pre-impregnated plies, bonded using a 25mm × 25mm bond overlap. Bond interface was exposed to cyclic moisture and temperature loads by introduction of 2mm sharp cracks at joint runouts. Pre-cracked joint specimens were subjected to hygrothermal cycles in environmental chamber under conditions representative of aircraft operational cycles. Testing proved that joint degradation occurred with increased cycle numbers. Strength reduced by 42% under static load after 714 cycles compared to unaged joints. Degradation accelerated in the initial 84 cycles, but was reduced for higher cycles attributed to adhesive bulk moisture saturation. Moisture diffusion parameters were characterised for both adhesive and composite subjected to hygrothermal cycles. Adhesive reached moisture saturation level of 1.54%wt, while composite laminate was 0.68%wt. In both cases, moisture diffusion followed Fick's second law. Displacement-diffusion analysis determined effect of moisture on elasticity of adhesive. This analysis plus the single-lap test data were coupled to develop degradation parameters required for CZM, demonstrating an 87% accuracy at 714 hygrothermal cycles