Mechanical degradation of composite bonded joints subjected to environmental effects

Abstract

The purpose of the current study was to evaluate the joint strength of a composite single lap joint (SLJ) adhesively bonded, exposed to a hostile environment. The aged joints under hygrothermal cycles were tested under static and fatigue load. A combined experimental-numerical cohesive zone model (CZM) predicted 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% RH.The mechanical test results showed that the strength degraded with the increase in the number of cycles. The strength was reduced by 42% under static load after 714 cycles when compared to unaged joints. The degradation was accelerated in the initial 84 cycles but fell away slightly after this point. The fatigue life was evaluated at 30, 40 and 45% (ultimate static load) to a maximum of 1,000,000 cycles, resulting in a continuous fatigue life reduction with the increase in the number of ageing cycles. A characterization of the moisture diffusion parameters was performed on the adhesive (FM94) and composite laminate (T800/M21) subjected to hygrothermal cycles. Finding that the adhesive reached its moisture saturation level of 1.54% wt, while composite laminate was 0.68% wt, in both cases, the moisture diffusion obeyed Fick's second law. A displacement-diffusion analysis was conducted to determine the effect of moisture on the elasticity of the adhesive, predicting an elasticity modulus reduction of 20%.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.