An experimental investigation on the influence of environmental ageing on strength and fatigue behavior of different single lap joints

It is essential to examine the performance of CFRP and aluminum specimens after being exposed to environmental aging and to develop a technique that can predict their mechanical function. However, there is limited research on the static and fatigue strength of adhesive, mechanical, and hybrid joints (adhesive/mechanical) after environmental ageing. The study aims to examine the effect of thermal and hygrothermal ageing on the static and fatigue performance of three-point bending joints. Three groups of samples were prepared: I) unaged, II) thermally aged, and III) hygrothermally aged. The bending performance of three different types of joints (bonded, bolted, and bonded/bolted) was evaluated by conducting bending tests using three distinct types of adherends: ATA (Aluminum-to-Aluminum), CTC (Composite-to-Composite), and ATC (Aluminum-to-Composite). Also,the strength of the joints was measured and assessed for both static and fatigue (at three different levels of loading using the new fixture) loading. Static tests showed that bonded/bolted CTC specimens had the highest strength, and hygrothermal and thermal conditions significantly affected the static performance of the three types of joints and substrates. Under fatigue load, the highest fatigue life was observed in bonded, bolted, and bonded/bolted specimens for ATA, ATC, and CTC, respectively. In a bonded/bolted single lap joint, the static and fatigue strength consists of two steps: the initial step corresponds to the strength of a bonded specimen, and the next step starts when the bonded joint fails and fits the strength of a bolted joint. Finally, both ageing and static load percentage reduced the fatigue life and shifted the failure mode of the joints.