On the interplay of surface pretreatment, hygrothermal ageing and failure mode of adhesively bonded CFRP/aluminium composites

This study investigates the interaction between near-infrared laser surface pretreatment and hygrothermal ageing of carbon fibre-reinforced polymer (CFRP) and aluminium adhesive bonds to identify their influence on shear strength and fracture behaviour under quasi-static and dynamic loads. Pristine single-lap joints with laser surface structures exhibit higher shear strength (14 – 15 % quasi-static, 13 – 23 % fatigue) then acetone cleaned samples due to enhanced absorption of multi-axial loads by the three-dimensional surface topography. Regardless of the surface structure, hygrothermal ageing reduced the quasi-static bond strength by 14 % and significantly decreased the fatigue strength at higher loads. Quantitative evaluation of the fracture behaviour, along with the development of a weakest link model describing the system-type bond strength behaviour, enabled in-depth semi-quantitative analysis of failure mechanisms with respect to the complex interplay of pretreatment, ageing condition and mechanical load. Critical damage accumulation at the adhesive-aluminium interface was observed for acetone cleaned samples in high cycle fatigue (HCF). Laser pretreatment led to a shift towards cohesive substrate failure of the CFRP, which is further aggravated in HCF. Hygrothermal ageing mainly affects the epoxy-amine-based adhesive and CFRP-matrix. Consequently, cohesive strength was the failure dominating chain-link independent of the surface structure and applied load after ageing.