Experimental investigation on mechanical behaviour of FRP-to-steel adhesively-bonded joint under combined loading – Part 2: After hygrothermal ageing

This paper presents the second part of the research project, focusing on the mechanical behaviours of the FRP-to-steel adhesively-bonded joins after hygrothermal ageing. To investigate the moisture and temperature effects on mechanical degradation of adhesively-boned joints, the adhesive joint specimens were immerged in a hot/wet environment (40°C water condition) for 4 months. After hygrothermal ageing, series of loading tests under six angle loading conditions were conducted by employing the specific tensile/shear loading device (developed in Part 1, the companion of this paper). Mechanical behaviours of aged adhesive joints were studied and compared with the un-aged adhesive joints (results of Part 1), regarding to failure modes, ultimate failure loads, stiffness as well as tensile–shear failure criterion. It revealed that the hygrothermal ageing significantly decreased the ultimate failure loads of aged adhesive joints under shear and tensile loading. However, for the tensile/shear combined loading, the environmental degradation was not that obvious. The failure criterion curves of un-aged and aged adhesive joints were close to each other. The stiffness of adhesive joints was significantly influenced by the hygrothermal ageing under all six loading conditions. With regard to the failure modes, for the tensile and tensile/shear combined loading conditions, the hygrothermal ageing switched the failure mode of adhesive joints, from the partial interfacial failure between the FRP sandwich deck and the adhesive layer to the full FRP delamination in FRP laminates and fully covered fibre breaking area. For the shear loading condition, the same failure mode (cohesive fracture in the adhesive layer) was obtained before and after the four-month hygrothermal ageing.