Prediction of the full debonding process of mixed-adhesive FRP-to-substrate joints through a new analytical method

Rehabilitation and retrofitting of existing structures using externally bonded fibre-reinforced polymers (FRP) have become increasingly popular. A common failure mode in such strengthened systems is the debonding of the FRP laminate from the substrate. To address this, various techniques have been developed to prevent or delay debonding failures. One such approach is the use of two adhesives with different elastic moduli, resulting in a mixed-adhesive joint. This technique is claimed to reduce stress concentrations at the plate ends, thereby delaying or preventing debonding failures. However, a detailed interfacial stress analysis, considering failure initiation and propagation within the bonded joint, has yet to be conducted to fully understand the effects of using a mixed adhesive. To address this gap, the present work proposes an analytical solution to describe the complete debonding process of FRP mixed-adhesive joints under mode II loading. This analytical solution is validated using the Finite Element Method (FEM), and several key parameters for mixed-adhesive joint design are identified. The results indicate that mixed-adhesive joints, compared to single-adhesive joints with a ductile adhesive, exhibit lower maximum load capacities. When the ductile adhesive is used as a loaded-end anchorage in the mixed-adhesive joint, the maximum load is higher than when it is used as an end anchorage. However, this configuration significantly reduces the ductility of the joint with the loaded-end anchorage.