Analytical solution of the full-range behavior of adhesively bonded FRP-steel joints made with toughened adhesives

Fiber-reinforced polymer (FRP) composites represent an effective solution to strengthen and retrofit existing steel members. Namely, bonded or unbonded carbon FRP (CFRP) plates have been employed to improve the strength, fatigue behavior, and durability of steel bridges. In bonded solutions, the effectiveness of the CFRP reinforcement strongly depends on the adhesive employed to bond the plate, as failure usually occurs due to debonding. Within this framework, the use of toughened adhesives is particularly attractive since they may improve the load carrying capacity of the CFRP-steel interface, also providing a certain ductility. Debonding in CFRP-steel joints was previously studied using a cohesive approach. However, solutions able to describe the full-range behavior of joints with toughened adhesives and finite bonded length are not available in the literature. In this paper, a trapezoidal (trilinear) cohesive material law (CML) is employed to model the bond behavior of pultruded carbon FRP-steel joints made with a rubber-toughened epoxy adhesive, which showed cohesive debonding within the adhesive layer. The analytical solution provided is employed to study the experimental response of nine CFRP-steel joints tested using a single-lap direct shear set-up. Comparisons of analytical and experimental results of joints with three different bonded lengths confirm the effectiveness of the solution proposed.