The influence of CFRP fatigue behavior on CFRP-to-steel bonded joints

The application of externally bonded CFRP reinforcements has shown its effectiveness in reducing crack growth in steel elements. However, externally bonded CFRP-to-steel joints may exhibit fatigue degradation when subjected to repeated cyclic loadings, consequently leading to cohesive debonding within the adhesive layer. Therefore, the cyclic degradation of both adhesive and the composite needs to be properly investigated. This work presents the preliminary results of an experimental and numerical investigation on the fatigue damage behavior of CFRP and its possible influence on the load response of CFRP-to-steel bonded joint. The overall fatigue degradation of the joint under single-lap direct shear (DS) conditions is numerically modelled considering separately the damage occurring in the adhesive layer and in the composite patch. The behavior of the adhesively bonded joint is described through a cohesive zone model approach, while the composite damage behavior is described by adopting a fatigue residual stiffness model based on macroscopic stiffness degradation. Experimental results of tensile fatigue tests on rectangular CFRP coupons and of cyclic single-lap direct shear tests are presented and used for the model parameters calibration. Then, the numerical response of DS tests obtained considering the composite material either elastic or accounting for the fatigue damage is compared. The interfacial behavior maintains its role as leading failure mechanism. The introduction of the composite damage behavior shows up in a reduction of the number of cycles at failure, especially for longer fatigue lifetime, while the relative slip between the two adherends is not or slightly affected.