Influence of composite damage on CFRP/steel bonded joint behaviour under cyclic loadings

High performance adhesives, with augmented fracture toughness, allow to enhance the stress-transfer mechanism in CFRP/steel joints, thus increasing the rate of exploitation of the reinforcement. While existing approaches assume that the mechanical properties of the composite are not affected by the cyclic loading, at large stress levels, close to their static strength, CFRP materials could instead exhibit fatigue damage. In this paper, the influence of CFRP cyclic damage on the fatigue behaviour of CFRP/steel bonded joints is investigated both experimentally and numerically. A residual stiffness method is adopted to model the cyclic nonlinear behaviour of the CFRP lamina. This model is coupled with a cyclic cohesive law accounting for the behaviour of the CFRP/steel interface. The models’ parameters are calibrated using cyclic tensile tests on CFRP coupons and single lap direct shear fatigue tests of the CFRP/steel joints. Parametric analyses are performed by varying the main design parameters. The results show that fatigue damage in the CFRP reinforcement plays a significant role in the joint response. Failure occurred by thin-layer cohesive debonding within the adhesive, localized near the CFRP side. Composite fatigue degradation was found to significantly reduce the number of cycles to failure, especially at low fatigue load ranges.