Fatigue performance of CFRP-steel joints bonded with toughened adhesive: experimental investigation and numerical modelling

Steel elements are prone to fatigue problems and the use of externally bonded carbon fiber reinforced polymer (CFRP) provides an efficient solution to enhance fatigue lifetime. Recently, a class of toughened epoxy adhesives was proposed in the literature, but their fatigue performance needs further investigations. In this study, the results of experimental tests on the bond behavior of externally bonded carbon fiber reinforced polymers plates to a steel substrate are then presented. A total of 9 CFRP/steel bonded joints, prepared using toughened epoxy adhesive, were tested using a direct shear single lap test set-up under monotonic, fatigue and post-fatigue loading scheme. Digital image correlation technique was used to measure the strain fields during the post-fatigue tests. Finally, a numerical model has been adopted, calibrated using the monotonic and fatigue tests results. Results have shown ductile behavior and high interfacial fracture energy of the toughened adhesive, resulting in high debonding load. Debonding occurred primarily at the composite/adhesive interface, during fatigue loading, then moved cohesively into the adhesive layer during subsequent post-fatigue stage. It has been also highlighted that, as the residual bond length is higher than the effective one, the post-fatigue strength was not affected by the cyclic loading. Experimental results are discussed in terms of load-global slip curve, joint strength, stiffness reduction, energy loss during fatigue cycles, CFRP axial strain distribution during the post-fatigue loading. Finally, numerical results fit well the outcomes of fatigue tests showing the reliability of the proposed numerical model.