The behaviour of CFRP-to-steel bonded joints under fatigue loading

Abstract

Flexural strengthening of steel beams using CFRP laminates has gained popularity within the structural engineering community due to the many advantages it offers over traditional strengthening methods. Such advantages include ease of construction, resistance to corrosion and minimal disruption to traffic. Performance of such strengthened systems relies on the interfacial shear stress transfer mechanism of the bonded interface. Many existing studies have investigated the behaviour of CFRP-to-steel bonded joints under quasi-static monotonic loading, however, few studies have been carried out on understanding the behaviour of such bonded joints under cyclic loading. Since behaviour of the bonded interface is of critical importance to the performance of such strengthened structures, studies aimed at investigating the behaviour of CFRP-to-steel bonded interfaces under fatigue cyclic loading is urgently needed.This study presents an experimental and theoretical investigation into the behaviour of CFRP-to-steel bonded joints under quasi-static cyclic and fatigue loading. The experimental study presented consists of thirteen single shear pull-off tests of CFRP-to-steel bonded joints, covering three different bond thicknesses and three types of loading (quasi-static monotonic, quasi-static cyclic and fatigue loading).The results are discussed in terms of their failure mode, load-displacement behaviour, CFRP plate axial strain distribution, interfacial shear stress distribution and bond-slip behaviour. Based on the existing experimental data on behaviour of CFRP-to-steel bonded joints under quasi-static cyclic loading, a damaged plasticity type bond-slip relation was developed to model the constitutive behaviour of the bonded interface under quasi-static cyclic loading. This was further extended by developing a model for damage accumulation rate as a function of the applied slip range and number of cycles, to account for damage evolution under fatigue cyclic loading.Following the current studies on the constitutive behaviour of the bond in this research under fatigue loading, an experimental and numerical study aimed at investigating and modeling the performance of CFRP strengthened cracked steel plates under fatigue cyclic loading was performed.The double-sided strengthened notched steel plates were used to simulate the cracked steel plates. CFRP plates were bonded on both sides of the plate over the crack. In total, five plate tests were carried out. The results were discussed in terms of their failure mode, fatigue life extension, CFRP plate axial strain distribution, shear distribution, bond-slip behaviour and the collaboration of debonding and steel crack. Based on the proposed fatigue damage propagation model, a simplified numerical methodology was developed to simulate and predict the fatigue life of the CFRP strengthened cracked steel plates. This methodology was verified with the test results. To achieve this goal, several stages and tasks were accomplished and documented as three distinct research papers.The main achievements of this thesis can be summarized as:· The damaged elasticity assumption commonly used on CFRP-to-steel bonded interfaces is not valid and a damaged plasticity type model is required to accurately model the full-range behaviour of CFRP-to-steel bonded joints under cyclic loading.· To account for damaged plasticity observed during the experimental tests for the single-shear pull-off test specimens, the damaged parameter was defined as a function of the ratio between the total dissipated energy and the critical fracture energy. Function for the damage parameter was then obtained using the best fit curve for the experimental results.· The Damage propagation rate for the specimens under fatigue loading was defined as a function of the damage parameter, maximum and minimum slip values of the loading cycle. Empirical parameters of the defined function were then calibrated using the experimental data.· Based on the proposed fatigue damage propagation model, a simplified theoretical model was developed to predict the fatigue life of the CFRP strengthened cracked steel plates. The simplified theoretical model was verified using the experimental data. This model could be used to design for CFRP strengthened cracked steel plates under fatigue cyclic loading.