Effects of temperature on the adhesive bonding in steel beams reinforced with CFRP composites

Abstract

The use of carbon fibre reinforced polymer (CFRP) composites in strengthening and repairing steel beams and girders in existing structural bridges has become wide spread in recent years. Though the previous studies had showed significant improvement in the load carrying capacity and the fatigue life of the strengthened structures, they did not take into consideration the effects of extreme summer temperature on the behaviour of the reinforced structure because they were carried out at room temperature. The change of temperature affects the mechanical properties of the adhesive epoxy. Moreover, thermal stresses are induced in the adhesive bond due to the significant difference in the coefficients of thermal expansion between CFRP and steel materials. The main aspects investigated in this thesis are the changes in the adhesive mechanical properties due to the change in the temperature and the impact of these changes on the failure mechanism, the failure load, the fatigue life and the stresses distributions of steel beams strengthened with CFRP plates.

In order to achieve the aims of the research, the following procedures are carried out: First, tensile tests on adhesive specimens were carried out at different temperatures to investigate adhesive properties temperature dependency. Second, static tests were conducted on both double-lap shear and three-point bending specimens at temperatures ranging between 20 to 60°C to examine the influence of temperature on the failure mechanism and the failure load. Third, an analytical model was developed to calculate the interfacial stresses in the reinforced beam by approximating the adhesive nonlinear behaviour at elevated temperatures with the elastic perfectly-plastic representation. Fourth, finite element analysis was conducted to validate the analytical results assuming bilinear elastic-plastic adhesive behaviour. Finally, fatigue tests were carried out on both double-lap shear and three-point bending specimens at different load and temperature ranges to identify their fatigue life temperature dependency.

Adhesive materials showed significant reduction in the strength and the stiffness coupled with nonlinear behaviour as the temperature reaches the adhesive glass transition temperature (Tg). Thus, the failure load and fatigue life of the strengthened structures were reduced significantly at Tg with a change in the failure mechanism. Both the analytical and finite element results were capable of defining the length of the plastic zone occurring at the plate end due to the high shear stress concentration. The length of the plastic zone depends on several parameters, including the elastic modulus and the thickness of the CFRP plate, the applied load and the temperature. The study of these parameters shows that it is possible to avoid CFRP plate debonding at extreme temperature by using thinner and longer CFRP plates with high modulus instead ultra-high modulus.

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