Creep of bonded FRP-strengthened metallic structures at warm service temperatures

Abstract

Bonded Fibre-reinforced polymer (FRP) plates are becoming a widely used method in rehabilitating and strengthening existing structures due to their structural and economic advantages. Research on the behaviour of FRP-strengthened structures at ambient temperatures has been conducted extensively in the past. Nevertheless, the performance of these strengthening systems at warm service temperatures (<100°C) still remains largely uncertain. The bonded strengthening method relies critically upon the structural adhesive to transfer load between the FRP plate and the metallic structure. When the ambient temperature approaches the glass transition temperature (Tg) of the adhesive; however, the adhesive becomes soft and shows more obvious viscoelasticity. The creep rate will increase dramatically, which could have a significant effect upon its long-term service performance. This thesis found that the viscoelastic creep of the adhesive at warm temperatures can significantly affect the performance of FRP-strengthened metallic structures, which is necessary to be examined in design.

The present thesis examines a typical strengthening adhesive to investigate the effect of adhesive thermo-viscoelasticity. The response of the adhesive was determined using a series of tests using the multi-frequency scanning mode of a dynamic mechanical analyser. The linear and nonlinear viscoelastic constitutive models were then developed, which were in turn used within two finite element beam models to examine the effects of linear creep and nonlinear creep in the adhesive at warm temperatures on the performance of a lab-scale carbon fibre-reinforced polymer (CFRP) plate strengthened steel beam and a real-scale CFRP plate strengthened cast-iron beam, respectively.

The study found that linear viscoelastic creep of the adhesive bonding layer causes an increase in the slip between the FRP and the structure, which could induce damage in the bonded joint and make the CFRP becomes less effective, potentially resulting in failure of the strengthening system during the long-term service. Compared to a model using linear creep, a model incorporating nonlinear viscoelastic creep does not predict higher joint damage proportion, and will only lead to a slightly larger joint slip (maximum 1.0% after 1 year) and a slightly lower CFRP axial stress (maximum 2.4% after 1 year), which has a limited impact on the structural performance. The effect of differential thermal expansion (DTE) is more significant than that of nonlinear creep, which may be potentially beneficial by maintaining the tensile stress in the strengthening FRP plate under the constant temperature condition. In most cases, the simpler linear viscoelastic constitutive model is sufficient to analyse the performance of the FRP-strengthened metallic beam at warm temperatures.

The study also found that under cyclic temperature and cyclic load, the performance of the FRP-strengthened structure during the first year of its service life is crucial, and the joint creep is mainly related to the applied cyclic temperature. A simpler equivalent approach is proposed, which is able to accurately predict the effects of cyclic temperature and cyclic load with an error of less than 5%. The effect of differential thermal expansion becomes detrimental to the effectiveness of strengthening at the cyclic temperature as it can increase the joint slip deformation when the temperature grows and bring a higher moment on the metallic beam when the temperature reduces.