Study On Interfacial And Fatigue Performance Of CFRP-strengthened Steel Structures Based On New Bonding Materials And Temperature Effects

In recent decades,severe fatigue cracks have been appearing one after another on in-service steel bridges.Externally bonded carbon fiber reinforced polymer(CFRP)can inhibit fatigue crack propagation and significantly improve the fatigue life of the structure.However,CFRP-strengthened steel structures are inevitably subjected to harsh environments(e.g.,high temperature,cyclic wet and dry and freeze-thaw cycles)during service.Especially under environmental high temperatures,the interfacial and fatigue performance is gradually degraded.This paper focuses on these critical issues and carries out a study on the interface and fatigue performance of CFRP-strengthened steel structures with a new bonding material considering the effect of temperature,which mainly includes the following:(1)A study of the mechanical properties of aluminum-to-aluminum epoxy interfacial nano-strengthening/toughing was carried out.The mechanical indexes of the adhesive were obtained by dynamic mechanical analysis(DMA)with the variation of temperature.Tensile-shear tests of aluminum-to-aluminum epoxy interfacial nano-strengthening/toughing single-lap joints were carried out at room temperature.The effects of nanofiber content and the number of nanofiber mat layers on interface performance were derived.By comparison of the bond strength,appropriate adhesive and the number of nanofiber mat layers were selected,and the interfacial nano-strengthening/toughing mechanism was revealed.(2)A study on the mechanism and temperature effect of CFRP-steel epoxy interfacial nano-strengthening/toughing was carried out.Tensile-shear tests for CFRP-steel double-lap joints bonded with nano-modified adhesive of different nanofiber content(0/1.5/4.5 gsm)were carried out at diffferent temperatures(25/40/55/70℃).The test results included the load-displacement relationship,failure modes and CFRP surface strain distribution.The degradation law of CFRP-steel interface mechanical properties at high temperatures was clarified.The bond-slip relationship of nanofiber content and temperature-dependent nano-strengthening/toughing interface was established.The excellent performance of the nano-strengthening/toughing interface in terms of bond strength,ductility and thermal resistance was demonstrated by comparing the ultimate load-bearing performance.The interfacial damage mechanism of CFRP-steel was revealed based on the digital image correlation(DIC)technique.(3)A study on the interface and fatigue performance of CFRP-strengthened damaged steel beams with film adhesive at different temperatures(25/40/55/70°C)was carried out.First,the thermal resistance of the film adhesive was demonstrated via DMA.Meanwhile,the bond-slip behavior between CFRP and steel at different temperatures(25/40/55/70°C)was obtained by tensile-shear tests of the bonded joints.Finally,the bond and fatigue performances of the CFRP-strengthened steel beams assembled with the film adhesive under elevated temperatures were investigated via numerical simulation.The results showed that the finite element model could accurately predict the static and fatigue properties of CFRP-reinforced damaged steel beams.As the temperature increased from 25°C to 40°C,55°C and 70°C,the fatigue life decreased by 26.3%,38.8% and 50.2%,respectively.Increasing the bond length and width could effectively improve the CFRP debonding resistance under the investigated temperature ranges.The fatigue life was hardly affected by increasing the bond length when the bond length exceeded 200 mm.