Research On The Effects Of Adhesive Properties On The Failure Behaviours Of CFRP-to-steel Interface

At present,a large number of steel structures,such as steel bridges,offshore platforms,large mining equipment and steel structural buildings,need be strengthened or repaired,due to corrosion or fatigue.The strengthening technique with externally bonded CFRP(Carbon Fiber Reinforced Polymer)is widely used in the field of strengthening steel structures,which possesses many advantages,such as easy installation,ignorable increased loading and without residual stress.Debonding of CFRP plate from the steel substrate is one of the main failure mode in CFRP strengthened steel structures.Therefore,the bond behavior of CFRP-to-steel interface is the most critical issues for CFRP strengthened technique.The properties of adhesive has large influence on the bond behaviours of CFRP-to-steel interface,so this paper focuses on the the bond behaviours of CFRP-to-steel interface with different kinds of adhesive.In additional,the effect of temperatures on bond behaviours of CFRP-to-steel interface is presented.Based on the research of bond behaviours,the flexural capacity of CFRP strengthened volume-distributed H-shaped beams is studied.The main contents of the dissertation are given as follows:According to the single lap shear test on CFRP-to-steel bonded joints,experimental study on the effect of adhesive properties on bond behaviours of CFRP-to-steel interface is presented.The damaged process and failure modes are analyzed.The law of load-displacement relationship,CFRP plate axial strain distribution,and bond-slip relationship are obtained based on the test results.The test results show that the failure of CFRP-to-steel with low strength adhesive is ductile failure and the failure of CFRP-to-steel with high strength adhesive is brittle failure.The stiffness of interface increases with the adhesive shear modulus.The bond-slip relationship depends on the properties of adhesive(linear or nonlinear).For linear adhesive,it contains ascending branch and descending branch.However,a plateau state exists except above two states.Based on the test results and combining to the literature data,four bond-slip constitutive model are proposed with four kinds of adhesive.Based on the proposed bond-slip constitutive model,an analytical solution is obtained.to interfacial stress distribution and the full-range behavior of CFRP-to-steel bonded joints is analysed.A method of using nonlinear spring element to simulate adhesive layer is proposed.Finite element analysis is adopted to analyse the bond behaviours of CFRP-to-steel interface using ANSYS software.According to the comparison of computed results and test results of load-displacement relationship,it is demonstrated that using nonlinear spring element to simulate adhesive layer is effective.The parametric study is adopted on the effect of FRP Young's modulus on the effective bond length and ultimate load.It is found that the effective bond length increases linearly with FRP Young's modulus and the ultimate load increases nonlinear with FRP Young' modulus.According to the single lap shear test on CFRP-to-steel bonded joints,the effect of temperatures on bond behaviours of CFRP-to-steel interface is studied,and the law of failure modes,load-displacement relationship,strain distribution,bond-slip relationship,and ultimate load are obtained.The theoretical degradation model and bilinear simply model of interfacial ultimate load is developed.Experimental study on flexural capacity of volume-distributed H-shaped steel beam strengthened with CFRP is conducted,including the failure process and failure modes.The strengthened effect is analysed in terms of the stiffness,ductility coefficient and ultimate load,it is shown that The strengthening technique with externally bonded CFRP is fit for strengthening steel structures.The high strength adhesive is proved to be suitable to strengthen steel structures.According to finite element analysis,it is found that the bond-slip of interface has a very limited influence on ultimate load.Based on above analysis,a design method,which doesn't consider bond-slip,is developed of flexural capacity on volume-distributed steel beam strengthened with CFRP for engineering application.