Fracture Mechanical Analysis Of The Failure Behavior Of Interface Between FRP And Concrete

The key factor leading to the failure of reinforced concrete(RC)structure strengthened with fiber reinforced polymer(FRP)is the debonding of the interface between FRP and concrete.The research on the failure mechanism of FRP–concrete interface has important scientific significances and wide application prospects in the development of FRP strengthening technique and FRP–concrete composite structure.The traditional strength theory cannot accurately describe scientific problems such as the failure behavior after interface fracture,which was based on the bond–slip relationship of FRP–concrete interface.Therefore,the fracture behavior of FRP–concrete interface was analyzed and evaluated with Fracture Mechanics.On the basis of experimental research,theoretical analysis,and numerical simulation,following issues were discussed:a new double shear experimental method of fully-bonded FRP–concrete interface;measurement method of the interfacial crack energy release rate,the initiation and propagation of interfacial crack,and the characterization and evaluation method of interface fracture behavior.The main contents and conclusions of this thesis were as follows:(1)Experimental study on the double shear test of fully-bonded FRP–concrete interface.Based on the theoretical analysis of the traditional double shear test results and several groups of comparative experiments,the double shear test of fully-bonded FRP-concrete interface was successfully realized,using the developed devices including a fastening cap to limit the axial displacement and three clamping devices to limit the lateral displacement of the concrete block.The displacement field and strain field of FRP surface were obtained by DIC technology,and the bond-slip experimental curves were obtained.Experimental results showed that:1)the debonding failure of FRP-concrete interface was between the penetration layer of adhesive and the surface of concrete block;2)the interfacial shear stress has a reverse phenomenon,and the descending section of the bond-slip curves has a large dispersion;3)the displacement field of FRP surface has a fluctuation distribution;4)the discontinuous microcracks at the interface are the fundamental reason for the reverse phenomenon of interfacial shear stress and the fluctuation of the displacement field on the FRP surface.(2)Single side J-integral method of interfacial crack.Based on the vision of interface fracture theory,according to the characteristics of FRP-concrete interface debonding failure,a single side J-integral method was proposed,of which periodic integral contours were only set on the concrete side of CFRP–concrete interfacial crack.On this basis,the influence of concrete crack near the interfacial crack and interface mismatch on the calculation results of single side J-integral were discussed,and a?J criterion of interfacial crack unstable propagation was established.(3)Critical energy release rate of mode II interfacial crack between FRP and concrete.According to the definition of energy release rate,on the basis of the above-mentioned double shear test method of fully-bonded FRP-concrete interface,a testing method for the critical release rate of mode II interfacial crack was proposed.Double shear tests of the fully-bonded FRP–concrete specimens with different pre-crack lengths were carried out.The corresponding load–displacement curves were obtained,and the critical energy release rate of FRP–concrete interfacial crack was determined.The research results not only provided reliable experimental data for existing semi-empirical fracture energy model,but also determined the threshold value for the?J criterion of the interfacial crack unstable propagation.(4)An evaluation method for debonding failure behavior of FRP–concrete interface.To verify the feasibility of the single side J-integral method,four groups of double shear specimens with different pre-crack lengths(a₀=0 mm,20 mm,40 mm,60 mm)were taken as the research objects.The displacement and strain fields of the concrete side of the interface were obtained by DIC technique.A calculation program of the single side J-integral of the interfacial crack was compiled,and the single side J-integral-displacement curves were obtained.The results showed that the relative error between the absolute value of critical?J and the critical energy release rate G_side-c of the 17 integral paths of 4 specimen groups was only 5.4%,which indicated that the evaluation method of FRP–concrete interface debonding failure behavior based on the established?J criterion is effective.(5)Numerical analysis of the failure behavior of FRP–concrete interface.The double scalar damage model of concrete was introduced to compile a nonlinear finite element program.The effectiveness of the program was verified by the experimental data of bearing load and crack propagation direction of different specimens.On this basis,a finite element model of FRP-concrete interface failure behavior was established.The model was verified by the data of single shear tests and three-point bending tests of concrete beams strengthened with FRP.The model was used to analyze the reason of different failure phenomena in double shear test of FRP–concrete interface under different boundary conditions.The precise expression of interfacial bond–slip relationship and the numerical solution of single side J-integral were obtained.It was proved that the concrete crack near the interface has interference on the single side J-integral,and the?J criterion can also be used to evaluate the unstable propagation behavior of interfacial crack in finite element analysis.In addition,compared with other numerical models,this model can also reproduce detailed information,such as the Strain Transition phenomenon of FRP in single shear specimen,and the first peak and valley values of load–deflection curves in three-point bending specimens.