Detection On Concrete Material Nonlinearity And Debonding Damage Based On Laser Ultrasonic Second Harmonic Generation Technology

Concrete is the most widely used building material in the field of civil engineering construction because of its high strength,simple operation process and low price.However,with the increase of service time,most of the concrete in service structure is in the early damage accumulation stage.Once the structure has obvious damage,the bearing capacity of the structure has been greatly reduced,so the early damage assessment of the concrete structure is particularly important.On the other hand,in the field of structural strengthening,the concrete members strengthened by FRP often have debonding damage at the interface due to the lack of reliable anchoring measures,which m akes the reinforced structure have obvious brittle failure.Aiming at the problems of early damage material characteristic evaluation of concrete and debonding damage nondestructive testing of FRP reinforcement,most of them are based on linear ultrasonic testing,which is not sensitive to early damage characteristics and small size debonding defects.Therefore,this paper proposes the principle of nonlinear behavior of materials and nonlinear interface contact of spring model to induce ultrasonic generation of second-order harmonics,which is based on laser ultrasonic simulation model The feasibility of this method to detect the early damage of concrete material and the debonding damage of FRP strengthened structure is demonstrated by the quasi equivalent p iezoelectric ultrasonic experiment.Based on the principle of virtual work,the equivalent integral form of the coupled differential equation is established,and the finite element form of the coupled equation is obtained by discretization.The fundamental and second harmonic displacements of the two-dimensional Rayleigh wave propagating in the nonlinear medium are derived by using the interface nonlinear reflection technique.The second harmonic response caused by the opening and closing effect of the interaction between the interface layered spring model and the ultrasonic wave is analyzed.The equivalent meso concrete model and the debonding damage model of FRP strengthened structure are established.In the meso concrete model,the three-phase random aggregate model is equivalent to a homogeneous unit with similar mechanical properties according to the specific size.The optimal unit size is 12.5mm through the probability statistical analysis,and the non-linear constitutive relation corresponding to the e quivalent mechanical parameters is given to each unit.The local delamination damage model of FRP concrete forms a delamination damage interface that allows contact but does not penetrate each other by creating boundary conditions of consistent pair and co ntact pair.In this paper,the technique of finite element simulation intensity modulation laser excited narrow band ultrasonic surface wave is proposed to detect the material nonlinearity of concrete models with different porosity and the debonding damage of FRP strengthened structures.It is found that with the degradation of the mechanical properties of concrete,the nonlinear mechanical behavior increases,which makes the nonlinear response more significant under the effect of ultrasonic perturbation.The nonlinear coefficients of concrete materials in different damage states are detected by the piezoelectric ultrasonic second harmonic experimental system,and the dynamic elastic modulus of concrete specimens is detected by the percussion resonance funda mental frequency system.The exponential function relationship between the nonlinear coefficients of concrete materials and the macroscopic mechanical properties is obtained by fitting.The sensitivity of the nonlinear coefficients measured by the second harmonic method is far greater than that of the total The linear frequency of vibration method verifies the feasibility and superiority of concrete material nonlinearity as early damage evaluation index.The interaction between intensity modulated laser ult rasound and peel damage model is simulated by finite element method.The location and size of peel damage are successfully located and detected by the nonlinear coefficient of FRP surface receiver signal and the spatial distribution characteristics of the second-order harmonic in the spectrum.A piezoelectric ultrasonic system was set up to detect the artificial debonding damage specimen strengthened by FRP.In the experiment,the position of debonding damage was successfully located according to the peak value of non-linear coefficient of the surface receiving point signal.In view of a misjudgment peak in the experiment,the improvement measure was to use non-contact probe to avoid the problem of low fundamental frequency amplitude caused by coupling.Furt her verify the feasibility of the method proposed in this paper to evaluate the early damage material characteristics of concrete and to locate the peel damage of FRP reinforcement.