| As the core vertical prestressed stress member of the bridge structure,the vertical prestress cannot be effectively controlled during the actual construction,resulting in vertical prestress that cannot meet the design requirements or even fail.In the process of long-term service,affected by environmental corrosion and load overrun,the effective prestress is further reduced,resulting in excessive downward deflection in the bridge span and cracks in the main beam,which seriously threatens the durability and safety of the structure.In this thesis,the effective prestress of rebar is detected by combining ultrasonic guided wave technology and acoustic elasticity theory,and the key issues such as guided wave propagation characteristics and acoustic elastic effects of rebar under different stress conditions are studied by combining theoretical analysis,finite element simulation and experimental research.According to the solid acoustic elasticity theory,the change law of guided wave L（0,1）modal wave velocity with rebar tension is studied,which provides a theoretical basis for non-destructive testing of rebar working prestress.This thesis mainly carries out the following research work.（1）The numerical simulation of ultrasonic guided wave propagation in spiral steel was carried out.Based on the theory of wave propagation in a slender cylinder,the frequency-wave number curve and theoretical dispersion curve of a circular bar are obtained by numerical calculation.The finite element model of screw steel was established by using finite element software,and the guided wave signal was extracted for time history and 2D-FFT analysis.The time domain and frequency-wave number curve of guided wave signal propagating in screw steel were obtained.The results show that the stress nemogram can intuitively observe the propagation law of guided wave in the bar steel,and the finite element frequency-wave number curve is very consistent with the theoretical solution in the range of frequency 0-0.6MHz and wave number 0-0.2mm-1,which proves the feasibility and correctness of the finite element simulation method adopted in this thesis.（2）Based on the equivalent acoustic elasticity method,the acoustic elastic effects of guided waves on bare bar steel are numerically simulated.According to the relevant indexes（equivalent Poisson’s ratio,equivalent density and equivalent elastic modulus）obtained by the method of equivalent acoustic elasticity,the acoustic elastic effect of the steel bar was studied.By extracting the acceleration time history signals under different equivalent tensioning forces,the wave velocity variation law of the steel bar was analyzed.At the same time,the change of the mode frequency-wave number domain of the screw steel under different equivalent tensile forces is analyzed,and the change law of phase velocity is obtained indirectly.（3）An experimental study on acoustic elastic effect in rebar was carried out.The center frequencies of 50k Hz,75k Hz and 100k Hz were used as excitation signals to carry out the tension test and guided wave propagation test of the bar steel,and then the variation rule between the head wave velocity and the tension was analyzed.The results show that the flight time of the head wave of the rebar decreases with the increase of the tensile force under different frequency excitation.The difference of flight time between loading and unloading under low stress is mainly caused by the residual stress of rebar after unloading.（4）The influence of excitation frequency,elongation and excitation power on guided wave propagation and acoustic elastic effect in rebar was investigated experimentally.With the increase of excitation frequency,the first wave velocity of rebar guide wave decreases,which is consistent with the theoretical dispersion curve.With the increase of tensile force,the influence of its elongation on the flight time of the first wave increases gradually.Under the working condition that has the least effect on the flight time,the change amount accounts for about 20%of the total change amount,and its influence cannot be ignored in the test.With the gradual increase of the excitation power,the amplitude of the guided wave only increases gradually in the time domain and frequency domain,while the shape and position of the waveform do not change,so the influence of the excitation power can be ignored in the test.（5）The numerical simulation and experimental study on the acoustic elastic effect of outsourced mortar steel bar are carried out.According to the equivalent acoustic elasticity method,the time-history signals of the node acceleration extracted from the finite element method of the outsourced mortar bar steel are analyzed.In the guided wave test,the center frequencies of 50k Hz,75k Hz and 100k Hz are taken as the excitation signals to analyze the influence of different outsourced thickness on the guided wave propagation and the sound elasticity effect.The results show that the relationship between the head wave velocity and the equivalent tensile force of the outer mortar of the bar steel is consistent with that of the bare bar steel,but the head wave velocity and energy are lower than that of the bare bar steel.The amplitude of the guide wave of the bar decreases with the increase of the thickness.Under the same thickness,the energy of guided wave decreases gradually with the increase of tensile force.Both the finite element and the experimental results show that the change law of first wave flight time increment and the change amount are basically consistent with that of bare bar steel. |
