| Orthotropic steel bridge deck widely exists in bridge structure.Because of the influence of alternating load all the year round,fatigue damage at the joint of longitudinal rib and roof is the main failure mode.In the whole fatigue damage process,the early fatigue accounts for nearly 90% of the whole fatigue life.At present,there is still a lack of early fatigue damage detection methods.In addition,the fatigue damage evaluation methods of orthotropic steel deck are limited,focusing on the theoretical research of model prediction and macro crack treatment.Based on the above two points,this paper proposes to use the nonlinear ultrasonic technology to detect and evaluate the fatigue damage of orthotropic steel bridge deck.The nonlinear ultrasonic guided wave is used as the detection method,and the nonlinear parameters,the first wave energy and the sound speed are used as the indicators to carry out the detailed research from the two aspects of numerical simulation and experimental verification.The main contents are as follows:Firstly,the modules of the nonlinear ultrasonic testing system are tested.The frequency response curve of the ultrasonic probe is obtained by the frequency sweep test;the filtering efficiency of the nominal low-pass filter is tested,and the influence of the frequency 500 KHz signal in the transmitted signal is excluded;the influence of the output voltage on the test is explored,and the relationship between the square of the fundamental amplitude value and the harmonic amplitude under different voltage is obtained.Secondly,the fatigue crack of steel plate is identified by nonlinear ultrasonic guided wave.In the numerical simulation,based on the combine39 spring element,a finite element model of steel plate with macro defects and closed defects is established.In the three-dimensional numerical simulation,the nonlinear ultrasonic guided wave transmission process is simulated.The step-by-step scanning is used to identify macro defects and closed defects,and the local area is scanned for crack imaging.In the experimental study,ram-5000 is used Snap highenergy ultrasonic testing system is used to detect the macro defects and closed crack defects of steel plate.The relationship between the first wave energy and pseudo sound velocity,nonlinear parameters and macro defects and closed crack defects is obtained.The results of numerical simulation and test show that the first wave energy,pseudo sound velocity and nonlinear parameters can be used to evaluate the macro defects and closed crack defects of steel plate members.Thirdly,the differences of fatigue defects between orthotropic steel deck structure and its roof base metal are compared.In the third chapter,the fatigue crack modeling method and evaluation index of steel plate members are used in numerical simulation and experimental research.The two-dimensional numerical simulation is used to simulate the transmission process of nonlinear ultrasonic guided wave in orthotropic steel bridge deck.Then,the characteristics of nonlinear guided wave signal before and after fatigue loading are compared in simulation and experiment,which further shows that the closed crack in orthotropic steel bridge deck can make the bridge super The nonlinear response of the guided waves is obvious.At last,the change of detection index of orthotropic steel deck structure and its roof base metal sample under different fatigue damage degree is analyzed.In the process of fatigue loading of orthotropic steel bridge deck and its roof base metal,the changes of sound velocity,energy and nonlinear parameters under different fatigue damage degree are summarized.The instantaneous amplitude and frequency of the signal are obtained by the experimental signal processing method of HHT transformation.The nonlinear parameters can be used to evaluate the fatigue life of the joint between the longitudinal rib and the roof of orthotropic steel bridge deck,and the HHT transformation is more conducive to obtain the local characteristics of the signal. |
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