| The bridge suspender is composed of several steel bars with uniform thickness.Under the continuous erosion of the natural environment and long-time high tension,the surface of the suspender will produce fatigue microcracks.How to identify micro-cracks effectively has important theoretical significance and engineering application value for the evaluation of the carrying capacity of the deterioration behavior of the bridge structure in the safety state.For this kind of contact defects of the bar,the echo of traditional ultrasonic test is difficult to identify the defect wave signal,and the nonlinear ultrasonic testing technology has a very good effect.Vibro-acoustic modulation technology is one of the nonlinear ultrasonic detection technology,which is particularly sensitive to contact type defects,especially fatigue cracks.However,the nonlinear detection technology can only judge the existence of damage,and cannot identify the specific location of microcracks.Therefore,the time reversal method is introduced in this paper,the nonlinear signals detected by vibro-acoustic modulation technology were focused to realize the detection and location of microcracks in bars.The main research work and conclusions of the paper are as follows:1.The basic theory of vibro-acoustic modulation technology and time reversal method is studied,and the dispersion curve formula of straight and spiral rods are deduced.2.The modulation analysis of straight bar is carried out by using several groups of low frequency excitations with different frequencies,and it is proved that the motion state of the crack under low frequency excitation is open-close type.By comparing the white noise response of the right end of the rod with the relative displacement of the crack surface in the Z direction,it is found that the closer the low frequency excitation frequency is to the resonant frequency of the rod,the better the modulation effect is.3.The bars with micro cracks and non-destructive bars are simulated and tested.It is found that there are obvious first-order sidelobes,second-order sidelobes,harmonics and other nonlinear signals in the spectrum of the detection signal of the damaged bars.Then,the time reversal focusing processing of the nonlinear signal is carried out to locate the microcracks of the bar,which provides a reference for the parameter setting of the subsequent experimental detection.4.An experimental detection platform for vibro-acoustic modulation was built,the microcrack detection and location tests for straight and screw rods were carried out based on the vibro-acoustic modulation control method.The ultrasonic signal is modulated at the crack interface,and the nonlinear damage signals appear in the test results.It is proved that the vibro-acoustic modulation technique is suitable for the detection of micro-cracks in straight and spiral rods.At the same time,the time reversal focusing processing is carried out in the finite element software.The three bars with different microcrack positions all have time-reverse focusing points,and the error is less than 6%.The method of combining vibro-acoustic modulation test with finite element time reverse focusing can effectively realize the location of microcracks in the rod.As a new type of ultrasonic nondestructive testing technology,the adaptive focusing ability of vibro-acoustic modulation and time reversal process provides a new idea and approach for enhanced detection and local localization of small damage of bridge suspender. |
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