| Bolt anchorage support technology is widely used in mining engineering and geotechnical engineering at home and abroad.Ultrasonic guided wave detection technology has the advantages of long detection distance,small attenuation and simple detection process,but ultrasonic guided wave also has the characteristics of multi-mode and dispersion,which makes its propagation in the structure very complex.There is great blindness in experimental research,which leads to the increase of research cost.Numerical simulation has the advantages of low cost,high repeatability and easy operation.Therefore,this paper mainly adopts numerical simulation method and supplemented by specimen test experiment to study the propagation characteristics of longitudinal ultrasonic guided wave in the bolt.Firstly,the basic theory of ultrasonic guided wave is introduced,the frequency and displacement equations of axisymmetric longitudinal mode ultrasonic guided wave in free bolt are solved numerically.Then,the numerical model of ultrasonic guided wave propagation in bolt is given and the propagation of specific high-frequency ultrasonic guided wave in free bolt in 1MHz ~ 3MHz is simulated.Then,the propagation of low frequency(10kHz ~ 180kHz)guided ultrasonic wave in bolts of different anchorage quality was studied by a specific experimental device and verified by numerical simulation,at the same time,the numerical simulation of bolt with different anchorage length and thickness is also carried out.Finally,the method of loading the displacement load at the excitation end of plate and free bolt respectively according to the wave structure of center frequency is used to try to excite the ultrasonic guided wave of a single mode.According to the research results of this paper,the important conclusions are as follows:1)The dispersion curves of phase velocity,group velocity and displacement distribution of each mode at a specific frequency are obtained by solving the equation.The phase velocity,group velocity curve and displacement curve of each mode are analyzed.It is found that the displacement distribution of the frequency corresponding to the maximum group velocity of each high order mode is very similar and its axial displacement is zero at the bar edge,these frequencies are suitable for bolt detection.2)The simulation results of high-frequency ultrasonic guided wave propagation in free bolt basically agree with the theory.When the group velocity of the high-order mode reaches the maximum,the propagation time of the wave packet corresponding to the mode in the time domain waveform is obviously shorter than that of other modes and will not overlap with the wave packet of other modes.Therefore,the frequency detection bolt corresponding to the group velocity of the high-order mode has great advantages in signal processing and analysis.At the same time,the simulation results also show that the ultrasonic guided wave of mode L(0,1)at high frequency has smaller attenuation and longer propagation distance.3)The experiment shows that the propagation speed and attenuation of ultrasonic guided wave of the same frequency are different in bolts with different anchorage quality,and the anchorage quality of bolts can be judged according to such difference.The ultrasonic guided wave diffracted at the junction of free bolt and anchor solid and the propagation of the diffracted wave was analyzed.The method of calculating anchoring length and thickness of bolt with time domain waveform and group velocity dispersion curve is presented.4)In a certain frequency range,the method of loading the wave structure according to the central frequency cannot excite the ultrasonic guided wave of a completely single mode in the plate and free bolt,but it can make the signal energy concentrate on the target mode to a certain extent,which is beneficial to increase the detection length. |
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