| Ultrasonic guided wave Non-Destructive Testing technology(NDT)has become one of the most important methods to determine the grouting condition of anchored rock bolts,because of its non-Destructive,efficiently,economically,etc.However,in practical applications,most of the excited test waves will rapidly attenuate in the anchored structure,and the reflected signal from the bottom end of the bolt cannot be obtained,and thus the quality of the anchored structure cannot be judged.The main reason for this phenomenon is the dispersion and multimodality of ultrasonic guided waves,which makes the propagation characteristics of ultrasonic guided waves of different frequencies in the anchored structure completely different.In addition,the size and material parameters of the anchored structure are also significant influencing factors.Therefore,studying the propagation characteristics of ultrasonic guided wave in anchored rock bolt is very important to select the excitation waves in NDT testing.Theoretical calculation,numerical simulation,and comparisons with experimental results in the literature are used in this paper to study the propagation characteristics of ultrasonic guided waves in the frequency-domain and time-domain respectively,and the results are compared with the experimental test results in the literature.the main results and conclusions of this research are as follows.(1)The dispersion characteristics of ultrasonic guided wave propagation in anchored rock bolts is the important theoretical basis for selecting excitation waves for non-destructive testing.Dispersion equation is a transcendental equation and usually contains three variables: circular frequency,real wave number and attenuation.Therefore,it is difficult to solve the dispersion equation.In order to obtain dispersion characteristics of ultrasonic guided wave,the global matrix method was used to establish the dispersion equation of ultrasonic guided wave propagating in anchored rock bolt.Then the quadratic extrapolation and bisection methods were used to accurately solve the dispersion equations by dealing with the difficult problems brought on by the categorization,intersection,and bifurcation of the dispersion curves.Furthermore,programmers were written to calculate dispersion equations,and the results of this paper agree with the results output by Disperse,which is a foreign copyright software.These results verify the rationality of the global matrix method to establish the dispersion equation,the efficiency of the algorithm for solving the dispersion equation,and the correctness of the solving program.(2)Longitudinal mode,torsional mode and flexural mode are the three modes of ultrasonic guided waves,which have different attenuation properties.In this study,theoretical calculation and numerical simulation are used to calculate and analyze the propagation characteristics of these three modes ultrasonic guided waves with frequencies less than 2.5 MHz in the grouting rock bolt structure in use.The results indicate that longitudinal ultrasonic guided waves with a frequency greater than 500 k Hz are the most applicable modes for NDT testing because of low attenuation and easy generation,while longitudinal modal ultrasonic guided waves with a frequency less than 500 k Hz,and torsional mode and flexural mode ultrasonic guided waves with all frequency are not applicable for NDT testing because the high attenuation.(3)The radial size of the anchored rock bolt structure has a great influence on the propagation of low-frequency longitudinal modal ultrasonic guided waves with the frequency range of 0-100 k Hz.The excitation wave obtained by the laboratory test using the experimental model of finite anchoring thickness cannot be applied to the site anchored rock bolt with infinite thickness.The research results indicate that low-frequency ultrasonic guided waves with low attenuation and long propagation distance can be obtained from experimental model with finite anchoring thickness.However,these “low-frequency optimal guided waves” are not appliable to on-site anchored rock bolts because of high attenuation.(4)The testing age of mortar rock bolts has a great influence on the Non-Destructive Testing results using low-frequency guided waves.At present,many non-destructive testers for mortar rock bolts designed based on experimental test results still use low-frequency waves with about 10 k Hz to test the quality of mortar rock bolts that have been grouted for 3 to 5 days.This paper studies the relationship between the attenuation of low-frequency guided waves and the age of grouting of anchored rock bolt.The results show that the low-frequency longitudinal guided wave of about 10 k Hz excited by the non-destructive testing instrument decays rapidly with the growth of the grout age of anchored rock bolt.Therefore,these low-frequency longitudinal guided waves only can be used to detect the anchored quality within eight hours after the completion of grouting,for anchored rock bolt with a rapidly increasing elastic modulus of grout. |
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