Research On The Character Of Guided Wave Propagation In Multi-layerscy Lindrical Grouted Structure

The multi-layer cylindrical waveguide structure is widely used in various production capacities,from the most common pipelines that transport various fluid media to pile foundations in geotechnical engineering and rock bolt anchoring structures used to support surrounding rocks in underground engineering.The structures of the embedded rock bolts will inevitably become damaged during use.If the viability of these structures cannot be quickly determined in real time,it will cause great harm to production operations.In particular,the grouted structure embedded in infinite rock is concealed,and the anchoring quality of the rock bolts cannot be determined by direct contact methods.Existing destructive methods such as the pull-out method and the overcoring method are inherently flawed,and the testing cost is high and the efficiency is low.An efficient non-destructive testing method for the anchoring quality of rock bolts has become a pressing need in the field of geotechnical engineering.The non-destructive testing of rock bolts generally entails the input of excitation waves at the exposed end of the rock bolt while recording the reflected echo from the bottom end of the rock bolt;and extracting the waveform parameters related to the anchor parameters of the rock bolt from the time-domain and frequency-domain curves of the complete waveform.Therefore,the basis of a viable non-destructive testing technique should be the minimization of energy loss and the ability to maintain a complete reflected waveform.In this paper,elastodynamic theories were used to study the propagation of ultrasonic guided waves in a multi-layer cylindrical structure with different media to establish general calculation model.First,the numerical method is used to obtain the dispersion curve of the guided wave propagation in different anchoring structures.Secondly,the wave structure of the potential detection guided wave with small attenuation is determined,and best guided wave mode suitable for the non-destructive detection of the anchoring structure of the rock bolt is selected.Finally use the finite element numerical simulation combined with physical experimental testing method to verify the best test modal obtained.The main research contents are as follows:1)Principles related to guided waves were discussed and an explanation was provided for the process of generating a guided wave dispersion function in an embedded rock bolt with a basic structure based on the theory of elastodynamics;furthermore,the process of obtaining a solution to the free bolt dispersion function and the resultant dispersion curves were analyzed.2)A general mathematical model for the propagation of ultrasonic guided waves in multi-layer of different cylindrical media structures is established.The propagation law of guided wave in free rock bolt structure,two-layer grouted structure(rock bolt and concrete mortar),and three-layer grouted structure(rock bolt,anchoring agent and surrounding rock)are analyzed,and the mathematical models for the respective structures are established.3)Using MATLAB software to numerically solve the mathematical models,the dispersion equations under three different anchoring structure conditions are obtained,and the spatial curves of three variables of frequency,wave number and attenuation in three different modes(F,T,L)are analyzed.Finally,the quantitative relationship between phase velocity and frequency,attenuation and frequency,respectively,are derived.In addition,longitudinal guided wave were determined to be the most optimal for non-destructive testing of anchoring quality according to the analysis of the attenuation characteristics of the varying guided wave modes.4)Analyze the different scenarios of anchor bolts on site.The influence of these parameters on the propagation law of guided waves was studied by changing the material parameters,thickness of the anchoring agent and the parameters of different surrounding rock materials.5)According to the attenuation characteristics of the guided wave,the wave structure of the selected mode is calculated,and the distribution law of the axial displacement and radial displacement of the test excitation wave on the cross section of the grouted structure of the rock bolt is determined.6)Using the calculated optimal excitation wave,to conduct the physical experiment test and finite element calculation simulation of the guided wave propagation in the grouted structure,respectively,and verify the results.It is found that the calculation,physical experiment and simulation results are in good agreement。7)The physical nature of a number of periodic reflected signals,obtained from numerous literary sources and experimental data form this research group,were determined using a combination of theoretical analyses and numerical simulations.These periodic reflected signals were formed in a finite guided wave structure,therefore they cannot be applied in the non-destructive testing of the rock bolts that are embedded in infinite wall rocks in the field.These research results have great significance as reference research of rock bolts.