| FRP bolts are usually used in railway,mine and other underground engineering support and slope care,which improves the safety technology level.In practical geotechnical engineering,FRP bolts essentially solves the problem of corrosion of steel bolts,but due to man-made damage and the influence of natural conditions,there will be many defects of FRP bolts,such as fracture,which causes potential safety hazards.Based on the magnetostrictive effect of ferromagnetic materials,a magnetostrictive transducer is designed in this paper.The magnetostrictive guided wave technology is applied to the nondestructive testing of FRP bolts.The quality of FRP bolts is analyzed by numerical simulation and experimental data.The test results provide an important guarantee for the practical application of magnetostrictive guided wave technology in the nondestructive testing of FRP bolts.The specific contents of this paper are as follows:(1)The basic principles of magnetostriction,the basic concepts of ultrasonic guided waves,the multimodal characteristics and dispersion phenomena of guided waves propagating in FRP bolts are described.Based on the dispersion equation and the stress equation of guided waves,the dispersion curves and the power flow and displacement distribution of guided waves in free FRP bolts are plotted,as well as the patterns of guided waves in the propagation process.(2)The longitudinal guided wave magnetostrictive transducer is designed according to the basic equation of longitudinal guided wave.The effect of magnetostrictive coefficient of ferromagnetic material on the efficiency of magnetostrictive transducer is analyzed theoretically,and three finite element models of different ferromagnetic material transducers are established.The effect of material on the efficiency of transducer is analyzed by numerical simulation.According to the displacement load formula of torsional guided wave,torsional guided wave is generated in FRP bolts by applying displacement force,and its optimal detection frequency is determined and its effectiveness is verified.(3)An experimental system for FRP bolts detection based on magnetostrictive excitation was built to stimulate and collect guided waves.For the problem that the signal-to-noise ratio of the echo signal is low and the feature is not obvious,the variational mode decomposition-minimum mean square error(VMD-MMSE)algorithm is used to complete the filtering.The analysis and comparison show that compared with the traditional empirical mode decomposition(EMD)algorithm,the echo signal-to-noise ratio after VMD-MMSE filtering is improved.By experimentally changing the number of turns of the excitation and receiving coils,adjusting the active area of the static and bias magnetic fields,the magnetostrictive transducer is optimized,and the torsional guided waves are generated in the FRP bolts and the FRP bolts is experimentally detected.The results show that magnetostrictive guided wave technology can be applied to FRP anchor detection. |
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