Research On Ultrasonic Guided Wave Detection Technology Of Steel Rod Based On Time Reversal

As the load-bearing part of structure,steel rod may fail during long-term application due to the factors such as fatigue,corrosion and process defects,resulting in serious accidents.Therefore,it is necessary to perform non-destructive testing on steel rod.However,in the traditional ultrasonic guided wave detection process,modal conversion phenomenon at the defect causes the change of the defect echo waveform and the decrease of the amplitude,which has a negative effect on the defect detection.In order to solve the problem of waveform distortion and amplitude reduction of defect echoes caused by mode conversion in the existing guided wave detection process,an ultrasonic guided wave detection method based on time reversal theory is proposed for defect detection of steel rod in this paper,which can focus the defect signal and improve the detection effect.Through the methods of theoretical analysis,simulated analysis and numerical simulation experiments,the process of guided wave detection based on time reversal is studied in the following aspects:The guided wave mode and the excitation frequency range suitable for detection are determined.According to the dispersion equations of the ultrasonic guided waves in the steel rod,the dispersion curves are drawn in MATLAB software,and mode L(0,1)is selected as the excitation mode of guided wave detection in this paper.The influence of the steel rod's diameter on the dispersion curves of the longitudinal mode guided waves is analyzed.The results show that with the increase of rod diameter,the frequency range suitable for detection will become smaller and smaller.A time-reversed ultrasonic guided wave inspection method for steel rod is proposed.The time reversal focusing process of multimodal guided waves in steel rod is derived and the components of focusing signal are analyzed.Simulation experiments are carried out in MATLAB software to study the focusing performance of time reversal.The results show that the time reversal method can solve the waveform distortion caused by the mode conversion of the guided wave at the defect,focus the defect echo signal and improve the effect of guided wave detection.The finite element simulation of the time-reversed guided wave detection process in steel rod is carried out to study the focusing effect of time reversal in the actual detection situation.By comparing the simulation results with those of conventional guided wave detection,it is verified that the time reversal method can focus on the defect signal and enhance the detection efficiency.The influence of the window length on the focusing effect is discussed by intercepting the echo with time reversal windows in different lengths.The results show that the more complete the defect information contained in the signal captured by time reversal window,the better the focusing effect of time reversal.Numerical simulations of guided wave detection for defects in different sizes are carried out.The variation of the defect reflection coefficient before and after time reversal process with dimensions of defect in radial,circumferential and axial directions is studied,which provides a basis for quantitative analysis of defects.On this basis,the damage index is defined to determine the damage degree of steel rod.The quantitative relationships between the defects with different radial depths and circumferential lengths and damage index are given respectively.