| Pipelines are widely used in industrial transportation devices such as oil,natural gas,and gas.Long-term work in high temperature,high pressure,and corrosive environments will inevitably cause damage and affect the safe operation of the pipeline.Electromagnetic ultrasonic detection technology has the characteristics of no contact,no couplant,high detection accuracy and flexible types of excited ultrasonic waves,which can meet the online detection of pipelines.Electromagnetic acoustic transducer(EMAT)is the core of the detection technology,which is used to excite and receive electromagnetic ultrasonic waves propagating in the workpiece.This thesis focuses on the problems of electromagnetic acoustic transducer in pipeline defect detection,defect location determination and engineering application.In this thesis,the pipeline defect detection direction,detection sensitivity,detection scale,unidirectional propagation EMAT defect location determination and EMAT detector engineering application were studied based on the basic theory of electromagnetic ultrasonic guided wave.The research on the principle of electromagnetic ultrasonic energy conversion and wave propagation characteristics are the theoretical bases for solving the problems of pipeline defect detection and defect location determination.The transducer coil is fed with high frequency alternating current,which induces eddy currents on the near surface of the workpiece.Due the action of bias magnetic fields in different directions,through the mutual conversion of electric field,magnetic field,force field,acoustic field and other multi-physical fields,the wave power source of lorentz force,magnetostrictive force and magnetizing force mechanism are formed.Under the action of three power sources,the particles in the workpiece vibrate mechanically to generate electromagnetic ultrasonic guided waves.This thesis analyzed and established the functional relationship between the three force sources that generate electromagnetic ultrasonic guided waves and related variables such as fluctuation displacement,eddy current density,magnetic induction intensity,magnetic field intensity,electric field intensity,stress tensor,strain tensor,and permeability.The propagation characteristics of circumferential guided waves and axial guided waves in the pipeline were respectively analyzed.The relationship of the displacement field function equation between the wave displacement of the circumferential and axial wave and the propagation direction was established.According to the stress free boundary conditions in the pipeline,the dispersion equations and dispersion curves of the circumferential guided wave and the axial guided wave were obtained.Aiming at the problems of defect detection direction,detection sensitivity and detection scale in electromagnetic ultrasonic guided wave pipeline detection,the propagation characteristics of electromagnetic ultrasonic guided wave and directivity of the radiated acoustic field were studied.Aiming at the problem of pipeline defect detection direction,this thesis established the propagation direction analysis model and propagation direction control model of the EMAT main ultrasonic beam based on Biot-Savart law according to the relationship between electromagnetic ultrasonic wave displacement and magnetic induction intensity.Through calculation and analysis,the propagation characteristics of the main ultrasonic beam generated by EMAT in ferromagnetic materials perpendicular to the wire where the excitation coil is located were obtained.By controlling the oblique angle of EMAT coil,the propagation direction of electromagnetic ultrasonic guided wave in the pipeline can be controlled.Aiming at the problem of defect detection sensitivity,this thesis analyzed the relationship between defect detection sensitivity and coil parameters based on EMAT ultrasonic beam propagation characteristic model.Results show that the strength of echo signal,namely the sensitivity of defect detection,is proportional to the length of transducer coil and the number of turns.Aiming at the problem of defect detection scale,this thesis proposed and established a mathematical model of the asymmetric radiation acoustic field directivity of oblique guided waves based on the research of the detection scale of the radiated acoustic field of the axial and circumferential guided waves.The asymmetric EMAT radiated acoustic field in the pipeline was calculated,and the angular distribution scale of the radiated acoustic field on both sides of the main acoustic beam were obtained,which provides a calibration method for the asymmetric acoustic field distribution in the pipeline and determines the defect detection scale of the asymmetric EMAT in the pipeline.Experiments verified the correctness of the above-mentioned defect detection problems,which provides a research foundation for electromagnetic ultrasonic pipeline defect detection.Aiming at the problem that the traditional electromagnetic ultrasonic guided wave propagates along the workpiece in two directions,and it is difficult to determine the defect location directly and accurately according to the echo time.The unidirectional guided wave EMAT with unidirectional propagation and double excitation coil structure was proposed and designed in this thesis.By adjusting the relative distance between the double excitation coils and the delay time difference,the intensity of the superimposed echo signal of the electromagnetic ultrasonic guided wave is controlled.When the intensity of the echo signal on the weakened side is zero,the unidirectional propagation control is realized,which provides a research basis for the accurate determination of the defect detection positon.In this thesis,based on the electromagnetic acoustic transducer mechanism and the relationship of magnetic field,displacement and other multi-physical fields,the mathematical model of wave superposition was built,and the unidirectional guided wave EMAT analysis and calculation were carried out.The finite element simulation analysis of the wave displacement of unidirectional guided wave EMAT was carried out,and the coil parameter characteristics experiment,unidirectional propagation control experiment,and defect location determination experiment were carried out.Through theoretical research,simulation analysis and experimental verification,it is concluded that the amplitude of the weakened echo signal of the designed unidirectional guided wave EMAT is almost zero,and the amplitude of the enhanced side echo signal is significantly improved,which compares with the ultrasonic amplitude of traditional structure transducer is 5.88 d B.When the relative distance between two excitation coils is ?/4 and the delay time is T/4,or the relative distance between double excitation coils is3?/4 and the delay time is 3T/4,the electromagnetic ultrasonic guided wave propagates unidirectionally,which can realize the accurate defect location determination.Aiming at the engineering application problems of electromagnetic ultrasonic guided wave pipeline detection,based on the research of electromagnetic ultrasonic pipeline defect detection and defect location determination,this thesis designed and developed the multi-channel electromagnetic ultrasonic guided wave internal detector with the function of axial guided wave,circumferential guided wave and ±45° oblique guided wave defect detection and defect location determination.A combined EMAT structure was proposed.The permanent magnets in the structure are multiplexed to form four unipolar magnetic field regions and four U-shaped magnetic field regions.It provides static bias magnetic field for eight-channel EMAT,and improves the utilization of the transducer structure.The COMSOL was used to simulate and analyze the combined EMAT to optimize the parameters of the transducer.The combined EMAT consists of two unidirectional circumferential guided waves,two unidirectional axial guided waves and four unidirectional oblique guided waves.The EMAT structure detector was used to conduct experiments on defective pipelines.According to the time and dispersion curve of the defect detection echo signal,the axial,circumferential and oblique defects in the pipeline can be detected and the position can be determined accurately.Through the PC-side multi-channel electromagnetic ultrasonic guided wave detection data analysis software,the detection parameters can be set,and the detection echo data of each channel can be viewed and called. |
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