| The pipeline has been widely used in the current industry and daily life.Faced with the problem of pipeline defect detection,numerous research has been carried out.However,traditional guided wave detection technology has some inherent limitations,such as the need for coupling agents,expensive materials and cleaning the surfaces of the detected objects.Hence,the electromagnetic ultrasonic transducer has advantages including simple structure and noncontact detection.Simultaneously,the guided wave detection has the characteristics of long detection distance and different sensitivity of guided waves of different modes to specific types of defects.Therefore,the combination of the two can give full play to their advantages and achieve faster and more convenient detection of defects.It can effectively overcome the main limitations of the existing guided wave detection technology.Thus,the prospect of electromagnetic ultrasonic technology is very broad.Firstly,the propagation characteristics of guided waves in cylindrical waveguide were studied,where the dispersion equation of guided waves was solved and the dispersion curve was drawn according to the dispersion equation.Then the relationship between the wave velocity of the guided wave and the phase velocity as well as the group velocity was analyzed.Furthermore,the concept of the multimodal characteristic of the guided wave and the product of the frequency with thickness were also interpreted.Additionally,the propagation characteristics of the torsional mode guided wave in the tubular waveguide were researched.Secondly,a theoretical model of electromagnetic ultrasonic transducer was established and the transduction mechanism was analyzed.The factors affecting the eddy current distribution were studied by the Comsol finite element simulation software,meanwhile a Lorenz force model and a magnetostrictive model were established and analyzed to verify the inference of Lorenz force as the main energy exchange mechanism in the steel tube.Afterwards,the testing system was built.then the experimental research was carried out on the aluminum tube and steel tube so as to verify the specific modes of the excited wave.The influence of factors including excitation frequency,the bias magnetic field intensity and the number of the converter coil turns on the energy exchange effect were researched.Besides,the location of the groove defect on the steel tube was tested.At present,the shape of the transducer coil is like a runway with 0.69 mm diameter copper enameled wire winding 50 turns.A rubidium iron-boron permanent magnet with the size of 30 mm × 10 mm × 5mm is selected,while the permanent magnet poles are periodically interlaced,with 8 pieces in each group and 4 sheets on each side in two straight lines in the loop of the runway.The optimum excitation frequency range of the transducer is determined by the experiment,which is within 200 kHz ±10kHz.Besides,the error of locating the groove defect of steel pipe is less than 1.8%,which meets the requirement of positioning accuracy.Additionally,the echo amplitude of the axially grooved defect is 32% higher than that of the circumferential grooving defect,which proves that the torsional wave is more sensitive to the axial defect. |
PDF Full Text Request