Comparative Study Of The Focusing And Imaging Methods For Plate Structure’s Defect Based On Ultrasonic Guided Waves

With the growing demand of electricity in China, the operation stablity of critical equipment in power plant is becoming more important. The defect imaging methods based on the guided wave sensor array, which can quickly and intuitively detect the defects within the devices of power plant, is showing great potential. The comparative study of different defect imaging methods to find one suitable for the power plant equipment, is important for the stable operation of power plant equipment.Based on the circular ultrasonic guided wave sensor array, this paper conducts a comparative study of those defect imaging methods for plate-like member within power plant, the main research works are as follows:First, based on the Navier wave equation, the ultrasonic guided wave dispersion equation is derived and the dispersion curves of the group velocity of the guided wave in steel plate are calculated; the mechanism and process of the phased focucing method, the time-reversal method and the synthetic aperture method are analized and derived, and the wave field reconstruction methods for three imaging methods are deduced.Secondly, a defect detection system for plate-like member is developed based on the ultrasonic guided wave, with the optimization of the excitation frequency of A0 mode. And the detection of crack defects in steel plate is conduct with this system. To compare those imaging methods more accurately, a finite element model of steel plate with the same parameters of experiment is developed and the defect feature of the experiment and simulation signals is compared, results show that the finite element simulation can effectively reflect defect’s characteristics and reduce the influence of other factors.Finally, the models of steel plate with a single hole defect and with multiple defects are developed. Based on those models, the simulation study of phased focusing method, the time reversal method and the synthetic aperture method is conduct. With the simulation signals, the defects are imaged. The imaging results of three methods are compared in the working mehod, the focusing effect, the defect location and the image quality; the results are analyzed with the intensity distribution of the image matrix. Comparative results show that:for single defect, three imaging methods can locate and image the defect; for multi-defects, three imaging methods are capable of locating and imaging the near-field defect, but not of the far-field defect; compared with other imaging methods, the synthetic aperture focusing and imaging method gets better focusing and imaging results and is more suitable for the defect detection of steel plate.