Study On Ultrasonic Leakage Rayleigh Waves Propagation Characteristics And Imaging Of Metal Surface Defects

Ultrasonic Leaky Rayleigh waves can be used to detect surface or near-surface defects of metal parts,which has the advantages of non-contact detection and high sensitivity.Compared with wedge-type surface waves,ultrasonic Leaky Rayleigh waves can realize automatic detection and defect imaging under water immersion through motion control devices.However,there are still some problems in the ultrasonic Leaky Rayleigh waves,such as the unknown influence of detection conditions and defect characteristics,low detection imaging accuracy,and small detection range caused by attenuation and acoustic energy diffusion.In this paper,based on the existing research,the ultrasonic Leaky Rayleigh waves simulation of metal surface defects,experimental detection and Block frequency domain Synthetic Aperture Focusing Imaging method is carried out to finally achieve high-resolution,large-area,high-efficiency detection and imaging of metal surface defects,the main work is as follows.(1)Established a Leaky Rayleigh waves propagation and defect response finite element model to simulate the propagation process of Leaky Rayleigh waves.From the perspective of simulation,the influence of detection conditions such as underwater acoustic distance,probe frequency,probe incident angle and defect characteristics such as defect depth,width,inclination angle and the distance between defect and probe on surface or near-surface defects detected by surface leakage wave was explored.(2)The experimental study of Leaky Rayleigh waves detection is carried out.Combined with the above simulation,the influence of underwater acoustic distance,probe incident angle and probe frequency on the echo of Leaky Rayleigh waves defect is analyzed from the experimental point of view,and the optimal detection parameters are obtained.The echo signal and B scan imaging results of hole and crack defects are analyzed,and a series of conclusions are obtained by simulation:the imaging size of hole defects with different diameters is similar,and the lateral resolution is low,which is difficult to distinguish the defect size.The multiple defect waves of crack defects come from the bottom reflection of the test block.Surface leakage wave can accurately detect the length of crack defects,but it is difficult to quantitatively detect the depth and width of defects.(3)The Synthetic Aperture Focusing Imaging method for Leaky Rayleigh waves detection is studied to improve the lateral resolution of the imaging.Compared with the B-scan imaging,the transverse size error of the defect is reduced by 31.3%in the time domain Synthetic Aperture Focusing Imaging based on the root mean square velocity,and the frequency domain Synthetic Aperture Focusing Imaging is reduced by 23.8%.However,the imaging efficiency is higher and the imaging time is only 1/36 of the time domain.(4)A block frequency domain Synthetic Aperture Focusing Imaging method is studied to improve the detection range of Leaky Rayleigh waves detection.DAC attenuation compensation,frequency domain Synthetic Aperture Focusing Imaging and combination superposition are carried out on multiple sets of B-scan data.The problem of small detection range of single B-scan of Leaky Rayleigh waves is solved,and the detection time is only 1/25 of C-scan under the same detection effect.Finally,The block frequency domain Synthetic Aperture Focusing Imaging method improves the imaging efficiency and resolution of surface defect detection.