Research On Defect Detection In Metal Plate Based On Time-reversal Guided Wave

Metal plate is one of the most important industrial raw materials.It is prone to cracks and corrosion during use,and it needs to be regularly inspected for non-destructive testing.The long-distance and large-scale detection of defects in the plate can be quickly realized by utilizing the characteristics of long-distance propagation and small attenuation of the guided wave in the metal plate.The electromagnetic ultrasonic guided wave SH₀mode has the advantages of non-dispersion,no mode conversion,and less influence on the surface contact material of the specimen at low frequencies.Therefore,in this paper,the SH₀guided wave mode is selected to detect crack defects across the weld in the plate and the high-order Lamb wave mode is more conducive to the detection of small defects due to its short wavelength and high detection sensitivity,so it is used for pinhole corrosion defect detection.However,when the SH guided wave encounters a crack defect after crossing the weld,the reflected signal is received again through the weld when it returns,and the signal energy is weak.In addition,because the pinhole corrosion is very small,its detection signal is submerged in noise and difficult to distinguish,so the detection effect is poor.Aiming at the above two problems,this paper studies the detection of crack defects by time reversal sh guided wave across the weld and the detection of pinhole corrosion by time reversal of high-order Lamb wave mode.The main research contents are as follows:(1)The SH guided wave acoustic field measurement experimental system is built,and the Periodic Permanent Magnet Electromagnetic Acoustic Transducer(PPM EMAT)with different geometric parameters was fabricated,The effects of geometric parameters such as the inner diameter,outer diameter and width of permanent magnet on the sound field distribution of SH guided wave are analyzed,and finally the optimal parameters are determined to guide the design of PPM EMAT.Secondly,the influence of weld structure factors such as weld widths and weld margins on the sound field distribution is studied.It is concluded that the weld structure has little effect on the directivity of the guided wave sound field,but it will attenuate the guided wave energy.(2)The SH guided wave detection experimental system is established,and the SH guided wave time-reversal detection method is proposed.After crossing the weld in the aluminum plate and the steel plate,the cracks with different lengths are conventional detected and time-reversal detected respectively.The results show that the minimum crack length detected in the aluminum plate is 1mm and the crack size in the steel plate is 2mm in the conventional inspection;In the time-reverse detection,the crack with a length of 1mm can be detected in both materials,and the amplitude of crack echo signal increases.Compared with the conventional detection,the time-reverse detection has enhanced the defect echo energy,and its maximum enhancement multiple is 3.9 times.(3)Aiming at the problem that pinhole corrosion is difficult to detect,the high-order Lamb wave S₂mode time-reverse focusing detection method is studied,and the conventional detection and time-reverse detection are carried out for circular hole defects with different diameters and depths in aluminum plates.The results show that the sensitivity of conventional detection reaches?0.5mm pinhole defects with a depth of 2mm,while the sensitivity of time-reverse detection reaches?0.5mm pinhole defects with a depth of 0.5mm,it is verified that time-reverse Lamb wave enhances the defect signal energy and improves the detection sensitivity.A segmented time-reversal detection method is proposed,and detection experiments are carried out when the interval contains complete defect echo,partial defect echo and no defect echo.The results show that when time-reversal guided wave is focused in a certain interval,it can be determined that there is a defect in the interval.In addition,the detection of pinhole defects under low voltage is studied.Conventional guided waves cannot detect defects,but time-reversal guided waves can detect defects.It is shown that the time reversal technique can reduce the hardware requirements and enhance the sensor detection capability.