Study On Electromagnetic Ultrasonic SH Guided Wave Crossing Weld Defect Detection

Plate and pipe are the core components of storage and transportation equipment.Due to the harsh working environment of mechanical equipment,metal plate and pipe will produce various corrosion defects under aging and environmental action.Therefore,it is necessary to conduct nondestructive testing regularly to confirm its service status.Especially in the steel plate weld and pipe welding support internal area.Due to the long-term effect of internal and external corrosion,it is often a serious corrosion area.However,the detection of such defects needs to overcome obstacles,so it is often difficult to use conventional methods to detect.Electromagnetic ultrasonic Shear Horizontal guide wave at low frequencies only a single mode SH0,detection has no dispersion,less attenuation and other advantages,very consistent with the need to detect defects across the weld.To solve the problem of detecting corrosion defects across steel plate welds and pipe supports,this paper proposes a SHO guided wave detection method,which is studied in depth from both simulation and experiment.Firstly,a two-dimensional model of Lorentz force and magnetostrictive force excited ultrasound was developed and solved,and the mechanism of wave generation in ferromagnetic materials was analyzed in depth.Based on this model,an electromagnetic ultrasonic sensor composed of a perpendicular bias magnetic field(1.8 T)and a single-turn runway-type coil is designed by means of finite element simulation,and the magnitude of the near-surface load on the material is calculated for both mechanisms at the moment of excitation.The near-surface forces on the material in the simulation model of the(1.2 T)dualmagnet runway-type coil were also analyzed for both mechanisms.The results show that the Lorentz force at the near surface of the material dominates in the present structural configuration.Based on the two-dimensional finite element calculation results,the excitation and propagation models of three-dimensional periodic permanent magnet electromagnetic ultrasonic sensor on 3 mm thin plate and 3 mm thin-walled pipeline are established.The excitation frequency is 500 kHz,and the simulation detection signals of SH0 guided wave in plate and CSH0 guided wave in pipeline are obtained.SH0 guided wave propagates bidirectional.Its energy is strongest along the acoustic axis.And the beam has an arc distribution.The experimental system of SH0 guided wave in plate and CSH0 guided wave in pipe is built.The experimental results show that the velocity difference between experimental and simulated SH0 guided wave is small,which verifies the accuracy of the simulation model.Secondly,for the problem of detecting defects across the weld in the steel plate,a simulation model and experimental system of SH0 guided wave across the weld to detect defects was established.The butt weld Q235 steel plate with 1-3 mm length cracks was first routinely detected.The simulated and experimental signals demonstrate that the SHO guided wave has good ability to span the weld seam.The conventional detection signal energy is small,so the time reversal method is applied to focus on the post-weld defect.From the simulation and experimental point of view,it is verified that the time reversal method can improve the sensitivity of detecting post-weld defects,so that the detection accuracy of SH0 guided wave across the post-weld is significantly improved.And the smaller the size of the detected crack,the larger the amplitude enhancement multiplier obtained after time reversal.The time-reversal detection method was able to enhance the simulated and experimental signal amplitudes for conventional detection of 1 mm longitudinal cracks to 2.91 and 2.97 times,respectively.Finally,a three-dimensional pipe support structure detection simulation system was established for the problem of detecting internal defects across the support structure in the circumferential direction in steel pipes.The acoustic field characteristics of CSH0 guided waves are studied,the capability of CSH0 guided waves across the support structure is analyzed,and the interaction between CSH0 guided waves and circular hole-type and slot-type defects is discussed.Through the simulation analysis,the reflection and diffraction direction of the CSH0 guided wave is promiscuous when detecting round-hole defects;when detecting slot-type defects,the reflection and diffraction direction of the wave is more concentrated and the reflected wave energy is stronger than that in round holes.The experimental system for detecting defects across the support structure of the pipe is built,and the accuracy of the simulation analysis is verified by comparing and analyzing the experimental detection signal with the simulation results.Both simulation and experimental results show that CSH0 guided waves can effectively detect defects in grooves of 5 mm or more.However,the signals of φ5 mm round hole defects and 3 mm slot defects have been completely drowned in the clutter.