Research On Electromagnetic Acoustic Transducer Oriented To Defect Detection On Steel Plate

Electromagnetic Acoustic Transducer(EMAT)is a non-contact,high efficient and environmental friendly testing method,which has become a research hotspot in recent decades.However,as the core of the electromagnetic acoustic testing technology,EMATs' transduction efficiency is relatively low,limiting the use of EMATs in modern industry.The traditional EMAT design is mainly based on the Lorentz force mechanism.These EMATs are effective in the detection of non-ferromagnetic materials such as aluminum plates.However,many problems may occur in the process of detecting steel sample.First of all,compared with aluminum,the steel plate possesses the property of higher density and lower conductivity,both of which tend to reduce the transduction efficiency based on the Lorentz mechanism.In addition,the permanent magnet used in the EMAT produces a strong magnetic force,which makes on-line scanning very difficult.In order to solve the above problems mentioned above,in-depth research on the transduction mechanism of EMAT in the steel plate has been carried out in this thesis.Novel EMATs are designed and the characteristics of the trandcuers are also investigated.In defect detection,there is a contradiction between the detection sensitivity and the range of detection.In order to achieve small size defect detection,one needs to improve the ultrasonic frequency.However,higher frequency accelerates signal attenuation in the ultrasonic propagation process,increasing the difficulty of signal analysis.In this thesis,an excitation source scanning method,which takes advantage of the broadband nature of the ultrasonic signal,is proposed to solve the problem mentioned above.The main innovations and research results are as follows:(1)A self-excited electromagnetic ultrasonic transducer model is proposed by studying the transduction mechanism in the steel plate.The newly designed EMAT does not require the magnet to provide a static magnetic field.The strong dynamic magnetic field generated by the large current pulse is utilized to achieve ultrasonic lamb wave generation.By investigating the mechanism and characteristics of the fundamental Lamb wave,the A0 mode Lamb wave is mainly generated by the dynamic Lorentz force,and the S0 mode is produced by the magnetostrictive force in ferromagnetic sample.Since the magnetostrictive force is only related to the dynamic magnetic field and independent of the induced eddy current intensity.self-excited transducers are able to achieve transduction at higher lift-off distances.In addition,the self-excitation method greatly reduces the weight of the EMAT.(2)The self-excited EMAT is optimized by using a ferromagnetic core and a bias static magnet respectively,and the optimization mechanism is analyzed and studied.The effect of the ferromagnetic core on the transduction efficiency is mainly reflected in two aspects:Firstly,the high permeability of the ferrtite material makes it easier to form a magnetic circuit on its surface,so the dynamic magnetic field on the surface of the metallic sample can be enhanced.Moreover,the ferrite core increases the EMAT inductance,changing the overall circuit impedance matching relationship.Using a bias magnet can also increase transduction efficiency by changing the magnetostrictive mechanism.For the self-excited EMAT transducer,magnetostrictive force is mainly caused by the magnetic field induced domain wall movement.For the EMAT with a bias magnet,the steel plate is magnetized and the magnetic domain wall completes the movement,so magnetostrictive force is produced by the rotation of the magnetic moment.(3)The EMATs based on the magnetostrictive effect and the inverse magnetostrictive effect can operate as generator and receiver respectively,both of which are able to operate at higher lift-off distances.The signal has a good signal-to-noise ratio when the working distances of the coils are 5 mm.This distance is much higher than conventional EMATs(less than 3 mm).This EMAT structure not only improves the operating distance,but also reduces the magnetic force between the magnet and the steel sample.(4)Based on the designed transducers,an excitation transducer scanning method is proposed to detect both the surface and hidden defect successfully.The scanning method makes full use of the broadband characteristics of the current pulse.In the process of surface defect scanning detection,the crack changes the dynamic magnetic field generated by the excitation transducer.This change can be obtained from the received ultrasonic signal and the energy of the signal can characterize the size of the defects.The hidden defect in steel samles can also be detected by So Lamb wave because the in-plane vibration displacement of S0 Lamb wave is relatively uniform throughout the thickness in low frequency-thickness region.The fast group velocity and less diepersive properties also make defect signal analysis simpler to some extent.Defect location and depth characterization can be achieved from the variation of peak-to-peak amplitude during the scanning process.