Study On Configuration And Lift-off Sensitivity Of EMAT

Due to the electromagnetic coupling mechanism,electromagnetic acoustic transducers(EMATs)can generate ultrasonic waves directly at the surface of specimen,instead of transmitting the ultrasonic waves generated at the transducer surface to the specimen by utilizing couplant.There can exist a gap,also called lift-off,between coil in EMAT and specimen.With this feature,EMAT can be applied to many special environments such as dry environment,high temperature and relative movement.The main obstacles of development of EMAT are poor conversion efficiency and high sensitivity of lift-off variation.In addition,the application of EMAT into detailed defect detection has been hard to implement for the difficulty of simultaneous detection of two components of surface wave.Therefore,the research based on the above key technical issues is of great importance in enhancement of conversion efficiency,weakness of lift-off sensitivity and determination of improvement route of meander-line-coil EMAT detection performance,which provides theoretical guidance for wide application and development of EMAT.The main works of the research are listed as follows:Aiming at the lift-off variation during detection process,it has been proposed that conversion efficiency of EMAT at single one lift-off can not represent the performance of EMAT.The nonlinear relationship between lift-off and peak-to-peak amplitude of the signal is transformed into a linear relationship by utilizing the lift-off effect.The conversion efficiency and lift-off performance of EMAT can be described quantitatively by two parameters.Compared with previous research which only considered the conversion efficiency of EMAT at single one lift-off or can only describe the lift-off performance qualitatively,this thesis compensates for the lack of related research,and lays foundation for enhancement of conversion efficiency and lift-off performance of EMAT.Aiming at the issue of poor conversion efficiency and poor lift-off performance,the models of electromagnetic coupling and propagation of ultrasonic wave process are derivated,and a muti-physics finite element model referring to the testing process of electromagnetic acoustic based on Lorentz forces is built.Then,the excitation,propagation and reception of ultrasonic wave are studied.For spiral coil EMAT,the effect of magnet-tocoil distance,magnet size,excitation current and remanent flux density of magnet on conversion efficiency and lift-off performance is studied and theoretically explained in detail.For meander-line-coil EMAT,the effect of EMAT parameters on the conversion efficiency and lift-off performance is also studied.The results show that the magnet-to-coil distance can hardly affect the conversion efficiency and lift-off performance of meander-linecoil EMAT.To explore the effect of magnet-to-coil distance on meander-line-coil EMAT,the effect of lift-off distance and magnet-to-coil distance on eddy current density and magnetic flux at specimen surface is studied by solving frequency domain model of EMAT.The results show that the eddy current density at specimen surface is the main cause of changes in conversion efficiency of EMAT.Therefore,an approximate functional relationship between eddy current density and lift-off distance is theoretically proposed.It has been proved that the amplitude of ultrasonic wave variation can be explained by eddy current density variation by comparing the variation of amplitude of ultrasonic wave and eddy current density with lift-off distance.The eddy current density at specimen surface generated by every conductor of meander-line-coil is calculated,which explains the reason for the weak influence of magnet-to-coil distance on the conversion efficiency and lift off performance of meander-line-coil EMAT.Therefore,the improvement route of meander-line-coil EMAT detection performance is determined.The conversion efficiency should be considered particularly when improving the performance of meander-line-coil EMAT.Even the lift-off performance can be ignored.To enhance conversion efficiency of meander-line-coil EMATs,the conventional design concept of meander-line-coil EMAT and the constructive interference of ultrasonic wave generated by meander-linecoil EMAT are studied.Meanwhile,the limitation of conventional design concept is pointed out.This thesis proposes a new design concept whose feasibility is proved theoretically.Based on the influence rule of conductor position on conversion efficiency,a type of non-equal spacing interval meander-line-coil EMAT is proposed.Both numerical calculation and experimental results reveal that the conversion efficiency of non-equal spacing interval meander-line-coil EMAT is much higher than that of periodic-permanent-magnet EMAT.To detect in-plane and out-of-plane components of surface ultrasonic wave,the effect of magnetic field direction on received signal is studied.Based on the research,a type of new EMAT is proposed which can measure the amplitude of surface ultrasonic wave components without complex data calculation.With this EMAT,repeated positioning or large area is not required during the measurement process.Compared with other detection EMAT,this EMAT is much more sensitive to the ultrasonic wave for the magnetic field generated by the EMAT is enhanced.