The Study On Electromagnetic Acoustic Transducer Response Characteristics Excited By Hihg Energy Pulses

Electromagnetic acoustic testing has attracted wide attention and has been applied to the detection of various metal materials because of the advantages of no coupling-agent,flexible excitation,simple design and strong environmental adaptability.However,some problems should be solved in EMAT technology.Such as the transduction efficiency of EMAT is low and the amplitude of the received ultrasonic signal is lower than piezoelectric ultrasound under the same conditions.The magnet in EMAT will attract materials when detecting ferromagnetic materials,and will restrict the relative position of the transducer and the specimen which affects the convenience of detection.In this paper,a novel high-energy pulsed electromagnetic acoustic excitation system without static-bias-magnetic field was designed,which was based on the principle of electromagnetic acoustic excitation.The core of the excitation system is the high-energy pulse excitation source.The high-energy pulse excitation source used LC oscillation circuit to apply transient high voltage to the excitation coil and thousands of amperes of current in the excitation coil was generated,which was much larger than the current in the conventional EMAT excitation coil.The strong magnetic field coupled with eddy current generated high-intensity stress in the materials,which could significantly improve the intensity of the excitation ultrasonic signal and low transduction efficiency could be solved.Firstly,the transduction mechanism of high-energy pulsed electromagnetic acoustic in ferromagnetic and non-ferromagnetic materials was analyzed.A simulation model of high-energy pulse EMAT for non-ferromagnetic materials was established.The distribution of eddy current,magnetic field and Lorentz force in the aluminum plate was analyzed.The relationship between lift-off distance and the logarithm of particle displacement amplitude of Lamb wave was obtained by simulation of lift-off characteristics.A simulation model of high energy pulse EMAT for ferromagnetic materials was established to simulate the distribution of electric field,magnetic field,force field and lift-off characteristics in steel plate,and the corresponding relationship between lift-off distance and the dominant role of electromagnetic-acoustic energy transduction mechanism in steel plate was obtained.Secondly,a high-energy pulsed EMAT excitation source with A0 as the main modewas designed.The structure design,device selection,parameter calculation and reliability verification were carried out.The corresponding relationship between circuit parameters and excitation current,voltage frequency was obtained by simulation analysis of the excitation source circuit.The spectrum of high-energy pulsed EMAT excitation current was analyzed.Finally,the relationship between Lamb wave frequency and excitation current frequency was studied by experiment of ultrasonic propagation characteristics in high-energy pulsed electromagnetic acoustic experiment of aluminum plate.The exponential relationship between Lamb wave amplitude and lift-off distance was studied by lift-off characteristic experiment.The influence of lift-off distance on electromagnetic acoustic transduction mechanism in steel plate was analyzed through the lift-off characteristic experiment in the ultrasonic experiment of steel plate.The experimental results verified the consistency with the simulation analysis.Then the metal defect detection experiment was carried out and accurate defect location was realized.The experimental results of EMAT ultrasonic characteristics and defect detection under high-energy pulse excitation showed that when the separation distance was small,the Lamb wave with A0 mode as the main mode excited by high-energy pulse EMAT had a high signal-to-noise ratio,which significantly improved the transduction efficiency of electromagnetic acoustics,and the low-frequency Lamb wave improved the identification of the detection signal.