| Ultrasound is widely used in the field of nondestructive testing,but both electromagnetic ultrasonic and piezoelectric ultrasonic have disadvantages.Piezoelectric ultrasound can only excite ultrasound of a single frequency and type,and there are limitations in the detection of defects by single frequency ultrasound.when the electromagnetic ultrasonic probe is used as the receiving probe,the amplitude of the received signal is only dozens of uV with low receiving efficiency.In order to solve the above problems,this paper has done the following work:Firstly,based on the basic equation of ultrasonic wave and the working mechanism of EMAT transducer,the equations of EMAT exciting and receiving surface waves under Lorentz force,and EMAT exciting and receiving surface waves under magnetostrictive force are derived.It is easy to understand the excitation process of electromagnetic ultrasound and provide theoretical basis for the simulation of electromagnetic ultrasound by concretizing the and complex process.Secondly,the magnet and coil of EMAT transducer are selected and designed.Simulation software COMSOL was used to simulate the magnetic field distribution of EMAT,the Lorentz force on the medium and the displacement change of each particle.The simulation data are basically consistent with the theoretical data.Thirdly,the EMAT surface wave detection system is designed.The technology of"generation of frequency adjustable signal+DC voltage to AC voltage by high voltage and full bridge " is adopted in the surface ultrasonic wave excitation module to generate a serious of burst pulses with adjustable frequency and maximum peak value of 60A,which can realize the excitation of different frequency surface ultrasonic waves.A composite probe composed of 0.5MHz and 1MHz piezoelectric surface wave probes is designed to receive ultrasonic surface waves of different frequencies.The echo receiving module is designed,which uses"pre-amplification circuit+band-pass filtering circuit+gain adjustable amplification circuit",to adjust the echo of 10mV to 5V and filter out the noise contained in the signal and improve the signal-to-noise ratio by 6dB.Finally,The factors which affected the performance of the transducer are analyzed to determine the best parameters of the transducer.To solve the problem of low detection accuracy of a single frequency ultrasonic,a fusion algorithm of multi-frequency detection result is proposed.The fusion algorithm was verified by 0.5MHz and 1MHz ultrasonic surface wave respectively.In terms of ranging,the error was reduced by 0.568%and 0.761%when compared with 0.5MHz and 1 MHz respectively.In terms of flaw detection,it is 0.2J higher than the 0.5MHz flaw detection,and the accuracy is increased by 3.375%;Detect defect depth doubled compared to 1MHz flaw detection to 3mm. |
