Modeling and Experimental Analysis of Electromagnetic Ultrasonic Testing Considering Effect of Rough Surface

Abstract

Abstract Electromagnetic acoustic transducer (EMAT) is widely used in wall thickness measurement, which is suitably used in periodic inspection or long-term monitoring of pressure equipment. However, the corroded surfaces appearing on the pressure pipes and vessels usually cause an increase of background noise and variation of sound field, which restrict the improvement of measurement accuracy. In this paper, a numerical model of EMAT containing random rough surface is established based on the constitutive equation of Lorentz force and magnetostriction force. The distribution of eddy current at the skin layer in the excitation or receiving process of EMAT are analyzed. The characteristics of energy conversion of multi-physical coupling fields under effect of rough surface is clarified. Then, the variation law of measurement accuracy of wall thickness for EMAT under rough surface is obtained and compared by simulation and experiment. The results show that more interference sound waves come into being under rough front surface and the attenuation of signal-to-noise ratio (SNR) is more significant comparing to rough back surface. When the roughness is within 120μm, a better wall thickness accuracy can be obtained by means of multiple measurements. When the surface roughness is larger than 120μm, the measurement error exceeds 0.1mm. Therefore, when the surface of pressure equipment is in good condition, the high-precision thickness measurement of EMAT without grinding process can be realized. Otherwise, local grinding can significantly improve the measurement accuracy.