MULTI-MODES ELECTROMAGNETIC ULTRASONIC LAMB WAVE TOMOGRAPHY IMAGING FOR VARIABLE-DEPTH DEFECTS IN METAL PLATE

摘要

This paper proposes a new cross-hole tomography imaging (CTI) method for variable-depth defects in metal plate based on multi-modes electromagnetic ultrasonic Lamb waves (LW). The dispersion characteristic of LW determines that different modes of LW are sensitive to different thicknesses of metal plates, and the sensitivity of a certain mode of LW varies with the thickness. Thus, it's nearly impossible for a single-mode LW to be highly sensitive to an arbitrary thickness. And the single-mode LW CTI method proves to suffer low resolution when it's used in variable-depth situations. In this work, the sensitivities to thickness variation of A0 and SO modes LW are theoretically studied and then demonstrated on the frequency-thickness plane, based on the dispersion characteristic of each mode. Theoretical results show that A0 mode LW is more sensitive to thickness variation when the product of frequency and thickness is relatively small, and SO mode LW is more sensitive when the product is relatively large. Based on the theoretical analysis, the principles and procedures for the cooperation of A0 and S0 mode LW CTI are proposed. And the superposition and data fusion method for imaging results of A0 and S0 mode LW is demonstrated. Besides, the experimental LW imaging system on a 3mm thick aluminium plate with a variable-depth defect is setup in this work. The proposed multi-modes LW CTI method is applied to the experimental platform, based on the A0 and S0 mode EMAT (electromagnetic acoustic transducer) cross-hole arrays and simultaneous iterative reconstruction technique. Then the traditional single-mode LW CTI method is used in the same experimental platform, with A0 and SO mode respectively. The imaging results show that the computed thickness distribution by the proposed multi-modes method has a better reflection about the actual thickness variation of the defect, while neither S0 nor A0 single-mode method could distinguish thickness variation around the defect region. And the quantification of the defect's thickness variation is more accurate by the multi-modes method. Therefore, theoretical and practical results prove that variable-depth defects in metal plate can be successfully quantified and visualized by the proposed multi-modes electromagnetic ultrasonic LW CTI method.