Multi-Mode Electromagnetic Ultrasonic Lamb Wave Tomography Imaging for Variable-Depth Defects in Metal Plates.

Songling Huang,Yu Zhang,Shen Wang,Wei Zhao

doi:10.3390/s16050628

Abstract

This paper proposes a new cross-hole tomography imaging (CTI) method for variable-depth defects in metal plates based on multi-mode electromagnetic ultrasonic Lamb waves (LWs). The dispersion characteristics determine that different modes of LWs are sensitive to different thicknesses of metal plates. In this work, the sensitivities to thickness variation of A0- and S0-mode LWs are theoretically studied. The principles and procedures for the cooperation of A0- and S0-mode LW CTI are proposed. Moreover, the experimental LW imaging system on an aluminum plate with a variable-depth defect is set up, based on A0- and S0-mode EMAT (electromagnetic acoustic transducer) arrays. For comparison, the traditional single-mode LW CTI method is used in the same experimental platform. The imaging results show that the computed thickness distribution by the proposed multi-mode method more accurately reflects the actual thickness variation of the defect, while neither the S0 nor the A0 single-mode method was able to distinguish thickness variation in the defect region. Moreover, the quantification of the defect’s thickness variation is more accurate with the multi-mode method. Therefore, theoretical and practical results prove that the variable-depth defect in metal plates can be successfully quantified and visualized by the proposed multi-mode electromagnetic ultrasonic LW CTI method.

Highlights

The dispersion and multi-mode characteristics of ultrasonic Lamb waves (LW) indicate that the group velocity of a particular mode of LW varies with the product of frequency and waveguide thickness [1,2]
The multi-mode LW cross-hole tomography imaging (CTI) method was implemented on a 3-mm thick aluminum plate with a variable-depth defect
The pure single-mode EMAT was realized by the configuration of the distance l between the two adjacent annular wires along the radial direction, which is supposed to be half of the wavelength of the desired mode of LW

Summary

Introduction

The dispersion and multi-mode characteristics of ultrasonic Lamb waves (LW) indicate that the group velocity of a particular mode of LW varies with the product of frequency and waveguide thickness (frequency–thickness product, FTP) [1,2]. When some LW rays pass through the meshes with defects, the group velocities will change with the area thickness, and this leads to the changes in the TOF of the LW rays. [7,8,9,10], but but the the research research objects objects had had detecting different defects of different depths, as opposed to varying depths within a single defect. LW for CTIvariable-depth method for variable-depth defects This and waveguide waveguide thickness, thickness, to to explore explore the the variation variation pattern pattern of the defined sensitivity, sensitivity, and and to to propose propose aa and novel multi-mode electromagnetic. CTI method for variable-depth defects in metal plates. Novel multi-mode electromagnetic LW CTI method for variable-depth defects in metal plates

Part 1

Research

Principles and Procedures of Multi-Mode LW CTI

Experiments and Results

Thickness

Conclusions