Lamb Wave Local Wavenumber Approach for Characterizing Flat Bottom Defects in an Isotropic Thin Plate

Guopeng Fan,Wenfa Zhu,Hui Zhang,Haiyan Zhang,Xiaodong Chai

doi:10.3390/app8091600

Guopeng Fan, Wenfa Zhu + Show 3 more

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https://doi.org/10.3390/app8091600

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Abstract

This paper aims to use the Lamb wave local wavenumber approach to characterize flat bottom defects (including circular flat bottom holes and a rectangular groove) in an isotropic thin plate. An air-coupled transducer (ACT) with a special incidence angle is used to actuate the fundamental anti-symmetric mode (A0). A laser Doppler vibrometer (LDV) is employed to measure the out-of-plane velocity over a target area. These signals are processed by the wavenumber domain filtering technique in order to remove any modes other than the A0 mode. The filtered signals are transformed back into the time-space domain. The space-frequency-wavenumber spectrum is then obtained by using three-dimensional fast Fourier transform (3D FFT) and a short space transform, which can retain the spatial information and reduce the magnitude of side lobes in the wavenumber domain. The average wavenumber is calculated, as a real signal usually contains a certain bandwidth instead of the singular frequency component. Both simulation results and experimental results demonstrate that the average wavenumber can be used not only to identify shape, location, and size of the damage, but also quantify the depth of the damage. In addition, the direction of an inclined rectangular groove is obtained by calculating the image moments under grayscale. This hybrid and non-contact system based on the local wavenumber approach can be provided with a high resolution.

Highlights

Lamb waves have shown great potential for structural health monitoring (SHM) in plate-like structures [1]
The interactions between Lamb waves and the damage have been analyzed by many researchers
Lamb waves and demonstrated that the damage in the plate acted as a non-axisymmetric secondary source [33]

Summary

Introduction

Lamb waves have shown great potential for structural health monitoring (SHM) in plate-like structures [1]. Their attractive features include sensitivity to a variety of damage types and the capability of traveling relatively long distances [2]. Damage imaging methods based on Lamb waves have been widely studied by many researchers, such as delay-and-sum (DAS) imaging [3,4], time reversal focusing imaging [5,6], diffraction tomography imaging [7], and ultrasonic phased array [8]. Lamb waves are dispersive and multimodal [9]. The propagating Lamb waves may include incident, reflected, and converted waves when they encounter a sudden thickness variation [10,11], such as circular flat bottom holes and rectangular grooves. Various wave modes make the interpretation of Lamb waves very difficult [12]

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