Abstract
Ultrasonic guided waves are widely used to inspect and monitor the structural integrity of plates and plate-like structures, such as ship hulls and large storage-tank floors. Recently, ultrasonic guided waves have also been used to remove ice and fouling from ship hulls, wind-turbine blades and aeroplane wings. In these applications, the strength of the sound source must be high for scanning a large area, or to break the bond between ice, fouling and plate substrate. More than one transducer may be used to achieve maximum sound power output. However, multiple sources can interact with each other, and form a sound field in the structure with local constructive and destructive regions. Destructive regions are weak regions and shall be avoided. When multiple transducers are used it is important that they are arranged in a particular way so that the desired wave modes can be excited to cover the whole structure. The objective of this paper is to provide a theoretical basis for generating particular wave mode patterns in finite-width rectangular plates whose length is assumed to be infinitely long with respect to its width and thickness. The wave modes have displacements in both width and thickness directions, and are thus different from the classical Lamb-type wave modes. A two-dimensional semi-analytical finite element (SAFE) method was used to study dispersion characteristics and mode shapes in the plate up to ultrasonic frequencies. The modal analysis provided information on the generation of modes suitable for a particular application. The number of point sources and direction of loading for the excitation of a few representative modes was investigated. Based on the SAFE analysis, a standard finite element modelling package, Abaqus, was used to excite the designed modes in a three-dimensional plate. The generated wave patterns in Abaqus were then compared with mode shapes predicted in the SAFE model. Good agreement was observed between the intended modes calculated in SAFE and the actual, excited modes in Abaqus.
Highlights
Lamb waves are widely used to inspect the structural integrity of plates [1]
This paper studies the excitation of well-designed guided waves in a rectangular plate
Dispersion curves for Lamb waves propagating in an infinite plate with a thickness of 10 mm and 400 mm are shown in Figure 3 for comparison
Summary
Lamb waves are widely used to inspect the structural integrity of plates [1]. The plate is assumed to be infinitely long and infinitely wide, so that the sound field is considered to be uniform in the direction perpendicular to wave propagation. For waveguides of other cross-section, such as cylinders, rectangular plates, rails, etc., modal solutions of the governing equations are more complicated due to the reflection of the waves from boundaries, and analytical solutions are often limited to simple geometries and/or fundamental modes [7,8,9,10,11,12]. This paper proposes a method to systematically excite three types of guided wave modes (shear-horizontal, flexural and extensional) and Rayleigh surface waves in a rectangular plate. These wave modes are relatively non-dispersive, and have the potential to propagate over long distances.
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