Abstract
Lamb waves are often used for damage detection in structures. Hot-spot monitoring via wave focusing is considered in this study, which can be achieved by using a plano-concave aspherical lens. Once attached to the plate, the lens modifies the effective plate thickness, and therefore changes the Lamb wave characteristics, such as wavenumber and phase velocity, providing a convenient way of controlling Lamb waves. Another possibility for focusing Lamb waves is to use multiple actuators and design dispersion pre-compensated excitation signals with an embedded time delay. These two approaches can be combined together. We conducted numerical simulations using the time-domain spectral element method based on the 3D elasticity theory and demonstrated that the A0 mode of Lamb waves could be focused on the desired focal point. We compared the efficacies of the plano-concave lens approach, the dispersion pre-compensated focusing approach, and a combination of both approaches, emphasizing the advantages and disadvantages of each method.
Highlights
Wu et al numerically demonstrated focusing of the A0 mode of Lamb waves in a gradient-index phononic crystal.2 Jin et al proposed using gradient-index phononic plates for simultaneous control of both the A0 and S0 modes in a broadband frequency range.3 Yan et al.4 proposed using surface bonded elastic metamaterials made of planar arrays of lead discs with varying thicknesses to focus the A0 mode of Lamb waves
We conducted numerical simulations using the time-domain spectral element method based on the 3D elasticity theory and demonstrated that the A0 mode of Lamb waves could be focused on the desired focal point
Three methods for A0 mode Lamb wave focusing were compared, namely the plano-concave lens approach, a focusing approach aided by dispersion compensation, and the combination of both
Summary
Wu et al numerically demonstrated focusing of the A0 mode of Lamb waves in a gradient-index phononic crystal. Jin et al proposed using gradient-index phononic plates for simultaneous control of both the A0 and S0 modes in a broadband frequency range. Yan et al. proposed using surface bonded elastic metamaterials made of planar arrays of lead discs with varying thicknesses to focus the A0 mode of Lamb waves. The practicality of such approaches for structural health monitoring (SHM) is questionable They involve the development of complex periodical phononic crystals or sub-wavelength resonant metamaterials which have to be attached or coupled to the structure to be inspected. In contrast to the phased array approach, the focusing method does not rely on the beamforming principle, in which constructive interference occurs only for compactly spaced piezoelectric transducers, which limits potential transducer configurations and excitation frequencies. We present a study which follows the designs of simple slice lenses made to control Lamb waves by modulating the wavefronts, which were presented by Tian and Yu.16 We modified their approach so that the piezoelectric transducer is bonded directly to the lens instead of the host structure. The results presented in this paper are based exclusively on numerical simulations carried out using an in-house software program, namely the time-domain spectral element method.
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