Simulation and measurement of the optical excitation of the S1 zero group velocity Lamb wave resonance in plates

Abstract

Recent reports on the thermoelastic generation of Lamb waves in isotropic elastic plates show that a laser source efficiently excites a resonance that occurs at the minimum frequency of the first order symmetric (S1) Lamb mode. The group velocity of the Lamb wave goes to zero at this frequency while the phase velocity remains finite, and the resonance is referred to as the S1 zero group velocity (S1 ZGV) resonance. The S1 ZGV resonance can be employed for the nondestructive evaluation of the elastic properties of plates or plate thickness. A model for the generation of elastic waves in plates using an intensity-modulated continuous wave laser source is developed and used to study the behavior of the S1 ZGV resonance. The effects of the laser source parameters on the generation of the S1 ZGV resonance are explored, and the spatial distribution of the displacement produced at the resonance frequency is determined. The predicted displacement spectrum of Lamb waves generated in micron scale plates is found to compare well with experimental measurements. In addition, experimental measurements demonstrate that the S1 ZGV resonance can be used to map subsurface features in thin (4μm) membranes at high ultrasonic frequencies (700MHz).

Experimental and numerical study of the excitability of zero group velocity Lamb waves by laser-ultrasound

“…Not surprisingly, the S 1 ZGV point dominates the spectrum for most of the investigated diameters and the response is maximal for D=h % 1:5; this result agrees with previous work. 17 With increasing spot diameter, however, the coupling efficiency of the S 1 ZGV reduces and the cutoff frequencies start to become visible, such as for the S 1 mode where the resonance peak shifts from the S 1 ZGV point to the cutoff frequency of the S 1 mode.…”

“…The different amplitudes of these oscillations, as a result of the different spot diameters, are due to the coupling mechanism of the thermoelastic excitation into the ZGV mode; it becomes optimal if the ratio D=h % 1:5. 17 In the simulations, we assumed that the maximum power density of the incident light is identical in all three cases. Consequently, the total energy of the incident pulse increases monotonically with increasing pulse diameter.…”

“…16 In previous numerical modeling of ZGV excitation using a laser source, the optimal optical spot size for exciting the S 1 ZGV resonance was found to be close to half the wavelength of the ZGV mode. 17,18 In this paper, we perform an a) Electronic mail: istvan.veres@recendt.at experimental study of the effects of spot size on ZGV excitation in a thin plate. The redistribution of acoustic energy into different modes is observed when the spot size is changed.…”

On the backward Lamb waves near thickness resonances in anisotropic plates

“…Already early systematic numerical modelling of Lamb waves in anisotropic plates (e.g., Solie and Auld, 1973;Li and Thompson, 1990) has revealed numerous examples of the ranges of negative in-plane group velocity occurring at the onsets of different dispersion branches. Identifying these branches in a way which would be less effort-consuming than numerical computing of the whole dispersion spectrum is of practical interest in view of particular significance of the backward Lamb waves for various evaluation and imaging techniques (Liu et al, 2000;Germano et al, 2002;Durinck et al, 2002;Parygin et al, 2000;Marston, 2003;Holland and Chimenti, 2004;Clorennec et al, 2006Clorennec et al, , 2007Balogun et al, 2007). They also bear important implications for numerical schemes involving the normal-mode decomposition and perfectly matched layers (Pagneux and Maurel, 2002;Castaings and Hosten, 2003;Skelton et al, 2007).…”

Imaging of transient surface acoustic waves by full-field photorefractive interferometry

“…They have attracted much attention, by virtue of their non-contact, non-destructive nature and high flexibility of implementation. [1][2][3][4][5][6] Moreover, possible bandwidth limitations of electronic detection can be circumvented by making use of an optical delay line. 7,8 Optical detection of SAW induced displacements at a single location can be done with high sensitivity, e.g., by interferometric detection, 9,10 laser beam deflection, 11,12 modulated optical reflection, 13 or heterodyne diffraction detection.…”