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The nonlinear interaction between a high-frequency probing Lamb mode, propagating through a bilayer containing an initially closed tangential delamination at the interlayer interface whose contact conditions are dynamically changing because of a high-amplitude Lamb pump wave with long wavelength, is modeled in a quasi-stationary approach. The proposed criterion for the delamination to open is based on the magnitude, with respect to a threshold value, of the pump-wave-induced normal stress components that are pulling on both sides of the interface during part of its cycle. The temporal evolution of the contact condition during the pump wave cycle is calculated and the spectral enrichment caused by cross-modulation spectral components between the probing wave and the modulating wave is predicted. The impact of the cross-modulation on the normal displacement at the externally accessible surfaces is investigated for two different types of incident probing wave in an absorbing bilayer structure, and for different variations of the contact quality modulation. The results can serve as a parametric guide for experimenters considering the use of nonlinear harmonic generation of Lamb waves as a tool for nondestructive testing of bilayers such as rubber-steel composite walls that are typically used in storage tanks and pipelines for corrosive liquids. [ABSTRACT FROM AUTHOR]

Full-field dynamic shearography and laser Doppler vibrometric scanning are used to investigate the local contact acoustic nonlinear generation of delamination-induced effects on the vibration of a harmonically excited composite plate containing an artificial defect. Nonlinear elastic behavior caused by the stress-dependent boundary conditions at the delamination interfaces of a circular defect is also simulated by a 3-D second-order, finite-difference, staggered-grid model (displacement-stress formulation). Both the experimental and simulated data reveal an asymmetric motion of the layer above the delamination, which acts as a membrane vibrating with enhanced displacement amplitude around a finite offset displacement. The spectrum of the membrane motion is enriched with clapping-induced harmonics of the excitation frequency. In case of a sufficiently thin and soft membrane, the simulations reveal clear modal behavior at sub-harmonic frequencies caused by inelastic clapping. [ABSTRACT FROM AUTHOR]

Three experimental laser ultrasonic configurations—line excitation with scanning detection, grating excitation with single point detection, and grating excitation with scanning probe beam—are shown to consistently reveal the modified propagation properties of Lamb waves on a polyethylene terephthalate (PET) film that is in contact with different liquids on its two sides. Theoretical predictions concerning the physical nature of different wave modes in symmetric and asymmetric film loading configurations (i.e., their existence, velocity, damping, and polarization) are confirmed by the experimental results. [ABSTRACT FROM AUTHOR]

A theoretical study of Lamb mode propagation through an absorptive bilayer consisting of a steel plate coated by highly absorptive rubber with a finite delamination dividing these layers is presented. Essentially, in such a bilayer structure, steel-like A0 and S0 modes still propagate, although with moderate absorption. Two different types of delaminations are taken into consideration: slip delaminations in which noncontact boundary conditions are assumed only for shear displacement and stress, and stress-free delaminations in which noncontact boundary conditions are assumed for all mechanical displacements and stresses. The calculations, which are based on a modal decomposition method, show that delaminations in absorptive bilayers result in a considerable change of the normal displacement amplitude at the bilayer surfaces inside the delamination region, and for an incident mode steel-like A0 mode, also in the transmission region. Stress-free and slip delaminations can be distinguished exploiting their different effect on the steel-like A0 and S0 incident modes. [ABSTRACT FROM AUTHOR]

An experimental and theoretical study of bending modes in a partially electroded circular piezoelectric quartz (AT-cut) with free edge is presented. The quartz is excited by a voltage pulse applied on the electrodes, and its surface is scanned by a laser vibrometer that measures the out-of-plane displacements. The classical theory of bending of thin disks is used to describe the flexural modes at frequencies lower than the first thickness shear resonance (6 MHz). A fairly good agreement is found between experimental and theoretical results for the forced mode shapes and for the resonance frequencies. However, it appears that the two springs used to maintain the disk in position introduce extra clamping conditions. Several source shapes were studied, among which a collection of an arbitrary number of forces is particularly useful. The two-dimensional wavenumber representation shows the presence of anisotropy related to the crystallographic axes at higher frequencies, which is not predicted by the model. The experimental phase velocities are compared to those given by the classical theory of disks and to those of Lamb A0 mode. This study confirms the correspondence at low frequencies between the A0 mode and the bending eigenmodes of a disk with finite size. [ABSTRACT FROM AUTHOR]

By removing the symmetry of a free plate configuration, fluid loading significantly modifies the nature of acoustic waves travelling along a plate, and it even gives existence to new acoustic modes. We present theoretical predictions for the existence, dispersive behavior, and spatial distribution of leaky Lamb waves in a fluid-loaded film. Although Lamb modes are often investigated by studying the radiated fluid waves resulting from their leakage, here their properties are assessed by detecting the wave displacements directly using laser beam deflection. By using crossed laser beam excitation, the detection and analysis of the different modes is done at a fixed wavelength, allowing one to verify the existence, the velocity, and the damping of each predicted mode in a simple and unambiguous way. Our theoretical predictions for the nature of the modes in a waterloaded Plexiglas film, including parts of looping modes, are experimentally confirmed. [ABSTRACT FROM AUTHOR]