Bart Sarens

Full-field imaging of nonclassical acoustic nonlinearity

The feasibility of full field shearographic detection of nonclassical acoustic nonlinearity is investigated. Traditional frequency analysis of the sinusoidally excited sample, as used in scanning techniques, turns out to be not practical due to the inherent optical detection nonlinearity of the shearography system itself. An alternative method, based on determining the asymmetry between shearographic images stroboscopically obtained for positive and negative displacements, is proposed. This approach allows us to easily and rapidly detect the tension-compression asymmetry which typically arises where nonbounded contact interface defects are present.

Transient space-time surface waves characterization using Gabor analysis

Laser ultrasonics allow the observation of transient surface waves along their propagation media and their interaction with encountered objects like cracks, holes, borders. In order to characterize and localize these transient aspects in the Space-Time-Wave number-Frequency domains, the 1D, 2D and 3D Gabor transforms are presented. The Gabor transform enables the identification of several properties of the local wavefronts such as their shape, wavelength, frequency, attenuation, group velocity and the full conversion sequence along propagation. The ability of local properties identification by Gabor transform is illustrated by two experimental studies: Lamb waves generated by an annular source on a circular quartz and Lamb wave interaction with a fluid droplet. In both cases, results obtained with Gabor transform enable ones to identify the observed local waves.

2D Finite Impulse Response filters for surface wave identification

Elliptical areas transfer function Finite Impulse Response filters are investigated (E-FIR). The main axis of the elliptical area is oriented along the selected surface wave dispersion curve. The advantage of such areas is the analytical formulas of the impulse responses as a function of the frequencies bandwidths parameters, k and ω, without the need to use Inverse Fourier transform. E-FIR filters are tested on experimental space-time signals corresponding to the propagation of Lamb waves generated by a pulse and observed by classic transducers on a cylindrical shell and by laser Doppler on a plate. E-FIR filters exhibit their potential to extract Lamb modes and their direction of propagation, even in noisy datasets. Due to their natural round shape energy distribution in k-ω space, E-FIR filtering adds few artefacts to the filtered signals. By using wide k-bandwidth and narrow ω-bandwidth E-FIR filter, Gabor like analysis is also reached.

Combined Particle Motion And Fluid Pressure Measurements of Surface Waves

In this paper we investigate the use of combined particle motion and fluid pressure measurements of surface waves. In particular, we show that the surface-wave impedance of the pseudo-Rayleigh (pR) wave, excited at a water/aluminum interface, can be successfully extracted from such a combined experiment at ultrasonic frequencies. The displacement is measured using a laser Doppler vibrometer (LDV) and the pressure with a needle hydrophone. The impedance of the pR-wave can be clearly distinguished in the wavenumberfrequency domain. Our results suggest that the surface-wave impedance can also be extracted from measurements on seismic or borehole logging scales. Further, the excellent agreement between the observed and predicted pR-waveforms in both the particle displacement and fluid pressure shows that laboratory scale experiments using laser ultrasonics and small pressure detectors can offer a useful link between theoretical models and real field experiments.

Lamb wave interactions through dispersion 2D filters

Acoustic surface waves are widely used to sense and map the properties of the propagation media. In order to characterise local space-time waves, methods such as Gabor analysis are powerful. Nevertheless, knowing which wave is observed, extracting its full bandwidth contribution from the others and to map it in the signal domain is also of great interest. In the Fourier domain, the acoustic energy of a wave is concentrated along the wave-number frequency (k-ω) dispersion curve, a way to extract one wave from others is to filter the signals by mean of k-ω band-pass area that keeps only the selected surface wave. The objective of the present paper is to propose 2D Finite Impulse Response (FIR) filters based on an arbitrary area shape designed to extract selected waves. FIR filtering is based on convolving the impulse response of the filter with the signals. Impulse responses derived from using k-ω elliptical areas (E-FIR) are presented. The E-FIR filters are successfully tested on three experimental space-time signals corresponding to the propagation of Lamb waves measured by standard transducers on a cylindrical shell, by laser Doppler on a plate and generated by a circular pulse and observed by shearography on a rectangular plate.

Film thickness determination by laser ultrasonics

The thickness of films deposited on substrates is crucial for their thermal, electrical, optical behaviour. These properties are essential in thin film applications, especially in the field of microelectronics. In this study, we are interested in thickness determination of silver and gold films deposited by evaporation on a silicon substrate by using an ultrasonic non‐destructive technique. In particular, the well‐known laser ultrasonic technique is used to generate and detect the surface acoustic waves. Results obtained by two complementary methods allowing a non‐contact measurement in a large bandwidth (from 5 MHz to 200 MHz) are presented, and the dispersion of the Rayleigh wave propagation velocity is analyzed to determine the film thickness.

Stroboscopic interferometric full‐field imaging of laser‐induced surface acoustic waves

Surface acoustic waves (SAW) have the interesting property that they specifically interact with surface and sub‐surface regions, rendering them suitable for non‐contact investigation of sub‐surface properties and heterogeneities. Transmission, reflection and diffraction effects of SAW propagation are analysed in order to reveal information on the region over which they propagate. Laser excitation allows to efficiently excite SAW with short wavelengths, enhancing both the lateral and the depth resolution. Typically information on the waves is collected by scanning a part of the surface of interest with a fast laser probe, using the surface displacement or slope as a real time witness of the wave field. Here we present results obtained by a full field imaging technique [1], in which the sample is repetitively excited by a pulsed pump laser, and the wave displacement field is stroboscopically gathered by illuminating the sample surface with an electronically delayed pulsed probe laser, whose displacement ind...

Shearographic Detection of Delaminations in Sandwich Structures: Investigation on Various Excitation Modes

Delaminations are a common problem that occurs between the core and the skins of sandwich structures. Such defects may originate either from malfunctions in the manufacturing process leading to a reduced adhesion strength or from certain types of load like impacts during the operation. This study was focused on the detection of such defects with the increasingly popular shearography technique (Butters and Leendertz [1]). Shearography is based on the speckle effect, which occurs when coherent light is scattered on a rough surface. Rays from different positions on the sample surface interfere randomly in space to create a granular kind of image, with distinct intensity fluctuations called speckles. In shearography, two images of the same area on the object are simultaneously recorded laterally sheared with one another (Fig. 1). Images are recorded before and after the excitation and their difference results in a fringe pattern, which is associated with the full strain field.