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In this paper, we present a method to recover the complex wavenumber dispersion relations using spatial Laplace transform from experimental spatiotemporal signals measured by laser ultrasonic technique. The proposed method was applied on zero-group-velocity Lamb modes in order to extract the ultrasonic attenuation in a polycrystalline aluminum plate of about 70 μm thickness. The difference between the experimental and theoretical Laplace Fourier transforms was minimized in the least square sense to extract the complex amplitudes and complex wavenumbers of the modes at about 40 MHz. The experimental results were compared to values reported in the literature that were measured by other means and those estimated by using the quality factor extracted from a single temporal signal. [ABSTRACT FROM AUTHOR]

The complex dispersion relation of surface acoustic waves (SAWs) at a lossy resonant metasurface is theoretically and experimentally reported. The metasurface consists of the periodic arrangement of borehole resonators in a rigid substrate. The theoretical model relies on a boundary layer approach that provides the effective metasurface admittance governing the complex dispersion relation in the presence of viscous and thermal losses. The model is experimentally validated by measurements in the semi-anechoic chamber. The complex SAW dispersion relation is experimentally retrieved from the analysis of the spatial Laplace transform of the pressure scanned along a line at the metasurface. The geometrical spreading of the energy from the speaker is accounted for, and both the real and imaginary parts of the SAW wavenumber are obtained. The results show that the strong reduction of the SAW group velocity occurs jointly with a drastic attenuation of the wave, leading to the confinement of the field close to the source and preventing the efficient propagation of such slow-sound surface modes. The method opens perspectives to theoretically predict and experimentally characterize both the dispersion and the attenuation of surface waves at structured surfaces. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

This paper presents a method for the mechanical characterization of isotropic foams at low frequency. The objective of this study is to determine the Young's modulus, the Poisson's ratio, and the loss factor of commercially available foam plates. The method is applied on porous samples having square and circular sections. The main idea of this work is to perform quasi-static compression tests of a single foam sample followed by two juxtaposed samples having the same dimensions. The load and displacement measurements lead to a direct extraction of the elastic constants by means of normalized stiffness and normalized stiffness ratio which depend on Poisson's ratio and shape factor. The normalized stiffness is calculated by the finite element method for different Poisson ratios. The no-slip boundary conditions imposed by the loading rigid plates create interfaces with a complex strain distribution. Beforehand, compression tests were performed by means of a standard tensile machine in order to determine the appropriate pre-compression rate for quasi-static tests. [ABSTRACT FROM AUTHOR]

The aim of the present study is to develop a multifunctional sugarcane fibres reinforced epoxy porous composite combining good acoustic and mechanical properties. The study of the bagasse fibres geometry has shown that fibres’ length and diameter distribution can be fitted by lognormal laws. Composites manufactured by thermo-compression process with an epoxy matrix were characterized using an experimental design whose parameters were the diameter of the fibres (between 0.5 and 4 mm) and their mass ratio (between 40 and 70 %). The study of acoustic properties showed that the sound absorption increases with the diameter of the fibres, while decreasing with their mass proportion, over a frequency range between 500 and 1000 Hz. Mechanical characterisation by bending tests, has shown a fragile behaviour, with deviations of stiffness and maximum stress around 36 %. The stereo-correlation image analysis confirmed the heterogeneity of the strain fields throughout the thickness, in relation to the fracture observation. [ABSTRACT FROM AUTHOR]