Volume 135, Issue 5, May 2014
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
Resonant ultrasound spectroscopy for viscoelastic characterization of anisotropic attenuative solid materials135(2014); http://dx.doi.org/10.1121/1.4869084View Description Hide Description
Resonant ultrasound spectroscopy (RUS) is an accurate measurement method in which the full stiffness tensor of a material is assessed from the free resonant frequencies of a small sample, and the viscoelastic damping is measured from the resonant peaks width. High viscoelastic damping causes the resonant peaks to overlap and therefore complicate the measurement of the resonant frequencies and the inverse identification of material properties. For that reason, RUS has been known to be fully applicable only to low damping materials. The purpose of this work is to adapt RUS for the characterization of highly attenuating viscoelastic materials. Spectrum measurement using shear transducers combined with dedicated signal processing is employed to retrieve the resonant frequencies despite overlapping. A probabilistic (Bayesian) formulation of the inverse problem, tackling the problem of correctly pairing the measured and predicted frequencies, is proposed. Applications to polymethylmethacrylate (isotropic) and glass/epoxy transversely isotropic samples are presented. The full set of viscoelastic properties is obtained with good repeatability. Particularly, elastic moduli of the isotropic samples are obtained within 1%.
135(2014); http://dx.doi.org/10.1121/1.4869691View Description Hide Description
Measuring guided waves in cortical bone arouses a growing interest to assess skeletal status. In most studies, a model of waveguide is proposed to assist in the interpretation of the dispersion curves. In all the reported investigations, the bone is mimicked as a waveguide with a constant thickness, which only approximates the irregular geometry of cortical bone. In this study, guided mode propagation in cortical bone-mimicking wedged plates is investigated with the aim to document the influence on measured dispersion curves of a waveguide of varying thickness and to propose a method to overcome the measurement limitations induced by such thickness variations. The singular value decomposition-based signal processing method, previously introduced for the detection of guided modes in plates of constant thickness, is adapted to the case of waveguides of slowly linearly variable thickness. The modification consists in the compensation at each frequency of the wavenumber variations induced by the local variation in thickness. The modified method, tested on bone-mimicking wedged plates, allows an enhanced and more accurate detection of the wavenumbers. Moreover, the propagation in the directions of increasing and decreasing thickness along the waveguide is investigated.
135(2014); http://dx.doi.org/10.1121/1.4869692View Description Hide Description
It is well known that the Rayleigh wave propagating along a permeable plane boundary of a poroelastic half-space may have a high-frequency cutoff beyond which the corresponding Rayleigh pole is absent. The present study investigates the specific features of the surface wave propagation during transition through this cut-off frequency. Using a set of experimentally determined mechanical parameter values for water-saturated sintered glass beads in the framework of Biot's theory, this theoretical investigation indicates the following. The Rayleigh wave upper cut-off frequency may occur within a physical frequency range over which the characteristic wavelength far exceeds typical pore size. Beyond the cut-off frequency, the Rayleigh pole migrates onto the non-principal, in other words, unphysical, Riemann sheet. As a consequence, during this transition, the Rayleigh wave transforms into a pseudo-interface wave and radiates part of its energy into the interior of the half-space in the form of P2-wave motion.
135(2014); http://dx.doi.org/10.1121/1.4869085View Description Hide Description
A model describing the sound propagation between two infinite adsorbing plates is proposed in order to investigate the extension to the audible sound range of the Frequency Response method applied to the measurement of diffusion in micropores. The model relates adsorption parameters (i.e., diffusivity and equilibrium constant) to an acoustic quantity (i.e., propagation constant). The equations describing sound propagation in the presence of adsorbing boundaries are obtained on the basis of the classical Kirchhoff theory [(1868). Ann. Phys. (Leipzig) 134, 177–193]. The solution is derived using the Low Reduced Frequency Approximation method [Tijdeman, (1975). J. Sound Vib. 39, 1–33].
135(2014); http://dx.doi.org/10.1121/1.4869684View Description Hide Description
In the present work, the interaction of Rayleigh waves with a delamination in a fiber reinforced composite plate was analyzed. Rayleigh waves, upon interacting with delamination mode, convert into Lamb waves in the delamination zone. These guided Lamb modes have the capability to mode convert back into Rayleigh modes when they interact with the edge of the delamination. A unidirectional glass/epoxy laminate with a delamination of known size was fabricated and tested using air-coupled ultrasonics. Finite element models were developed to understand the mode conversions occurring at various sections of the delamination. Particle displacements along with numerical and experimental velocities were considered to identify each mode. Conclusions were drawn based on the velocity analysis.