Index of content:
Volume 13, Issue 1, January 2013
- Physical Acoustics
13(2013); http://dx.doi.org/10.1121/1.3646292View Description Hide Description
This paper reflects a presentation made at the 16th International Conference on Nonlinear Elasticity in Materials (the XVI ICNEM) held in Prague, Chech Republic, on June 5-11, 2011. It includes an outline of nonlinear dynamical models and experimental facts related to "slow time" phenomena in rock and non-consolidated geomaterials, including our field experiment, where a logarithmic restoration of Rayleigh wave phase velocity was observed. Several simplified models of stress-strain relation in media with relaxation and wave propagation in such media is also considered.
13(2013); http://dx.doi.org/10.1121/1.3646380View Description Hide Description
The composite samples of Al-alloy B95 reinforced with nanoparticles of a synthetic diamond have been fabricated by direct-melting the starting B95, adding the nanodiamond powder in the melt, and hashing it by an Ar-barbotage, followed by cooling and quenching. The linear and nonlinear elastic properties of the prepared samples have been investigated. The second order elastic constants (SOEC) have been determined on the basis of the measured sound velocities and densities of the samples. The data on the sound velocities of the longitudinal VL and transverse VT waves were obtained with an accuracy of ~1%; the elastic modules ~2-3%. For a quantitative characterization of the nonlinear elastic properties of a material, we measured the third order elastic constants (TOEC) of the composite samples by using a Thurston-Brugger method. Some relative changes of the elastic waves velocities in the tested samples from magnitude of the homogeneous uniaxial stress applied to it have been measured experimentally. The measurements have allowed to determine all independent TOEC C111, C112, C123. A relative comparison of the TOES data for B95/Nanodiamond composite with the data for the starting B95-alloy is made and discussed.
13(2013); http://dx.doi.org/10.1121/1.3664299View Description Hide Description
Here are represented the results of studying of nonlinear acoustic phenomenon in 1-D model unconsolidated granular medium. The medium was modeled as a system composed of identical steel spheres with the same radius, placed in a tube. Nonlinear acoustic effects in structures with the contact interaction are described by Hertz law.Investigation of nonlinear properties of the system was carried out using the spectral method. We experimentally investigated characteristics of the second and the third acoustic harmonics generation. The deformation of the spheres e during the experiment changed as we applied different static external force. Elastic nonlinearity of the system of spheres decreased with static strain e increasing. Analysis of the experimental results showed that if spheres experience elastic deformation, the amplitude of the second harmonic is proportional to 1/e, and the amplitude of the third harmonic is proportional to 1/e^2. These results are in agreement with theoretical analysis based on Hertz theory. However, the presence of dispersion in the radius of spheres leads to a drastic increase of the amplitudes of harmonics in a number of experiments when strain e of the system was equal to 2*10-5 - 3*10-5.We present a discussion of the results of the experiment.
13(2013); http://dx.doi.org/10.1121/1.3670734View Description Hide Description
Frictional interaction of two elastic spheres is a fundamental problem of contact mechanics. The tangential displacement depends on the normal and tangential force in a complicated and, in general, hysteretic way. The key characteristic of the system is the traction distribution in the contact zone that has a complex piecewise-smooth character. We propose a method of memory diagrams that enables to extract all memory information from traction distributions and store it in a compact manner. Then, instead on considerations on the case-to-case basis of a particular loading history, a general algorithm applicable for any loading history can be formulated. Finally, we used the obtained solution to prove that friction of two spheres is the Preisach system. The Preisach formulation of the problem enables to replace the custom-made algorithm of the memory diagrams method by a standard routine used over more than 70 years in ferromagnetism.
13(2013); http://dx.doi.org/10.1121/1.4789475View Description Hide Description
Protection against surface and underwater threats from swimmers constitutes one of the most challenging aspects of port security. The envisioned risk mitigation consist of passive acoustic detection and localization and active diver deterrent, which can be done by focusing high intensity acoustic waves. The main goal of this study is to explore the possibility of using the Time Reversal Acoustics (TRA) sound focusing system for non-lethal swimmer neutralization. This breakthrough technology enables the precision targeting of a hostile diver with minimum impact to the marine environments. The acoustic noise radiated by the diver is used to focus the acoustic energy, while moving diver acts as an active self disclosing acoustic beacon. In a shallow water environment the radiated noise, direct and multi-path interference is rich and is exploited through the use of TRA. The effectiveness of the TRA focusing and the diver deterrence zone are modelled taking into consideration the frequency range, harbour depth, geometry of the transducer array. It is demonstrated that outside the focal region, the TRA system produces acoustic intensities that are not harmful for marine life.