Volume 80, Nº 10 (2016)

The problem of reconstructing local low-contrast inhomogeneities in the Earth’s surface layers by means of coherent Rayleigh surface waves is considered. It is shown that analysis of the frequency characteristics of shear projections in this wave on the surface allows construction of the function of inhomogeneity distribution in a specified depth range. The results from seismoacoustic reconstruction of inhomogeneities correlate with data obtained using standard geophysical methods and are confirmed by direct observations.

Results from studying the interaction between gas bubbles and the field of a flow-through acoustic resonator, and the Raman scattering of acoustic waves by moving bubbles, are presented. The structure of the distribution of bubble concentration in the resonator is studied. It is shown that nonlinear scattering by moving bubbles can be used to image bubble objects.

This work considers the capabilities of a hydroacoustic antenna based on principles of nonlinear acoustics to perform hydrophysical studies on long-distance paths. The antenna is described and results from initial experiments at sea are reported. The antenna is installed at a depth of 40 m at the site of investigations by the Hydrophysical Institute, Abkhazia Academy of Sciences. At a pumping frequency of 20 kHz, it allows the medium to be probed in the 300–3000 Hz range of parametric radiation on long-distance routes. The sharp directional pattern (2° angular resolution) of the broadband parametric radiation ensures single-mode excitation of the marine waveguide throughout the frequency band. Generation of a specific signal by a parametric antenna provides conditions for using the frequency dispersion of the velocity of acoustic signal propagation in the marine waveguide to compress the signal as it travels along a long-distance path.

Strength characteristics of carbon fiber reinforced plastics (CFRPs) are investigated by nondestructive means as microstructural changes in a material’s bulk under external mechanical loads. CFRP microstructure is studied experimentally via pulsed ultrasonic microscopy at the level of mechanical deformation resulting in degradation of a material’s properties. The process of composite deformation is studied by means of stepped stretching. Acoustic emissions are used to identify the stage preceding final destruction (the accumulation of microcracks, fibers breaking, and delamination) as an indicator of a material’s degradation. Pulse acoustic microscopy is used to observe the accumulation of microcracks in individual layers of a material. To study the behavior of a CFRP microstructure upon mechanical loading, tensile stress was applied to samples with cross-ply packing of fibers (0°, 90°) and (45°, −45°). It is shown that the brittle fracturing of reinforcing fibers is typical of CFRPs with fiber orientation (0°, 90°), and is accompanied by growing areas of stress concentration and a rise in of acoustic emission activity, with a subsequent increase in the signal energy and the formation of extensive interlaminar delamination. Acoustic emission shows a low level of activity for CFRP samples with fiber orientation (45°, −45°), which is accompanied by the formation of structural microdefects that are clearly visible in acoustic images.

Three types of reconstruction algorithms for the optoacoustic tomography of biological tissues, based on delay-and-sum beam-forming, Fourier transform, and time reversal, are proposed. The comparison, based on both numerical and experimental data, shows the advantages of the delay-and-sum beam-forming method, which ensures acceptable computation time and improved quality of the reconstructed image.

Structural changes induced in fused silica subjected to focused femtosecond laser radiation in the heat accumulation regime are studied. The spatial profile of the induced refractive index and the effect its amplitude has on such effects as the self-focusing of the laser radiation, and thermal and photochemical processes are investigated. Individual contributions from each of these effects are separated and their effect is described on the basis of experimental dependences for the first time. It is also shown that the use of heat accumulation regime substantially increases the induced refractive index, compared to the nonthermal regime.

A mathematical model of thermomechanical processes in the output window of a high-power continuous gas laser is developed and used to examine the windows of a СО₂ laser. The dependence of the maximum allowed output radiation power on the beam diameter and the distributions of temperatures and mechanical stresses are obtained, and the divergence of radiation is studied for windows made from ZnSe, KCl, and polycrystalline diamond (PD). In addition, the damage threshold of a composite output window made from PD with a single-crystalline region at the center is considered.

The interface relief of copper–titanium compounds and its evolution upon the intensification of explosion welding are investigated. Typical structures are revealed: splashes, waves consisting of ledges, interrupted waves, and transient states generated by interrupted waves combined into bands. The effect the mutual solubility of the initial elements has on the formation of a welded joint’s structure is determined. Intermetallic compounds are detected both on ledge surfaces and inside areas of local melting.

Grain growth in thin silver films is investigated experimentally. It is shown that the duration of the incubation period preceding linear or parabolic grain growth depends on the annealing temperature and average grain size. It is found that the lower the isothermal annealing temperature and the smaller the grain size, the longer the period of incubation, i.e., the longer the period of microstructure stability.