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An experimental study is presented on the wear and failure of initial coarse-grained and modified ultrafine-grained Ti-6Al-4V alloy with a TiN protective coating, subjected to high-speed dynamic erosion by solid corundum particles with an average size of 109 μm in an air flow at a speed of 150 m/s and an erosion time of 30, 60, 180, 300 and 600 s. The experimental results are used to determine the erosive wear rate and the shear area percentage, as well as to measure the worn layer depth and erosion-induced changes in microhardness and structure of the alloy near the coated and uncoated surface. It is shown that all alloy samples are prone to wear and failure under the given high-speed erosion conditions, but their behavior is closely related to the erosion time and the structure of the substrate. The protective coating deposited onto the surface of ultrafine-grained Ti-6Al-4V titanium alloy significantly reduces the erosive wear rate compared to a similar coating on as-received coarse-grained alloy. [ABSTRACT FROM AUTHOR]

This paper is an experimental and theoretical study of changes in a laser-induced photoacoustic signal near a hole in metal plates of D16 aluminum alloy. The process of photoacoustic signal generation from metals is analyzed within the classical thermoelasticity framework, with account for only the thermal effect of laser irradiation on metal lattice deformation. A comparison is made of photoacoustic experimental and theoretical results obtained for a mechanically stressed aluminum alloy plate with a hole. The effect of external mechanical stresses on the behavior of laser photoacoustic signals near the hole is studied experimentally and theoretically. It is found that the classical theory of thermoelasticity is not enough to correctly describe the stress dependence of the photoacoustic signal near a hole in aluminum alloy plates. In order for the obtained experimental and theoretical results to agree, along with the thermal effect of laser irradiation on the lattice, it is necessary to account for the additional effect of the electron gas. A calibration procedure of the laser photoacoustic signal with respect to stress is described. It is shown that internal stresses in metals can be estimated by the photoacoustic method in combination with the hole drilling method. [ABSTRACT FROM AUTHOR]

A two-component model which accounts for electron gas pressure is proposed for describing the dynamics of thermoelastic and thermoacoustics effects in laser-irradiated conductors. The model medium represents two interpenetrating continua such that interacting particles of both exist at each point of the medium. The electron gas in the model comprises free and bound electrons of which the former obey the laws for perfect metals and the latter obey those that account for electron trapping to localized levels and for electron transitions from level to level, i.e., for jump diffusion and hopping conductivity. Unlike the classical model of thermoelasticity, the proposed model is the first to show that the electron gas pressure depends strongly on the temperature difference between the electron gas and the conductor lattice and on the change in the density of free electrons as localized species become free by the Mott mechanism. The duration of acoustic pulses in the conductor lattice is essentially dependent on the time of laser irradiation and on how long the gas and the lattice differ in temperature, with the longest acoustic pulse falling on a certain localized electron density. The model data are compared with experiments. [ABSTRACT FROM AUTHOR]

Numerous experimental studies on shock wave loading of metals have shown by electron microscopy that the crystal structure of the material can undergo transformation in a certain impactor velocity range. At the macroscale, these changes are observed as energy losses associated with the formation of a new structure. The losses are manifested on the time-velocity profile of the rear target surface which contains key information about the material properties. In this paper, a two-component model of a material with a nonlinear internal interaction force is proposed for the description of structural transformations, taking into account the periodic structure of the material. Dynamic equations are written with respect to the motion of the center of mass of the components acting as a measured macroparameter, as well as with respect to their relative displacement serving as the internal degree of freedom responsible for structural transformations. The proposed model is applied to solve a quasi-static problem of the kinematic extension of a two-component rod in order to determine the parameters of a nonmonotonic stressstrain curve, which is often used in describing materials subjected to phase transformations. By solving a dynamic problem of nonstationary impact on the material by a short rectangular pulse, the effect of nonstationary wave damping is demonstrated which is associated with the wave energy dissipation in structural changes of the material. An analytical expression is obtained on the basis of a continuous-discrete analogy for estimating the duration of structural transformations and the parameter characterizing the internal interaction force between the components. The conclusions are confirmed by a numerical solution of a nonlinear Cauchy problem within the finite difference framework. [ABSTRACT FROM AUTHOR]

The paper proposes a new graphene resonator circuit which operates on the principle of a self-oscillator and has no drawbacks typical of nanoresonators as mass detectors and associated with their law quality factor, eigenfrequency errors (measurements from resonance curves), and dependence of quench frequency on oscillation frequency (curves with quenching for nonlinear systems). The proposed circuit represents a self-oscillator comprising an amplifier, a graphene resonator, and a positive feedback loop with a graphene oscillation transducer, and its major advantage is in self-tuning to resonance frequency at slowly varying resonator parameters, compared to oscillation periods. The graphene layer with a conducting substrate beneath it forms a capacitor which is recharged by a dc voltage source as its capacitance varies due to graphene deformation, and the recharge current is an oscillation- dependent signal transmitted from the transducer to the amplifier input. The graphene layer is placed in a magnetic field and is deformed when a current from the amplifier output is passed through. By properly choosing the magnetic field direction and the amplifier gain, it is possible to provide swinging oscillation whose amplitude is limited by the amplifier nonlinearity. For the proposed system we present an electromechanical model, dimensionless equations of motion, and numerical data demonstrating the generation of steady-state oscillations with eigenfrequency. Also presented is an analysis showing that the system can have only one limit cycle and that this cycle is always stable. The proposed resonator circuit can be used as a mass detector which determines the added mass from a change in self-oscillation frequency. [ABSTRACT FROM AUTHOR]

The paper considers a new 3D nanostructuring technology of next-genergtion ceramic composites based on a ceramic matrix reinforced with titanium nitride (TiN) gnd silicon carbide (diC). Research data are reported on the formation of TiN and diC nanostructures on the surface of disperse alumina during successive gas chemisorption of organic Ti(N(CH)) (tetrakis-dimethylamino-titanium, TDMAT) and ammonia NH in the first case and ClSi(CH) and methane CH in the second. Such chemisorption increases the number of surface-attached Ti-N groups crystallized on annealing at 1100°C with the formation of a TiN or a SiC nanoparticle layer. According to X-ray diffraction and electron microscopy, TiN nanoparticles with an average size of about 40 nm are formed on the surface of alumina particles after TDMAT and NH treatment for 2 h and subsequent annealing at 1100°C. The mechanical properties of compacted α-A1O-based ceramics reinforced with TiN and SiC nanoparticles excel the properties of the best ceramic materials provided by different manufacturers. [ABSTRACT FROM AUTHOR]

The goal of this study is to construct simple electromechanical models of nanoresonators as mass detectors. A major obstacle in the achievement of sufficient measurement accuracy for the resonant frequency associated with the adsorption of additional mass onto the graphene layer is a low quality factor of the oscillatory system containing the graphene layer. A graphene resonator can be considered as an elastic system with distributed parameters. The application of the Galerkin method to study nearly resonant vibrational modes reduces the problem to considering an oscillatory system with a few degrees of freedom with pronounced nonlinear properties. These properties are, first of all, due to the nonlinear dependence of the forces produced by the electric field on the graphene deflection and, second, due to the nonlinear dependence of the graphene layer tension on its deflection. Taking into account the nonlinear properties leads to the appearance of characteristic drops in the resonance curve which allow for a more accurate resonant frequency measurement. Resonance curves with such characteristic drops can be obtained using a demonstration experimental macromodel of the resonator. Two absolutely new layouts are proposed, such as a differential resonator and resonator with parametric excitation. The oscillations excited in the differential resonator that contains two graphene layers resemble beats. In this case, small changes in the mass of the main layer correspond to significant changes in the frequency of the envelope. This effect is illustrated by oscillograms obtained for an experimental macromodel of the differential resonator. The parametric resonator has one graphene layer between two conducting surfaces. Parametric excitation of steady-state high amplitude oscillations is possible in this resonator only in a narrow frequency band close to the eigenfrequency. The band width reduces with a decrease in the quality factor of the oscillatory system. The latter fact can be useful for the improvement of eigenfrequency measurement accuracy at a low quality factor of the oscillatory system. [ABSTRACT FROM AUTHOR]

Alumina micro- and nanopowders with the particle size from 200 μm to 40 nm synthesized by the sol-gel method are studied. The particle size dependence of γ-AlO→α-AlO phase transformation is studied by differential thermal analysis, X-ray diffraction method, and transmission electron microscopy. X-ray diffraction data show that for alumina nanoparticles γ-AlO→θ-AlO phase transformation occurs at 900°C, and for micro-particles it occurs in the temperature range 1150-1200°C. The alumina ceramics produced of alumina nanoparticles is shown to have higher flexural strength under three-point bending than the ceramics produced of micro-particles. The obtained results demonstrate that alumina particle size reduction stabilizes the formation of α-AlO at lower temperatures, due to which the grain growth rate decreases and the flexural strength of monolithic oxide ceramics increases. [ABSTRACT FROM AUTHOR]

The paper presents a selection of simulation results obtained for nanosized objects in continuum and structural mechanics. Eigenfrequency measurements as a tool to study nanostructures composed of a multitude of like nanoelements are discussed. The structures under consideration are ordered arrays of nanoobjects (nanocrystals, nanoshells) fixed on an elastic substrate. It is shown that from the eigenfrequency spectrum of this type of mechanical systems one can determine several eigenfrequencies of a single nanoobject. Also considered in the paper is accounting for surface stress and associated modification of effective properties of nanomaterials. [ABSTRACT FROM AUTHOR]

The article presents documentation from a plan proposed by scientists in the Soviet Union from the 1980s to develop a spacecraft that could be used to study the Sun, interplanetary space, and other planets of the solar system. It provides a detailed description of the design of the proposed probe, referred to as the Tsiolkovsky project, and explores how automated spacecraft such as the Voyager 1, Pioneer 10, and Viking 1 have previously been successful in providing reliable information to scientists. It describes how the design of the probe will have to accommodate for issues such as the gravitational pull of planets, the heat of the Sun, and the large distances it will need to travel, and provides detailed drawings on the multiple parts of the spacecraft.

In recent years, virtual worlds have become increasingly popular in education and social life. Using a 'city' metaphor, we are developing a 3D virtual world enriched with social and educational tools. This virtual world allows performing a number of activities, such as exploring the city, learning and communicating with its citizens, building a social network. In this paper we report the results of case studies we have performed to explore users' activities and behavior in a virtual city context. Our goal was to investigate how such a city can facilitate learning and socializing, also in a cross-cultural context. Based on the empirical results, we derive a list of recommendations for a virtual city as a place for social and educational activities. [ABSTRACT FROM AUTHOR]