Влияние адгезии на трение качения и скольжения: эксперимент

Normal and tangential contact between a cylindrical steel indenter (wheel) and an elastomer with high adhesive properties is investigated. In the case of indentation in the normal direction, a computer simulation of the process of indentation and detachment was carried out, which shows good coincidence with an experiment. For the rolling friction mode, when analyzing the measured dependences of the tangential component of the contact force on the wheel displacement, the adhesive component of the friction force was determined. The situation of sliding friction, in which the rotation of the wheel was impossible, is considered. In the presence of adhesion, the sliding friction force is proportional to the contact area. In the absence of adhesion (the elastomer is covered with a chalk dust), a stick-slip friction mode is realized. The frequency and amplitude of stick-slip transitions depend on the indentation depth of the indenter into the elastomer.

STUDY OF THE CORRELATION BETWEEN THE CONTENT OF NANOOBJECTS AND SURFACE FEATURES

It is well known that at the present time two basic aspects of a material’s nano-objects sensitization should be considered. The first aspect relates to the change of the material’s basic physical-chemical properties when the concentration of the nanoobjects in the material’s body is varied. The second aspect is in regard to the modification of surface properties on the content of the nanoobjects in the material’s body. In the current paper the correlation between the concentration of the nanoobjects and wetting phenomena of the material’s surface is considered. Different experimental instruments and supporting models are presented with good coincidence.

The European Beech Annual Tree Ring Widths Time Series, Solar–Climatic Relationships and Solar Dynamo Regime Changes

In this study, the results from the analysis of annual ring widths (‘Dm’) time series of two “very sensitive” to the climate and solar–climate relationships of long lived European beech (Fagus sylvatica) samples (on age of 209 ± 1 and 245 ± 5 years correspondingly) are discussed. Both series are characterized by very good expressed and relating to the solar magnetic Hale cycle 20–22-year oscillations. A good coincidence between the changes of ‘Dm’ and the growth or fading of the solar magnetic cycle is found. The transition effects at the beginning and ending of the grand Dalton (1793–1833) and Gleissberg minima (1898–1933) are very clearly visible in the annual tree ring width data for the one of beech samples. Some of these effects are also detected in the second sample. The problem for the possible “lost” sunspot cycle at the end of 18th century is also discussed. A prediction for a possible “phase catastrophe” during the future Zurich sunspot cycles 26 and 27 between 2035–2040 AD as well as for general precipitation upward and temperature fall tendencies in Central Bulgaria, more essential after 2030 AD, are brought forth.

THE STUDY OF TEMPERATURES IN A TUNGSTEN ELECTRODE AT REVERSE POLARITY ARCING

The paper considers the features of energy release in a tungsten electrode under the reverse polarity TIG welding. The study substantiates the statement that the chemical composition of an electrode does not significantly affect the transfer of anode power to it. The specific effective power of an electrode is substantiated and taken as 6 W/A. The authors analyzed the features of arcing on the flat tip of a 3 mm diameter electrode using high-speed video. The analysis identified that at limiting currents ensuring tip melting, the tip heating is uniform over the cross-section. As a design scheme, the authors selected a continuous flat heat source on the semi-infinite rod surface with surface heat transfer. The authors obtained averaged values for volumetric heat capacity сρ=3.2 J/(cm3∙°С) and heat transfer coefficient а=0.3 cm2/s. The current at which the tip melting temperature is reached was taken as a limiting current. Using the limiting current value and start time of the electrode tip melting, the authors calculated the electrode heat transfer coefficient value b. The calculated melting depth for the over-limiting current welding mode showed good coincidence with an experiment. The authors recalculated the b value for the electrodes of 4-, 5-, and 6-mm diameter and calculated limiting currents for these diameters. The design limiting currents for these diameters also showed good coincidence with experimental results. The study showed that the increase of a coefficient up to 0.4 cm2/s does not cause changes in temperature and limiting currents at simultaneous сρ adjustment according to the constant thermal and physical properties сρа0.5. As a result, the authors obtained temperature dependencies for the electrode over time and length. Time dependence of the electrode tip heating allows calculating limiting currents with the decrease in arcing time.

Method of measuring antenna axis ratio close to unit

On purpose to increase efficiency of an orbital-frequency resource the usage of polarization-division multiplexing (PDM) is extended in satellite communication systems. For this aim the axis ratio (AR) of polarization ellipses on satellite radio lines with circular (rotating) polarization must be not less 0,94. Nowadays widespread techniques of experimental defining AR, are based on measurements of orthogonal components intensity of electromagnetic fields created by aerials. But, at high, more than 0,85 axis ratios, the information about a difference in components intensity loses on errors of the measurements realizing these techniques. The method of defining AR, close to extremely achievable value (nearly equal to unit) is presented in the article. The basic idea of this method consists in inclusion to the measurement scheme of two identical devices (DUT), working one towards to another. Also the scheme of counter inclusion of two identical DUT, similar used for measurements, may be applied for designing antenna elements with high AR. Good coincidence of results of modeling, laboratory measurements and field tests have confirmed suitability of the offered method for the designing and testing of aerials with high axis ratio.

Numerical Research of Fracture Toughness of Aged Ferritic-Martensitic Steel

Generally, material properties such as the modulus of elasticity, yield strength or fracture toughness are determined by conducting an experiment. Sometimes experimental determination cannot be done due to specific experimental conditions, lack of testing material and so on. Also, experiments are time consuming and costly. Therefore, there arises the need for alternative determination methods. A numerical method for the fracture toughness determination of steel P91 is suggested in this paper. For this purpose, the universal finite element software ABAQUS was used. The numerical simulation of the C(T) specimen tension test was carried out using non-linear simulation for a conditional load PQ determination, and linear simulation for fracture toughness value KQ determination. The suggested method is validated by comparing numerical and experimental tests results. The secondary aim of the paper is the evaluation of the ageing effect on the fracture toughness of steel P91. Thermal ageing of the steel was carried out in an electric furnace at 650 °C up to 11,000 h. As the numerical results had a good coincidence with experimental data at room temperature, the prediction of fracture toughness at elevated temperature, i.e., 550 °C, using numerical method was carried out.

SOME ASPECTS OF THE MATERIALS’ OPTICAL LIMITING FEATURES

Due to the effective use of the fullerenes and other nanoparticles in the materials for the general optoelectronic, laser and display technique, as well as for biomedicine, the different mechanisms to attenuate the light intensity are considered with good advantage in order to protect the human eyes and technical devises from high laser irradiation. Reverse saturation absorption, complex formation, scattering, etc. are taken into account. In the current paper the influence of the content of the nanoobjects on the interface relief is shown and considered as an additional possible optical limiting mechanism. Moreover, the diffraction from the nanostructured materials via high frequency Kerr effect is added to extend the numbers of the optical limiting mechanisms. Different experimental instruments and supporting models are presented. VIS lasers, AFM, OCA devises are applied to test and visualize the results. Theoretical and experimental data are in good coincidence.

Si-Ge whiskers for thermoelectric sensors design

The paper deals with studies of thermoelectric properties for Si1-xGex (x=0.01-0.05) whiskers doped with boron during their growth by CVD method. Temperature dependences of the resistance and the Seebeck coefficient for Si1-xGex whiskers were measured in the temperature range 275–550 K. A method for determination of thermoelectric parameters of the whisker was proposed with use of the whisker joints, which allows us to define a ratio of Seebeck coefficient to thermal conductivity a/k. Taking into account the obtained values of Seebeck coefficient, the whisker conductance and estimated values of thermal conductivity, parameter ZT was calculated for the whiskers and consists of 0.15 for T=200oC. The obtained value of ZT is in good coincidence with literature data for hop pressed Si-Ge nanocomposites. The humidity sensor was designed base on Si-Ge whiskers.

The Determination of Radon Diffusion Coefficient and Radon Speed Advection in the Conditions of Instantaneous Source with Diffusion-Advective Transfer in Rocks

Experimental determination of diffusion parameters in the upper part of geological section (presented by clayey weathering crust) was made by a point instantaneous source method in a diffusion mode and a linear instantaneous source diffusion-advective mode. The results obtained by these methods showed a fairly good coincidence of the obtained diffusion characteristics of the medium. The time to determine the diffusion characteristics of the medium is significantly reduced by more than an order of magnitude when using the advective method. This is a prerequisite for the widespread use of methods for determining radon hazard based on measurements of the vertical distribution of radon volume activity in the upper part of the geological section.

Theoretical and experimental studies of composite materials reinforced by carbon fabrics. Part 5: Simulation and experimental research of deformation of the carbon fabric structure

Results of simulation and experimental research allowed us to obtain values of deformation characteristics of carbon fabric structure. Experimental data were processed according to methods of measurement of state system for ensuring the uniformity of measurements. Theoretical and experimental data have good coincidence, confirming high quality of mechanic-analytical model for deformation of the carbon fabric structure.