No. 5, 1997


Ivo Dolezel, Josef Hodl, Rudolf Tesar, Bohus Ulrych: Contribution to study of dielectric stress in insulation of a buried telecommunication cable from galvanic overvoltage influences, 539–555

The paper presents new knowledge concerning effects of faults on hv overhead lines on near buried telecommunication cables. First, several analytical and numerical methods have been used to find the potential distribution near the cable and on its surface, which represents the first step for determining the stress distribution within its insulation. Second, some simple measures are suggested to decrease the harmful influences and their effectivity is evaluated.

Jaroslav Kucera, Maciej Hajduga: Time dependence of oxidation of Fe-wires in ambient air at 1100°C, 557–568

The oxidation of Fe AREMA wires is investigated in this paper. Iron samples were oxidized at 1100°C in ambient air. The periods of oxidation time varied from 10 hr up to 80 hr. Weight and size measurements, metallography, and chemical microanalysis were used as experimental methods. In has been found that the Fe wires being of 3 mm in diameter are partly oxidized at the oxidation times 10 up to 20 hr, and completely oxidized at >= 40 hr. Metallographic investigations have shown that the partly oxidized samples contain Fe remainders of cylindrical shape. Microchemical analysis has revealed that the scale mass produced in the course of oxidation process, 10–20 hr, contains approximately 54 at. %O, and cca 58 at. %O in completely oxidized samples, 40–60 hr.

Josef Mrazek: Thermally activated processes in a disordered polymer structure, 569–581

Thermally activated processes in a disordered polymer structure are approached as a superposition of reversible chemical reactions with distributed activation energies. A modified activation volume concept is used and the relevant distribution function is derived inclusive of its temperature and pressure dependence. A symmetrical exponential distribution of activation energies is identified as the only possible cause that i) brings about the power-law dependence of the polarisation and depolarisation current after a step-wise change of the electric stress and ii) shapes the frequency characteristics of the dielectric in the form of more or less flat arcs of the complex permitivity. The existence of the skewed arcs of the frequency dependences is attributed to the presence of an unoccupied free volume that is distributed exponentially in the polymer at temperatures higher than the temperature of glass transition. It is the free volume that causes a not symmetrical distribution of activation energies. The paper presents examples of the DC and AC characteristics, which have been calculated according to the discussed models. The skewed arcs of the frequency characteristics are compared with the classical data of Negami and Havriliak.

Zdenek Knesl, Karel Kubacek, Josef Cadek: On the double shear creep test technique using specimens of a very simple geometry, 583–589

This paper reports the results of finite element calculations carried out on a double shear creep specimen geometry suggested in [5]. The stress and strain distribution during the creep exposition under conditions corresponding to the applied experimental set up is presented and discussed. It is concluded that the suggested geometry of the specimen and corresponding experimental set up are suitable for double shear testing.

Frantisek Kroupa: Quenching residual stresses in continuously thermal sprayed coatings and plates, 591–612

An analytical theory of quenching macroscopic residual stresses in thermally sprayed thick coatings and free-standing plates (FSP) is presented. The theory is based on a simplified model of continuous spraying. Layers of infinitesimal thickness are gradually deposited and immediately cooled down to the constant bulk deposit temperature. The general formulation is given for inhomogeneous coatings and FSP with the material properties depending on the coordinate perpendicular to the plate plane. The cases of homogeneous coatings and FSF are discussed in detail. The secondary stresses formed during final cooling to the ambient temperature are commented.

Ales Tondl: To the quenching of a kinematically excited body, 613–621

A basic body suspended on two rods and capable of swinging as a pendulum in a horizontal direction is excited kinematically through a harmonic motion of its suspension points. An additional mass is elastically attached to the body, to function in either vertical (System I) or horizontal direction (System II) similarly as a dynamic absorber. The conditions for an optimal tuning and damping are investigated.

Werner Eberle, Ulrich Margull, Jurgen Grebler, Jurgen Hirschinger, Wilhelm Prettl, Helmar Kostial, Vit Novak: Rotating filaments in a n-GaAs Corbino disk, 623–632

Self-organized current filaments between two concentric contacts in an n-GaAs epitaxial film have been investigated in a magnetic field perpendicular to the sample. Two-dimensional spatial images of the current density distribution have been obtained by the method of quenched luminescence. It is shown that under the influence of an increasing transversal Lorentz force the static filaments are at first bent, then destabilized and finally the filament image gets completely blurred. Time resolved recordings of the sample voltage in the last stage reveal very complex but strictly periodic oscillations. Their analysis as well as other experimental results indicate that these oscillations are caused by filaments rotating around the central contact.

Jan Hlina, Vaclav Nenicka: Detection of nonlinear transitions in an arc discharge oscillating in magnetic field, 633–642

Various methods of diagnostics of nonlinear transitions were tested on experimental data acquired in a mercury vapour arc discharge exposed to the external magnetic field. The time series reflecting the arc current and optical radiation from two different parts of the arc column were analyzed using various methods of signal processing including the FFT, correlations, correlation dimensions and wavelets. The capabilities of the applied methods are illustrated on an example of the system evolution with transitions between oscillation regimes characterized by different types of subharmonic frequencies.