No. 4, 2002


Frantisek Chvala : A note on velocity distribution of free electrons in a gaseous host medium, pp.349-356

Mild solution of a spatially inhomogeneous initial-boundary value problem for the nonlinear Boltzmann equation is used for mathematical modelling of the velocity distribution function of free electrons in a partially ionized gas. Alternative results on existence, uniqueness and continuity of local solution to those presented earlier are established. The problem is assumed spatially inhomogeneous with external electric field and internal source. The solution is constructed iteratively using functional-analytic properties of the terms involved in the Boltzmann equation.

Kveta Kucharova, Shijie Zhu, Josef Cadek : High temperature creep behaviour of copper dispersion strengthened with fine alumina particles, pp.357-378

The creep behaviour of copper strengthened with fine alumina particles is investigated in two distinctly different temperature intervals. In both intervals the creep exhibits threshold behaviour; the true threshold stress decreases with increasing temperature. In the lower temperature interval (673-773K) the creep strain rate is found to be dislocation core diffusion controlled and the true stress exponent is close to 7. In the higher temperature interval (923-1023K) the creep is controlled by the lattice self-diffusion and the true stress exponent is close to 5. In both temperature intervals the apparent activation energies are much higher than the respective activation enthalpies of diffusion, i.e. the true activation energies of creep, and the apparent stress exponents are much higher than the respective true stress exponents. The differences of apparent and true activation energies are fully explained by the observed temperature dependences of the true threshold stresses. The differences between the apparent and the true stress exponents are shown to be inherent to the true threshold creep behaviour.

Hedaya Al Asooly, Josef Tlusty, Viktor Valouch : Optimal control of system with series converter only, shunt converter only, or with both series and shunt converters, pp.379-400

Recent advances in the power system handling capabilities of static switches have made the use of the voltage source inverter (VSI) feasible at both transmission and distribution levels. In this paper, comparison of the optimal control of system with unified power flow controller, shunt converter only and series converter only have been demonstrated. Tracking strategy according to steady state behaviour was applied.

Hanus Seiner, Michal Landa : Evaluation of all elastic moduli of anisotropic solids from ultrasonic wave inversion, pp.401-418

The paper presents description and comparison of various approaches for the determination elastic constants of an anisotropic solid from acoustic wavespeeds measured in symmetry and general directions. A direct numerical procedure is used for evaluation of wave phase and group velocities in an arbitrary anisotropic homogeneous media. The proposed inversive method for optimization of values of elastic constants in sense of least squares is based on the two-stage iterative algorithms utilizing a simplex method.

At first, the studied methods are tested with synthetic wave ray-surfaces. Next, the evaluation of all elastic constants is demonstrated on ultrasonic measurements of the unidirectional CFRP composite. The wavespeeds data were determined from time delay variations of wave arrivals with angular changes at fixed distance between the point source and the point-like receivers. Relations between accuracy of optimized elastic moduli and properties of used wavespeed data are analyzed and discussed.

Jaroslav Kucera, Borivoj Million, Karel Stransky : Redistribution of Al, C, Cr, Fe, Ni in steel weldments at hight temperatures 500 up to 110 C, pp.419-430

In the present paper there are reported the results of measurements of chemical diffusion of Al, Cr, Fe, Ni and redistribution of C in steel weldments formed by steel A 12050 additionally alloyed by Al, and by austenitic steel N 17242. The measured values of chemical diffusivities show that [`(D)](Cr) @ [`(D)](Fe) @ [`(D)](Ni) and that [`(D)](Al) is slightly higher than these values, approximately 2-times in ferritic region and 3.5 times in austenitic region. The choice of A and N steels yields the possibility to judge the Al diffusivity [`(D)](Al) in both BCC and FCC steel structures and its influence on C redistribution in steel weldments. The knowledge of thermodynamic eCi and of diffusion bCi interaction parameters yields the possibility, together with carbon diffusion coefficients D(C) in steels A and N, to calculate carbon redistribution N(C, x) and carbon activity a(C, x) in steels weldments A/N.

Ryszard Nawrowski, Konrad Skowronek : Operating conditions of a random constant voltage source in an electrical power system, pp.431-444

The paper presents a mathematical model of a system for obtaining energy from a real constant voltage source with random characteristics of equivalent electromotive force variations. Basic reasons for random characteristics of electrical power sources are discussed. Based on statistical linearization the probabilistic parameters of the source current are determined. Examples of the analysis of operating conditions of a random voltage source are presented for a selected form of correlation function.

Miros Pirner, Shota Urushadze : Structural damage assessment using dynamic response, pp.445-466

Use of the change of dynamic characteristic for the determination of the magnitude and location of damage or other imperfections. Comparison of the CAMOSUC (Change of Mode Surface Curvature) method with earlier methods. Examples of identification of changes of real structures and on models.