ACTA TECHNICA CSAV |

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.

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.

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.

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.

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 e_{C}^{i} and of
diffusion b_{C}^{i} 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.

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.

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.