No. 4, 2001


Michal Landa, Premysl Urbanek : Anisotropy detection by lens-less cylindrical ultrasonic probe, pp.337-358

The principle of time-resolved acoustic microscopy using line-focus probe is briefly described on the basis of the simplified ray approach. The lens-less, line-focus polyvinylidene fluoride (PVDF) transducer with large aperture enables generation and detection of bulk, guided and surface waves propagation in a tested material in the broad range of ultrasonic frequencies. The defocus technique yields precise velocity measurements and mechanical properties (elastic moduli) determination. Directional measurements is adopted to detect material anisotropy.

Experimental results on the guided wave forming in plates (glass) and surface wave velocities in the single crystal (Si) exhibit advantages and limitations of the non-destructive method.

Frantisek Chvala : On velocity distribution of free electrons in a gaseous host medium, pp.359-373

The velocity distribution function of electron component of a partially ionized gas is treated as a solution of a spatially inhomogeneous initial boundary value problem for the nonlinear Boltzmann equation. Existence and uniqueness of local solution are established. External electric field and internal sources are assumed. The solution is constructed iteratively.

Josef Cadek, Kveta Kucharova, S.J. Zhu : Threshold stress and load transfer effects in creep of an Al-8.5 Fe-1.3 V-1.7 Si-15 SiCp composite, pp.375-398

The creep behaviour of an Al-8.5 Fe-1.3 V-1.7 Si-15 SiCp composite (the subscript p stands for particulates) in two temperature intervals, namely 623 to 723K and 773 to 823K is investigated using the isothermal constant stress creep test technique. The measured minimum creep strain rates cover six to seven orders of magnitude. Occasionally, this creep behaviour is compared with that of an unreinforced (matrix) Al-8.5 Fe-1.3 V-1.7 Si alloy. For the temperature interval 623 to 723K the true threshold creep behaviour is characteristic. The true threshold stress sTH decreases with increasing temperature more strongly than the shear modulus and is higher in the composite than in the matrix alloy. With the strong temperature dependence of the true threshold stress, extremely high values of the apparent activation energy and the apparent stress exponent are associated. However, the minimum creep strain rate is matrix lattice diffusion controlled and depends on the fifth power of the effective stress se = s-sTH (s is the applied stress). The creep behaviour is interpreted in terms of athermal detachment of dislocations from fine incoherent Al12(Fe, V)3 Si phase particles in the composite matrix. At temperatures ranging from 773 to 823K the true threshold stress is absent. The apparent stress exponent increases with increasing applied stress, the apparent activation energy decreases with both increasing applied stress and temperature. The true activation energy of creep is higher than the activation enthalpy of the matrix lattice diffusion. The creep is interpreted in terms of thermally activated detachment of dislocations from fine Al12(Fe, V)3 Si phase particles. The load transfer effect does not play any role in creep either at temperatures 623-723K or at temperatures 773-823K.

You Xiaojie, Pavel Pivonka, Viktor Valouch : Design and build of an active power filter based on digital control, pp.399-407

With the development of microprocessor, the digital control of active power filter becomes promising. There is a tendency that it will replace the conventional analog control. This paper presents a design and build of active power filter based on digital control. The experimental results are also presented.

Roman Jezdik, Jan Cerv : Dispersive properties of media discretized by lisa-2D code in wave modelling, pp.409-422

By means of the numerical procedure dispersive properties of the LISA-2D program package were evaluated in dependence on a grid parameter and a direction of wave propagation. Criteria for the selection of a grid parameter are then determined. The results are compared to those obtained from the literature considering finite elements with modified mass matrix.

Bohumil Stadnik, Stanislav Javorsky, Oto Köhler, Jiri Kocarek, Frantisek Zatura : Very sensitive diagnostic fluorometer of human cancerous tumours with an acoustical warning system, pp.423-429

Very sensitive, stable, reliable and portable fluoremetr for the medical photodynamic research of human cancer tumours in inner hollow organs as are the ureter, esophagus, colon, and etc. has been designed and manufactured. This apparatus is suitable for detection of tumours and measurement of very weak optical fluorescence and autofluorescence signals generated by these tumours at the photodynamic diagnostics and therapy. Moreover, a warning acoustical signalling system is included in this apparatus. The apparatus test results made on artificial ureter cancer tumours are shown.

Miroslav Knob : Properties of three-phase metal-enclosed lines at a different resistance of enclosures, pp.431-442

The calculation of longitudinal electrical parameters of a three-phase metal-enclosed line, of losses arising in phase conductors and enclosures and of the external magnetic field at a different magnitude of the resistance of enclosures is carried out and the values obtained are analysed. The calculation is based on a mathematical model of the skin effect in a system of parallel direct conductors carrying the harmonic current. The calculation is performed by using a modified numerical solution of Fredholm equations.

Marian Pasko, Krzysztof Sztymelski : Elimination of higher harmonics of source currents by a use of series filters, pp.443-458

Periodic nonsinusoidal waveforms can be filtered by filters of higher harmonics. Parallel filters of higher harmonics have got big install power | SN|. Minimization of deformation of the input currents can be carried out by means of series filters, too. This paper shows simulations of parallel and series filters of higher harmonics of currents in full-wave rectifiers, which are done in SPice program.Install power | SN| is smaller in series filters than in parallel ones and this is one of the most important advantages of these filters.