No. 2, 1998


Josef Cadek, Karel Milicka, Shije Zhu: On the threshold creep behaviour of aluminium dispersion strengthened by fine alumina particles, 119–133

Results of an investigation of creep in aluminium strengthened by 2.6 vol. % fine alumina particles – ODS 2.6 Al alloy – are presented with an emphasis on threshold creep behaviour. At temperatures ranging from 623 to 723 K the measured minimum creep strain rates covered six orders of magnitude. The true threshold stress sTH decreases with increasing temperature approximately linearly and the same holds for sTH–/–G ratio, where G is the shear modulus of the matrix metal-aluminium. Nevertheless, the threshold stress is suggested to originate from attractive dislocation/particle interaction although the detachment stress sd scales the temperature dependence of the shear modulus. The minimum creep strain rate em is matrix lattice diffusion controlled and the true stress exponent, n, of the minimum creep strain rate is very close to 5.

The result for ODS 2.6 Al are compared with similar ones for ODS Al-30SiCp composite. From the comparison it follows that the load transfer effect is absent in this composite with heavily dispersion strengthened matrix and that the dominant role plays the threshold stress effect, which is very similar to that in ODS 2.6 Al alloy.

Stanislav Vojtasek, Igor Zolotarev: A note on the Hurwitz problem for polynomials with complex coefficients, 135–145

When solving various technical problems we meet often the necessity of investigating the roots that belong to a polynomial with real or complex coefficients. Especially, in investigating the stability of mechanical and electrical systems (we consider, namely, linear and linearized systems), we must devote our attention to real parts of roots of corresponding characteristic polynomials that must be all negative. In such a case we can guarantee that the physical quantities, representing the behaviour of the system under investigation, cannot grow in an unlimited way. The conditions for the mentioned properties of studied systems are involved on the basis of the well-known Routh-Hurwitz theory [4], [5] and by use a method based on the construction of some special Hermitean forms [3]. On the example of an electrical circuit it is shown the practical advantage of the complex accession to the solving of these problems. Simple recommendations concerning application of the introduced theorems are presented in CONCLUSION of the paper.

Marian Pasko, Krzysztof Debowski: Selection coefficient for optimum symmetrization in n-phase networks, 147–159

The aim of the paper is the present principles of symmetrization of asymmetrical n-phase load supplied from ideal symmetrical nonsinusoidal voltage source. Symmetrization for any finite number of harmonics is carried out by means of reactance LC one-ports.

Miros Pirner, Stanislav Pospisil, Sota Urushadze: Diagnostics of the Troja footbridge by means of dynamic response, 161–174

The idea of using dynamic response for the defects indication is not frequent for civil engineering structures. Both static and dynamic loads induce cracks which may influence their response. It could be then desirable to monitor the state of the structure during its entire life of exploitation. One mean of such monitoring is by measurements of structural eigenparameters and analysing them by the methods of modal analysis. The paper deals with such a diagnosis of the unusual structure characterized by it low bending rigidity and by the geometrical form stabilized by its own weight only.

Jiri Klima: Analysis of a three-level PWM inverter fed induction motor in frequency and time domain, 175–188

Paper presents mathematical model, which enables analytical study of three-level Space Vector Modulated (SVPWM) inverter-fed induction motor, both in time and frequency domain. With these inverters, good quality of AC drives can be achieved with high voltage devices.

Jaroslav Feda: Bonding of a cement-treated sand, 189–203

Cement-treated sand was tested in an oedometer and shear box with the aim to study the decay of the brittle bonds in naturally cemented soils. Small cement content (2 % to 8 %) made it possible to study the problem within the engineering stress level. Nonlinear strength envelopes and index of brittleness depending on the stress level and cement content documented the course of debonding. This was successfully compared with the natural clay behaviour. High cement content (8 %) changes the cement-treated soil into a soil-cement with the properties of weak rocks. Considerable increase of the deformation modulus due to cementation was registered.

Ales Tondl: Dynamic absorber for an externally excited system having the pendulum form, 205–215

The action of a dynamic absorber attached to an externally excited system having the pendulum form is analysed. The restoring moment of this pendulum is given not only by gravitation but also by an elastic element. The effect of tuning, absorber position and mass ratio is an