–––ACTA TECHNICA CSAV––– |

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 s_{TH} decreases with
increasing temperature approximately linearly and the same holds
for s_{TH}–/–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 s_{d} scales
the temperature dependence of the shear modulus. The minimum
creep strain rate e_{m}
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.

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.

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.

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.

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.

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.

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