No. 2, 1997


Daniel Mayer, Bohus Ulrych: A numerical solution of networks with distributed parameters, 115–127

In the paper we carry out numerical analysis of transients in uniform lines. A mathematical model in the form of a system of partial differential equations of hyperbolic type is solved by the finite differences method using Wendroff's approximation. The method is illustrated by three examples. The paper represents the introductory part of the research concerning numerical solution of overvoltage in three-phase networks.

Jiri Bendl, Ludek Schreier: Disconnection of one phase of asynchronous generator and one-phase operation, 129–144

Island operation of an asynchronous generator supplying a power system is investigated at the instant when one phase of the machine is disconnected from the rest of the network. The transient processes aroused in this way are evaluated to show that preservation of the subsequent one-phase operation necessitates increasing the capacity of that part of the battery of capacitors that was not disconnected. Even in this case there appears decrease of phase voltages in remaining parts of the system.

Robert Kral, Pavel Lukac, Michael Zehetbauer, Milos Janecek: Mechanisms controlling plastic behaviour in Al-Mg and Al-Zn-Mg alloys: A comparison of contemporary models, 145–174

Deformation behaviour of two pure Al-Mg alloys with actual weight concentrations of 2.6 % Mg and 4.8 % Mg, respectively, and a commercial aluminium alloy of type 7020 with 5 wt. % Zn and 1.2 wt. % Mg was investigated in the temperature range 193–523 K. Tensile tests with constant cross-head speed were performed for initial strain rates from the interval 4.76 × 10-6 s-1 <= e0 <= 2.38 × 10-2 s-1. Experimental work hardening curves are compared with the predictions of theoretical models proposed in the literature. Among them the models proposed by Malygin and lately by Lukac and Balik were found to be applicable in the whole temperature range for all strain rates: The shape of the work hardening behaviour is described satisfactory in both models. The description by the Lukac and Balik model is better in the case of Al-Zn-Mg alloy where also the stage IV of deformation is reached. The parameters of the models, which were obtained from the fits of experimental data and which have connection to various microstructural processes, are compared with the theoretical predictions. The role of various hardening and softening processes at different temperatures is considered. Parameter values were found to be in a good agreement with theoretical values, observed microstructure (TEM) and measured dislocation density (XPA).

Knesl Z., Kubacek K., Bednar K.: Evaluation of the biaxiality parameter using the finite element method, 175–201

Procedures for evaluation of the biaxiality parameter B (or, equivalently, the T-stress) by applying the finite element method are presented and discussed. It is shown that one of the most effective and precise methods of calculating the B (or T) parameter uses a crack tip hybrid element approach. General rules for reliably estimating of B (or T) by using direct methods in connection with standard finite elements are suggested. It is concluded that the application of quarter point elements near the crack tip in connection with the extrapolation procedure gives accurate and reliable estimation of the T -stress. Numerical results necessary for a two-parameter fracture mechanics application are given for six different types of test specimen. The results obtained by a crack tip hybrid element approach are completed by values of the third and forth terms in the Williams series.

Ales Tondl: Vibration absorber for a system with self-excitation synchronized by external excitation, 203–212

The effect of the vibration absorber on a self-excited system which is synchronized due to harmonic excitation is analysed. The van der Pol type self-excitation is considered. It is proved that through the absorber action this synchronized vibration can be substantionally diminished. For absorber tuning and damping similar rules are valid as for the case of mere external excitation, i.e. the optimal tuning is where the natural frequency is equal to the excitation frequency and the absorber damping is very small.

Slavka Mitic, Katica (Stefanovic) Hedrih: Nonlinear oscillations of the torsion oscillator with impact masses, 213–226

This paper deals with nonlinear oscillations of the torsion oscillator with reciprocal rigidly connected impact masses. It is assumed that two impulses occur at one interval of the disturbing torsion moment. The asymptotic Krilow-Bogolyubov-Mitropolyskiy method is applied, along with the stereomechanical impact theory for the inclusion of impact conditions, to the determination of the primary approximation of the torsion system nonlinear oscillations. Phase trajectories are drawn on the basis of the numerical results.

The mathematical model of the vibroimpact system is written in the form of an autonomous nonlinear system of the first order differential equations. The integral curves and the phase trajectories are obtained by means of the Runge-Kutta method, of the Turbo-Pascal program and with the aid of the computer graphics.