ACTA TECHNICA CSAV |

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.

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.

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} <= e_{0} <= 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).

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.

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.

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.