No. 1, 2001


David Vokoun, Vratislav Kafka: A modified model of shape memory phenomena with return point memory, 1–14

An amount of martensite phase in shape memory alloys (SMAs) depends not only on temperature and externally applied stress, but also on the previous thermomechanical loading history. In our paper, the constitutive model for hysteresis phase transition proposed by Ivshin and Pence is modified to obtain a model that is consistent with "return point memory", a property of shape memory alloys that has been experimentally verified by a number of authors, as well as by our own experiments. The model is completed by equations that enable to calculate recovery tensile stresses generated in a constrained shape memory alloy specimen during thermal cycling. An illustrative example of the recovery stress transformational loops due to constrained thermal cycling is presented. It is shown that the simulated stress-temperature loops include return point memory behavior.

Josef Cadek, Kveta Kucharova, Josef Brezina, Vladimir Sustek: Creep behaviour of a 2124 Al-20SiCp composite at temperatures ranging from 623 to 773 K, with special regard to temperatures above 700 K: An overview, 15–39

The creep behaviour of a 2124 Al-20SiCp composite was investigated at temperatures ranging from 623 to 773 K. At temperatures 623 to 698 K the creep is associated with the true threshold stress sTH that depends on temperature more strongly than the shear modulus of the composite matrix. The minimum creep strain rate is matrix lattice diffusion controlled and depends on the fifth power of the effective stress (s - sTH); s is the applied stress. Assuming the relaxation factor kR in the relation sd = sOB (1 - kR2) (s OB is the Orowan bowing stress) to increase with temperature, the detachment stress sd is identified with the true streshold stress following from the analysis of the experimental em(T,s) creep data. At temperatures 723 and 748 K the true threshold stress is absent. The creep behaviour at these temperatures is interpreted in terms of the Rösler-Arzt model of creep (Acta Metall. Mater. 38 (1990) 671) assuming thermally activated detachment of dislocations from small incoherent alumina particles present in the composite matrix. At 773 K, the true threshold stress sTH reappears. However, its origin is different from that at temperatures 623–698 K. The normalized creep strain rate depends on the 2.5th power of the normalized effective stress (s - sTH) / G and is interpreted in terms of superplastic flow.

You Xiaojie, Pavel Pivonka, Viktor Valouch: Active power filter using predicted current control, 41–50

The conventional control strategy of active power filter is based on the instantaneous reactive power theory. Its disadvantage is that the strategy is strongly influenced by the distortion and frequency fluctuation of source voltage. To overcome the disadvantage, this paper presents a new control strategy based on phase-locked circuit and predicted current control with fixed switching frequency. The simulation and experiment results are presented.

Jaroslav Kucera, Krysztof Adamaszek: The relation between decarburization and oxidation of the steels heated at high temperatures in ambient air, 51–60

The results of investigation of decarburization and surface oxidation of (Fe-C-Cr-Mn-Si) steels at high temperatures are reported. The oxidation anneals were carried out at the temperatures 940 up to 1100C. The time periods of decarburization anneals amounted to 6075–7830 min and to 180–910 min in oxidation anneals. The measured carbon concentration profiles N(C, x, t) enabled us to state the carbon diffusion coefficients D(C), and the measured scale thicknesses l(T, t) the oxidation coefficients kp. Both the values D(C) and kp satisfy Arrhenius plots and D(C) is directly proportional to kp.

Boglou A.K., Arvanitis K.G.: A multirate control technique for improving the performance of a three phase hydrogenerator, 61–83

An optimal control strategy based on Two-Point-Multirate Controllers (TPMRCs), is used to design a desirable excitation controller of a hydrogenerator system, in order to enhance its dynamic stability characteristics. In the TPMRCs based scheme, the control is constrained to a certain piecewise constant signal, while each of the controlled plant outputs is detected many times over a fundamental sampling period T0. The proposed control strategy is readily applicable in cases where the state variables of the controlled plant are not available for feedback, since TPMRCs provide the ability to reconstruct exactly the action of static state feedback controllers from input-output data, without resorting to state estimators, and without introducing high order exogenous dynamics in the control loop. On the basic on this strategy, the original problem is reduced to an associate discrete-time linear quadratic (LQ) regulation problem for the performance index with cross product perms (LQRCPT), for which a fictious static state feedback controller is needed to be computed. Simulation results for the actual 117 MVA hydrogenerator unit in Sfikia, Greece, show the effectiveness of the proposed method which has a quite satisfactory performance.

Vilma Polouckova, Karel Milicka: Constant structure creep aluminium and two its solid solutions. Part II: Pure aluminium, 85–105

Creep behaviour of pure aluminium and two of its solid solutions, i.e., Al-13.7Zn and Al-5.5Mg, were investigated extensively using constant structure creep experiments performed in the steady state creep stage. Results of the investigations of pure aluminium are summarized in this second part of the paper. An analysis of the constant structure creep has shown that the creep rate is composed of two contributions resulting from different dislocation mechanisms acting simultaneously in the course of the steady state creep. One of these mechanisms is dominant over the whole interval of experimental conditions. However, the ratio of these two contributions changes with experimental conditions of the creep test, i.e., the applied stress and temperature. Some characteristics of both mechanisms were determined. Both mechanisms are probably controlled by diffusion along high diffusivity paths, most probably diffusion along dislocation cores. However, the activation areas of these mechanisms are different.

Radoslav Nabergoj, Ales Tondl: Self-excited vibration quenching by means of parametric excitation, 107–118

This contribution supplements the paper presented on the colloquium Dynamics of Machines 2000, in which the possibility of self-excited vibration quenching by means of parametric excitation due to the spring stiffness variation has been analysed. There a two-mass system excited by flow has been treated. In this contribution the results of the analytical analysis are compared with numerical solutions of the governing differential equations. It is shown that there really exists an interval of the parametric excitation frequency where the vibration is fully suppressed and the equilibrium position is stable when certain conditions are met. This interval lies around the excitation frequency h = W2 - W1 where W1, W2 are the natural frequencies of the abbreviated system. A substantial vibration reduction can be achieved even when one of the conditions is not met.