ACTA TECHNICA CSAV |

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.

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 s* _{TH}* 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 - s

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.

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 1100–C. 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 *k**p*. Both the values *D*(*C*) and *k**p* satisfy Arrhenius plots
and *D*(*C*) is directly proportional to *k**p*.

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 *T*_{0}. 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.

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.

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 = W_{2} -
W_{1}
where W_{1}, W_{2} are the natural frequencies of the
abbreviated system. A substantial vibration reduction can be achieved even
when one of the conditions is not met.