| ACTA TECHNICA CSAV |
The creep in a 2124 Al alloy reinforced by 20 vol.% silicon carbide particulates – 2124Al-20SiCp particulate composite – is investigated in a temperature interval ranging from 623 K to 748 K and at applied shear stresses ranging from 2.5 MPa to 40.0 MPa depending on the testing temperature. The double shear creep test technique is used.
It is shown that the composite exhibits the threshold creep behaviour at least up to the temperature of 698 K. Up to this temperature, the true activation energy of creep is very close to the activation enthalpy of matrix lattice diffusion and the value of the true applied stress exponent of minimum creep strain rate is almost exactly equal to five. The threshold stress decreases with increasing temperature linearly and if extrapolated to temperatures above 700 K it disappears at a temperature of about 735 K. At the testing temperature 748 K and even at the testing temperature 723 K the apparent applied stress exponent of minimum creep strain rate increases with applied stress and depends slightly on temperature. But, most probably, the creep strain rate remains to be matrix lattice diffusion controlled.It is assumed that the origin of threshold stress is closely related to its temperature dependence and to its disappearance at a relatively well defined high creep testing temperature. An attempt is made to account for the origin of threshold stress accepting the concept of load transfer to particulates and the long range internal back stresses generated in creep in the composite matrix, which reduce the load transfer to an approximately applied stress independent value at any given temperature, except the high temperatures at which the internal back stress is very close to the applied stress. Under the latter conditions the threshold stress can be expected to disappear.
The following paper presents a system of asynchronous machine equations, suitable for solving transient conditions in unsymmetrical regimes. The influence of the iron nonlinearity is considered.
Two two-mass systems are considered where one mass is self-excited (m1 - see Fig. 1 and 2) and the other represents an elastically mounted foundation mass (m2). The self-excitation is of van der Pol type depending either on the absolute motion of mass m1(Model I) or on the relative motion (Model II). Self-excited vibration is synchronized by external harmonic excitation acting either on mass m1(alternative (a)) or mass m2 (alternative (b)). The vibration amplitudes are defined for the following cases: Systems without external excitation, synchronized vibration for Model I and II and for both alternatives of external excitation.
The viscoelastic behaviour of isotropic and anisotropic bodies is described by the ordinary differential tensor equations with variable and constant coefficients or coupled multiple integrals. The presented relations follow from the analogies between the rheological and mathematical structures. In this manner, a close connection between the ordinary differential equations and multiple integrals is analysed and rational solutions of these equations obtained. For this purpose, the special resolving functions are introduced. The viscoelastic behaviour of anisotropic bodies is described on introducing the transformed stress and strain tensors connected with tensorial coaxiality.
The unsteady flow inside a square cavity, generated by a uniform oscillatory motion of the upper wall has been studied. The Navier-Stokes equations and the equation of continuity governing the flow have been solved using a boundary element method. The elements of the boundary and domain are generated as in the case of the finite element method and the resulting integral equations are then solved. The result have been compared with those of the finite difference and boundary element methods and found to be in good agreement. It is observed from the results that the steady streaming component has two stronger contra-rotating eddies located in the upper half of the cavity and two weaker contra-rotating eddies situated in the lower section of the cavity.
The characteristic features of the onset of chaotic oscillations in series resonant RLC circuit with ferroelectric capacitance have been studied depending upon temperature of a ferroelectric crystal as well as upon amplitude of sinusoidal electric field applied to the crystal. The well known model ferroelectric crystal triglycine sulphate (TGS) was used as a non-linear material of the capacitance. Emergence of chaos in the non-linear dissipative dynamical system under investigation can be related to the process of the domain structure switching in this crystal under the influence of strong external AC electric field.