Issue 4

JTAM, Sofia, vol. 25 Issue 4 (1995)

Influence of Electromagnetic Fields on Human Organism

G. Brankov
Institute of Mechanics, Bulg. Acad. Sci. Sofia

JTAM, Sofia, vol. 25 Issue 4 pp. 015-024 (1995), [Full Article]

Augmented Lagrangian for Optimal Control Problems of Parabolic Systems

S. Grozdev
Institute of Mechanics, Bulg. Acad. Sci., Sofia

An augmented Lagrangian is introduced for an optimal control problem of parabolic type. It is shown that the direct minimization of this Lagrangian solves the optimal control problem.

JTAM, Sofia, vol. 25 Issue 4 pp. 025-030 (1995), [Full Article]

A Mechanical Model on the Dynamics of Spiroid Gears

Al. Kazakoff, V. Abadjiev
Institute of Mechanics, Bulg. Acad. Sci., Sofia

The aim of this paper is a creation and investigation of monofrequency and polyfrequency spatial vibrations of a double mass vibration dynamic system with a suitable adjustment of the mechanical system. The physical object is a spiroid gear-pair. The mechanical modelling is created on the basis of a double mass spatial dynamic model with twelve degrees of freedom - six for each mass. The influence of the elastic links between the two masses and also the energy mutual linkages between the coordinates of the two bodies are investigated. Natural frequencies and amplitudes of motions could be defined by means of computer experiment in spatial aspect. The obtained results can be used for optimization of the constructive parameters of the spiroid gear set and also for improving of harmful re-sonance effects.

JTAM, Sofia, vol. 25 Issue 4 pp. 031-037 (1995), [Full Article]

On the Mathematical Modelling of Impact in a System of Bodies

L. Stoimenov
Higher Military Transport School, Sofia

The need and possibility to use Sequential Analysis in natural and numerical experiments on impact in systems of bodies are shown. The results can be used directly to define the number of tests when experiments should be stopped.

JTAM, Sofia, vol. 25 Issue 4 pp. 038-040 (1995), [Full Article]

A Dynamical Model of Device for Testing of Glass Packages in Fatigue Strength

S. Vassilev, N. Markova
Higher Institute of Food and Flavour Industry, Plovdiv

A dynamical model of original device for testing of glass packages has been suggested. By reading of the essential peculiarities of the construction the model presents a oscillating system with three degrees of freedom. Based one's arguments on facts considera-tions by composition of the dynamical model have been analysed. The differential equations describing the motions of the model have been composed by means of the Lagrange's method in the analytical dynamics. The system of frequency equations based on the differential equations has been obtained and the natural frequencies of device have been determined.

JTAM, Sofia, vol. 25 Issue 4 pp. 041-045 (1995), [Full Article]

Simulation and Dynamical Analysis of Random Vibrations in Railway Vehicles

A. Ivanov1, V. Chobanov2, P. Philipov3, K. Ishtev4
1Higher Military Transport School, Sofia
2University of Architecture, Civil Engineering and Geodesy, Sofia
3Institute of Mechanics, Bulg. Acad. Sci., Sofia
4Technical University, Sofia

In this paper is presented an approach for simulation and dynamic analysis with computer of random vibrations in railway vehicles, founded of the frequency methods. The described dynamic investigation in the frequency area, combined obligatory with using of modern powerful computer, would lighten the calculations and designing of the contemporary railway vehicles considerably, especially with regard to selection of the geometric, elastic and damping characteristics of the spring suspension with purpose – dynamic synthesis, minimum vibrations, as well as avoiding of the harmful resonance effects.

JTAM, Sofia, vol. 25 Issue 4 pp. 046-050 (1995), [Full Article]

Determination of the form Factor of the Slender Beams Loaded by Bending

A. Čalkovský, I. Ŝafář
Military Academy Brno, Czech

This work deals with the form factor determination of the beam cross-section. The analysis respects not only the shape of the cross-section, but the material characteristics too. The practical applications of the results are demonstrated in the conclusion of the article.

JTAM, Sofia, vol. 25 Issue 4 pp. 051-063 (1995), [Full Article]

Statical Investigation of a Double-Curved Arch Dam, Applying the Method of Large Fragments (MLF)

Hr. Ganev, Ch. Dimitrov
Institute of Water Problems, Bulg. Acad. Sci., Sofia

The most precise and comprehensive theory of arch dams can be created on the basis of shell theory. As is shown, the curvature of shells is one of the most essential factors for the favourable statical behaviour of these structures: shells with double curvature and thickness both variable in either direction, with arbitrary configuration, elastically clamped on the supporting contour and free of any forces along the crest. Thereupon an adequate computer aided numerical discretizational procedure is applied – the Method of Large Fragments. An example illustrates the effectiveness of MLF.

JTAM, Sofia, vol. 25 Issue 4 pp. 064-081 (1995), [Full Article]

Interaction of Elastic Waves in an Isotropic Multi-phase Material

J. Rushchitsky
Institute of Mechanics, Ukr. Acad. Sci., Kiev

Nonlinear elastic waves in an isotropic multiphase material are analysed. Harmonic waves in slightly nonlinear mixtures are considered, nonlinearity being of quadratic Murnaghan's type. Plane waves and a case of longitudinal waves excitation only are discussed in detail. The analytical solution obtained is analysed consequently, and microstructural effects (mode doubling, geometrical dispersion, mode blocking etc.), and nonlinear effects (new harmonics generation, dispersion of wave profils etc.) were discussed. An appearance of new wave was discovered, and it has been generated by the interaction of different modes.

JTAM, Sofia, vol. 25 Issue 4 pp. 082-093 (1995), [Full Article]

Transient Natural Convection in a 2-D Enclosure with a Heat Source at the Bottom

P. Minev, F. van de Vosse, A. van Steenhoven
Eindhoven University of Technology, The Netherlands

The transition to an weakly chaotic state of a 2-D natural convection flow in an enclosure with a heat source at the bottom is studied numerically. The numerical technique used combines the high spatial accuracy of the spectral element method with a second-order splitting time integration which allows a relatively cheap and accurate solution of the system of the Navier-Stokes and energy equations. Results for the first bifurcation of the flow (swaying motion) and the consequent pitchfork bifurcation (deflection of the flow towards one of the side walls) are presented. The development of the resulting flow pattern with the increase of the Rayleigh number is also studied. It undergoes a transition to-wards a weakly chaotic state which can be recognized in the presented power spectra.

JTAM, Sofia, vol. 25 Issue 4 pp. 094-108 (1995), [Full Article]

Variational Principles for Cosserat Elastic Shells

P. Cermelli1, J. Ivanova2, F. Pastrone3
1Institute of Physics Mathematics, "J. L. Lagrange", Torino, Italy
2Institute of Mechanics, Bulg. Acad. Sci., Sofia
3University of Torino, Italy

In this work we give an intrinsic formulation of the basic variational principles for Cosserat nonlinear elastic shells. We generate the analogs of the four fundamental principles of the classical theory of elasticity, namely the energy, Hu-Washizu, Castigliano and Hellinger-Reissner principles, and reformulate them in terms of invariant (under rigid-body displacements) kinematical measures.

JTAM, Sofia, vol. 25 Issue 4 pp. 109-121 (1995), [Full Article]

Wave Disturbance of Rod Stability

V. Baranov1, I. Lopa1, Hr. Hristov2, K. Boiadjiev3
1Tula State Technical University, Russia
2Unit 42660 - Ministry of Defence, Sofia, Bulgaria
3Vazov Engineering Plants Ltd, Sopot, Bulgaria

In case of impact loading of a rod, the high intensity of the longitudinal compression waves causes a longitudinal stability loss of the loaded rod part. This phenomena takes place only within a limited range of impact velocity variation, which is proved by the experiment. This problem is mathematically interpreted as a wave task of a solid deformable body dynamics and criteria of a core stability loss during the process of a thin plate penetration are formulated. Using the developed mathematical model, equations, describing the bended core axis and the critical value for the contact stress (impact stress) are obtained.

JTAM, Sofia, vol. 25 Issue 4 pp. 122-130 (1995), [Full Article]

Elastic-Viscoplastic Deformation of Structure Elements in Non-Isothermal Processes of Loading

Yu. Shevchenko, R. Terekhov
Institute of Mechanics, Ukr. Acad. Sci., Kiev

The general formulation is presented of the problem for investigation the elastic-viscoplastic deformation of structure elements in nonisothermal processes of complex loading. Constitutive equations are presented, describing the complex nonisothermic processes of deformation for metals. These processes depend essentially on the speed of their development. Basic experiments are formulated, on the basis of which scalar functional is specified in these equations. Results are presented of experimental investigations for substantiation of the constitutive equations and results of computations of the stress-strain state of concrete structure elements.

JTAM, Sofia, vol. 25 Issue 4 pp. 131-139 (1995), [Full Article]

On the Mechanism of Sulphate Ion Diffusion in Cement Stone

P. Gospodinov1, R. Kazandjiev1, M. Mironova2
1Institute of Mechanics, Bulg. Acad. Sci., Sofia
2Central Laboratory for Physico-Chemical Mechanics, Bulg. Acad. Sci., Sofia

The paper studies sulphate ion diffusion in cement stone. The penetration of S04 ions in the material is experimentally followed by keeping cement specimens in a water solution of 0.5 Na2SO4 for different periods of time and by recording the ion loss from the solution. The depth of ion penetration is estimated by solving a non-steady equation of mass transfer, considering the chemical reactions which accompany the diffusion process and choosing an appropriate value of the diffusion coefficient and of the interfacial resistance.

JTAM, Sofia, vol. 25 Issue 4 pp. 140-148 (1995), [Full Article]

Aluminium Alloy Fatigue Crack Growth under Variable Amplitude Loading

Y. Davidov
Institute of Metal Science, Bulg. Acad. Sci., Sofia

The fatigue crack growth behaviour of two modifications with different iron quantities of AlSi7Mg cast alloy for some types of variable amplitude loadings superimposed on constant amplitude cycling has been investigated in this study. The test results show that both AlSi7Mg cast alloy modifications are responding qualitatively in a similar way to different variable amplitude loading conditions but quantitatively the modification containing 0.31% Fe shows better behaviour in all types of tests carried out.

JTAM, Sofia, vol. 25 Issue 4 pp. 149-154 (1995), [Full Article]