JOURNAL OF THEORETICAL AND APPLIED MECHANICS
Volume 30 Number 3 2000
CONTENTS

G. Gospodinov.

University of Architecture Civil Engineering and Geodesy,
1, Smirnenski Str., 1421 Sofia, Bulgaria,

A Boundary Element Linear Solution Applied to Reinforced Concrete

Abstract. A simple boundary element linear model is proposed in this paper, capable to handle the problem of inhomogeneity in plane reinforced concrete due to presence of reinforcement. The approach is based on the discretization of steel reinforcement as a series of linear translational springs of a given stiffness. This new physical model is converted then into a Boundary Element (BE) computational model by discretizating the boundary and introducing new unknown displacements in the domain at the points of intersection of reinforcement bars. It is shown that the method is a powerful alternative of Finite Element Method (FEM) for linear analysis of reinforced concrete structures and may serve as a tool for nonlinear analysis of Reinforced Concrete R/C structures.
Key words: boundary elements, reinforced concrete.

Vl. Damgov, Iv. Popov.

Space Research Institute, Bulgarian Academy of Sciences,
6, Moskovska Str., 1000 Sofia, Bulgaria,
e-mail: vdamgov@bas.bg

Regular Modes and Bifurcations in Kick-Excited System

Abstract. The class of kick-excited system proposed in this paper is based on nonlinear oscillators under periodic forcing with a special form, namely acting as short impulses (kicks) in regards to the coordinate. The main features of these systems are studied both numerically and analytically on the example of kick-excited pendulum. A two-dimensional discrete map is obtained for this example.

Key words: nonlinear dynamics, pendulum, bifurcations, oscillations, chaos, strange attractors, discrete maps.

A. B. Kazakoff.

Institute of Mechanics, Bulgarian Academy of Sciences,
Acad.G. Bonchev Str., Bl.4, 1113 Sofia, Bulgaria,
e-mail: alex_kazakoff@yahoo.co.uk

A Mechatronic System for Vibration Technological Processes

Abstract. In the present work, evaluation of some analytical methods of designing and creating mechanisms and carrying systems used in construction works is made. These methods are not easy for application in the case of complex mechanical systems.
We introduce here a new approach presenting the so-called ``n-angle functions'' or ``n-dimensional polygon functions'' to be applied instead of the widely used now trigonometric functions (without totally rejecting them). The term n-angle is introduced by Bachmann and Schmidt, [2]. In this paper we discuss their properties and develop the theory of their application. A numerical example is solved to show the use of the functions in particular cases.

Key words: analysis of mechanism, kinetic, kinetostatic.

S. B. Pavlov.

Technical University  of Sofia,
8, St. Kl. Ohridski Str., 1756 Sofia, Bulgaria

Spatial Vibrations of a Two Masses System Machine-Foundation

Abstract. In the presented work, the principle of action of the unit Vibrating Servomotor (VS) created by Bogdanova and Grinberg [1] is applied in order to create a mechanism using the working vibrations of the ore mining hammers (perforators). An automatic feeding of the working instrument is realized. In this way, the miner is not subject to the vibrations harmful effect. The system of differential equations describing the work of the different elements of the vibrating servomotor is obtained.

Key words: vibrating servomotor, forced harmonic oscillations.

St. Tasev.

Institute of Water Problems, Bulgarian Academy of Sciences,
Acad. G. Bonchev Str., Bl. 1, 1113 Sofia, Bulgaria,

The Consideration of Water Effect on the Dynamic Characteristics of Concrete Gravity Dams

Abstract. A method of taking into account the hydrodynamic pressure on concrete dams is represented. A FORTRAN program called TRIDIDAD is made. Ordinary volume finite elements, tetrahedrons, are used. The integration, due to that type of finite elements, is made in a closed type; just as it is made about finite elements of the contact interface (dam - water), taking into account the hydrodynamic pressure. The program TRIDIDAD is used for calculating two real concrete gravity dams.

A method of comparing the results of the numerical investigations and the experimental tests is given. It permits prediction and identification of dynamic characteristics of dams, as well as the place of application of the external excitation during dynamic field tests.

Key words: concrete gravity dams, dynamic characteristics, hydrodynamic pressure, frequencies of vibration, mode shapes.

M. Mukntar Ali, Gh. Nabi Parrey, Rashid Ali.

Department of Applied Mathematics,
Z. H. College of Engineering and Technology,
Aligarh Muslim University, Aligarh-202002, India

A Punch Problem for Initially Stressed Neo-Hookean Solids

Abstract. The paper deals with indentation of a semi-infinite initially stressed elastic medium under the action of an axisymmetric rigid punch pressing the medium normally. The problem has been considered within the framework of incremental deformation theory for neo-Hookean solids. Using the Hankel's transformation, the distributions of incremental stress and strain have been obtained. Indentations by a flat-ended cylindrical punch and a conical punch have been obtained as special cases and these effects have been studied numerically and presented in the form of curves.

Key words: initially stressed medium, incremental deformation, neo-Hookean solids, Hankel transformation, flat-ended cylindrical and conical punches.

H. Argirov, K. Kalchevska.

Institute of Metal Science, Bulgarian Academy of Sciences,
67, Shipchensky prohod Str., 1574 Sofia, Bulgaria,
e-mail: kami@ims.bas.bg

Macroscopic Creep Crack Type 38HN3MAFA Nitrogen Steel

Abstract. Behaviour of steel 38HN3MAFA with optimized chemical composition and alloyed with super-equilibrium nitrogen concentration, in the process creep rupture strength was investigated. The nitrogen influence on the characteristics of the creep at 480degC, 540degC and 590degC was determined. At increase of the nitrogen content increase of the time till tensile fracture and decrease of the elongation of the specimens was established.

Key words: creep crack, nitrogen steel.

E. Bogdanova, A. Grinberg, V. Slavov.

University of Chemical Engineering and Metallurgy,
8, St. Kl. Ohridsky Str., 1756 Sofia, Bulgaria,

Vibrating Servomotor with Mobile Piston

Abstract. This paper solves the problems concerning the development of a method and a device for investigation and control of multi-frequency spatial vibrations of a vibration system of one single unbalanced (eccentricity) axle. A method is designed for the mathematical modeling of plane and spatial vibration movement of a single mass dynamic model in the presence of a variable mass rule and a reactive force arising after the adduced mass and a harmonic excitation force. An investigation of the effect of the nonlinear geometrical coordinate links is made, requiring a suitable adjustment of the single mass dynamic system. A method for spatial vibration control of a single mass dynamic model is developed and thus, by means of an adjusted frequency feedback, an optimal frequency spectrum of vibrations is selected. The obtained results can be applied in many cases in vibration technics and for solving of ecological problems of vibration protection and human engineering.

Key words: multi-frequency spatial vibrations, spatial vibration control, non-linear geometrical coordinate links, vibration protection, human engineering

N. Iontcheva.

Technical University of Sofia,
8, St. Kl. Ohridski Str., 1756 Sofia, Bulgaria

System of Functions for Geometric Description of Mechanical System Motion

Abstract. A research on the dynamic behaviour of a machine foundation system with twelve degrees of freedom is presented. Lagrange equations and yield equations of vibrations motion are applied. The stability of motion and the existence of resonances are studied by means of characteristic equations.

Key words: mechanical vibrations, stability, nonlinear systems.

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