JOURNAL OF THEORETICAL AND APPLIED MECHANICS
Volume 27 Number 2 1997
                                                                    CONTENTS
 

GENERAL

MECHANICS

Al. Cheremensky

Institute of Mechanics, Bulg. Ac. Sci., Sofia

Meaning of Feedbacks with the Highest Derivatives

Various effects produced by the feedbacks with the highest derivatives of output have been pointed out in order to explain why they may not be put in practice. Within the framework of the concept of negligible small time-delay, this paper makes clear the meaning that these feedbacks have themselves. It is shown that the known methods of the model diagonalizing and reducing its inaccuracy are not robust with respect to a small time-delay.


S. Grozdev
 

Institute of Mechanics, Bulg. Ac. Sci., Sofia

Examination of a Nonstability

A state equation problem of parabolic type is considered in the paper under a nonlinear perturbation. A sufficient condition is given when the state could be determined by the control in an unique way.



A. Vakhidov
 

Institute of Theoretical Astronomy, Russ. Ac. Sci. St. Petersburg

Influence of the radiation pressure on the motion of a satellite with large eccentricity

Analytical theory of the first-order perturbations from the solar radiation pressure is developed for artificial Earth satellites with large eccentricities using trigonometric expansions of satellite disturbing function in multiples of the elliptic anomaly.



Pl. Raikov
 

Central Laboratory of Mechatronics and Instrumentation, Bulg. Ac. Sci., Sofia

Some Aspects in the Design of Adaptive Measuring Systems
 

The development of adaptive measurement systems for geometrical shape recognition of mechanical parts during each stage of production is a design problem which cannot be solved without developing new methods for using conventional and advanced-construction measurement devices and applying sensor technology. A concept for design of adaptive measuring systems, applicable to simultaneous quality control of multiple machine parts in small and medium series production is presented. The general structure of the system and the major characteristics of its subsystems are discussed from the viewpoint of Mechatronics. The results of experiments are summarized.



CONTINUUM

MECHANICS
 

V. Vassilev
 

Institute of Mechanics, Bulg. Ac. Sci. Sofia

Symmetry Groups and Equivalence Transformations in the Nonlinear Donnell - Mushtari - Vlasov Theory for Shallow Shells
 

In the case of transversely only loaded shallow shells, the nonlinear Donnell - Mushtari - Vlasov theory for large deflection of isotropic thin elastic shells leads to a system of two coupled nonlinear fourth-order partial differential equations known as Marguerre's equations. This system involves two arbitrary elements - the curvature tensor of the shell middle-surface and the function of transversal load per unit surface area. In the present note, the point symmetry groups of Marguerre's equations are established, the corresponding group classification problem being solved. It is shown that Marguerre's equations are equivalent to the von Kármán equations for large deflection of plates in the time-independent case and in the time-dependent case as well. It is also observed that the same holds true in respect of the field equations for anisotropic shallow shells.



M. Mironova
 

Central Laboratory of Physical-Chemical Mechanics, Bulg. Ac. Sci., Sofia

Elastic  Modulus  of  Cement  Stone  Beam  Subjected  to  Sulphate  Attack
 

The paper presents a theoretical-experimental method for assessing the elasticity modulus of a cement stone, the latter affected by sulphate corrosion. The specimen is treated as having a two-layered srtucture (an external layer affected by corrosion and a pure cement core) owing to the corrosion effects on the material. The material mechanical characteristics are determined by performing fractureless experiments on compression and bending. A relation between the thickness of the external layer and its specific elasticity modulus is also determined. Good agreement between the theoretical assumptions and the experimental results is obtained.



M. Mihovski

A. Metchkov

A. Popov

S. Metchkova
 

Institute of Mechanics, Bulg. Ac. Sci., Sofia

Non-destructive Testing of Iron Base PM Materials
 

The deformation of sintered powder metallurgical materials on iron base was studied. The acoustic emission was monitored during the process of samples bending. It was founded that acoustic emission activity in the elastic region is successful criterion for the recognition of the structure and chemical compositions of the sintered powder metallurgical materials.



SOLID

MECHANICS

A. Baltov

M. Popova

V. Iliev
 

Institute of Mechanics, Bulg. Ac. Sci., Sofia
University of Chemical Technology and Metallurgy, Sofia

Simulation of the Mechanical Behaviour of Textile Composites 
 

In this paper a method for modelling of the mechanical behaviour of the textile composites is developed. A convenient model is adopted and it takes into account the effects of the structural anisotropy and the loading type sensitivity of the investigated materials. The idea of the model is to introduce a six-dimensional vector space of the stresses which consists of subspaces of the different types of loading and also to construct the hyper-quadratic surfaces in each subspace. The elastic characteristics of the material are identical in every subdomain. The computer simulation method is developed for obtaining of the elastic characteristics. The simulation is carried out on the representative element substituted in a suitable way by a lattice system. The rigidities of the various bars are determined on the basis of test data from comparatively simple experiments. The method suggested is applied for determining the elastic characteristics of a woven composite material on the base of glass fibres and a polyester resin in the case of two dimensional stress state.


G. Kolarov

R. Tenchev
 

Technical University of Sofia

Error Estimates for the Finite Prism Method

There are investigated the possibilities to estimate the error in the finite prism solution, that is a semianalytical method for solving elastic problems for 3D prismatic bodies. There is proposed an a-priori error estimator for the analytical part of the solution. For the numerical parts of the solution, performed by finite elements, there are tested with some modifications two well known a-posteriori error estimators - these of Zienkiewicz-Zhu and of Babu ska-Miller. A detailed example for the application of the error estimation is considered.



H. Sheiretov

University of Mining and Geology, Sofia

Influence of the Number of Plies and the Elastic Moduli of the Rubber and the Plies on the Stresses Distribution in a Conveyor Belt

Equations are obtained and an algorithm is worked out for determination of the location of the neutral line of the cross-section of a conveyor belt at pure bending and in the presence of external axial force. The influence of the number of textile plies and the ratio of the elastic moduli of the rubber and the plies on the stresses distribution is examined.



C. Kouyumdjiev

P. Hovsepyan

I. Ivanov

R. Tanov

University of Rousse, Rousse

Determination  of  the  Stresses  in  Isotropic  Multilayer  Thin  Coatings
 

A new algorithm for determinig the distribution of the initial and residual stress in multilayer coatings by the bending strip method is proposed. The algorithm is applicable for coatings obtained by consecutive deposition by various technological processes of arbitrary number of elastic, homogeneous and isotropic layers of different materials. An example about the determination of the stresses in composite Ti and TiN vacuum coating deposited on a steel substrate is given.



R. Savova
 

Institute of Mechanics, Bulg. Ac. Sci., Sofia

Propagation of Transversal Shock Waves in a Rod

In the present paper we consider the propagation of transversal shock waves in an elastic rod, which is in ideal contact with a semibounded elastic medium from where the shock wave in the rod comes from. On the boundary between the rod and the semibounded medium we have a reflection and a refraction of the shock wave coming from the semibounded medium. The moving wave then reflects from the other end of the rod, which is stress free and returns back to the first end of the rod and etc.The formulas for the stresses, the strains and the velocities of displacement in the rod during these numerous reflection and refraction of the shock wave are obtained. Generalized results of the case when the rod structure is not homogeneous but consist of n sectors with different mechanical characteristics are obtained.



FRACTURE

MECHANICS

V. Baranov

G. Kalujny

P. Poltev

H. Hristov

Z. Chiwikov

K. Boyadjiev
 

Tula State University, Tula, Russia
GNPP -Splav, Tula, Russia
Military Research Institute, Sofia, Bulgaria
Dunarit-Ltd., Rousse, Bulgaria
Vazov Engineering Plants-Ltd., Sopot, Bulgaria

Experimental - Theoretical Determination of Physical Constants Included in a Phenomenologic Model of Destructions

In this paper has been developed and realized an experimental theoretical method for determination of the numerical values of the physical constants included in the structure of the phenomenologic model for destruction of real material which allows the application of this model for evaluation of strength of structures in case of tensile step loading. The authors present the constant value of aluminum.



back to main page