Issue 4

JTAM, Sofia, vol. 26 Issue 4 (1996)

Generalized Forces and Robustness of Mechanical Systems

Al. Cheremensky
Institute of Mechanics, Bulg. Acad. Sci., Sofia

This article reveals interdependence of different forces ensuring zero equilibrium asymptotical stability of mechanical systems. Given is the sufficient condition when this property is robust with respect to small perturbation of these forces. Time quantizing the stabilizing forces is considered.

JTAM, Sofia, vol. 26 Issue 4 pp. 09-18 (1996), [Full Article]

On the Wear of a Tribosystem

V. Diamandiev
Faculty of Mathematics and Informatics, Sofia University

This paper considers problems of tribology. The wear, as well as the full wear of a tribosystem are considered. The friction depends on the contact temperature. The law of the wear is taken to be that of Preston. The results can be used in solving different contact problems of thermoelasticity.

JTAM, Sofia, vol. 26 Issue 4 pp. 19-23 (1996), [Full Article]

Concentration of Movements on the Links of Manipulation Systems for Robots

V. Pavlov
Technical University, Sofia

The robots are evolving in many directions. Such directions are the increase of the degrees of mobility and improvement of the manipulation qualities in the working area. An approach for achievement of these qualities by concentration of movements on the links is suggested. For concentration of movements is assumed implementation of more than one elementary relative movement in the active and passive kinematic pairs, i. e. implementation of complex movements. These relative movements are presented in two ways – as a sum of two and more elementary movements (the geometrical relation between the links is fulfilled by surfaces with axis of symmetry) or by appropriate space geometrical relation between the links. Some solutions for illustration of the method are presented.

JTAM, Sofia, vol. 26 Issue 4 pp. 24-30 (1996), [Full Article]

Mathematical Modelling for a Joint of Implanted and Host Blood Vessels

I. Selezov1, O. Avramenko1, G. Fratamico2, G. Pallotti2, P. Pettazzoni2
1Institute of Hydromechanics, Kiev, Ukraine
2University of Bologna, Bologna, Italy

A New physical-mathematical model is presented to describe the phenomenon near the joint of blood vessels of slightly different physical and geometrical properties such as Young modulus E, density ρ and thickness h. This is a typical situation for implantation of donor or artificial blood vessels. Even slight differences of properties leading to high stress concentration at the suture line between graft and host artery can be responsible for originating a stenosis, taking into account a great pulse reiteration. It should be especially noted that the danger of bending stresses is the most possible due to the presence of compressed and stretched tissue fibres across a relatively small thickness size and to proportionality of the bending moment to the third power of h, while the usual circumferential (chain) force is proportional only to the first power of h, and consequently is less sensible to changing thickness h.

JTAM, Sofia, vol. 26 Issue 4 pp. 31-42 (1996), [Full Article]

Study of Mechanical Behaviour of Selfstanding Inflatable Dams

M. Popova, N. Velkov
University of Chemical Technology and Metallurgy, Sofia

The basic principles of the performance of self-standing membrane dams are analyzed. A mathematical model for determining the form of the cross section of multi-chamber membranes based on the condition of forced closing of the contours is suggested in this paper. A computer simulation method for studying the form, the conditions of self-standing and the stability of multi-chamber structures under varying operational conditions is developed. The recommendable from the point of view of the self-standing geometric and force characteristics for two- and three-chamber dams are given. The obtained results establish a basis for the design and introduction of light, portable membrane back watering structures in irrigation and drainage and emergency cases.

JTAM, Sofia, vol. 26 Issue 4 pp. 43-56 (1996), [Full Article]

A New Approach to Determine the Yield Loci of Heterogeneous Media

A. Baltov, M. Datcheva, R. Iankov
Institute of Mechanics, Bulg. Acad. Sci., Sofia

The method is presented for analyzing the influence of the geometry of the inclusions on the plastic properties of the heterogeneous structure. It is based on the homogenization procedure but the yield locus is constructed without completing the homogenization. Only a proper estimation of the average plastic strain is used to determine the yield stress surface. That is an effort to avoid the mathematical difficulties in elastoplastic case where the fixed relation between strains and stresses is absent. The FEM is applied and the numerical solution of the elasto-plastic problem for the concrete micro structure is analyzed. The influence of the orientation of the elliptical inclusions is illustrated.

JTAM, Sofia, vol. 26 Issue 4 pp. 57-67 (1996), [Full Article]

The Homogeneous-invariant Theory for Thin Elastic Shells. Applications for Dynamic Calculus

V. Visarion, A. Iarovici
Institute of Solid Mechanics, Rom. Acad. Sci Bucharest

Based on the statical-geometrical analogy introduced in [1] and the complex homogeneous-invariant formulation from [2], a more elaborated homogeneous-invariant theory for thin elastic shells is presented. Using special functions derived from Legendre polynomials [3] one presents the dynamic calculus for rotation type shells loaded on normal direction of the surface.

JTAM, Sofia, vol. 26 Issue 4 pp. 68-81 (1996), [Full Article]

Cohesive Quadratic Load Distribution Arresting Crack Opening – the Dugdale Approach

R. Bhargava, S. Agrawal
University of Roorkee, Roorkee, India

The problem of an infinite, homogeneous, isotropic, elastic-perfectly plastic matrix containing two equal, symmetrically situated collinear straight cracks is investigated in the present paper. The matrix is subjected to remotely applied uniform tension acting normal to the rims of the cracks. The cracks faces open because of these loads, developing plastic zones ahead of the tips of the cracks. These plastic zones are subjected to cohesive quadratically varing yield point stress, consequently the cracks are arrested. The variation of load required for the cracks closure and corresponding crack opening displacement (COD) at the crack tips is studied varying crack length and the inter-crack distance between the cracks. The complex variable technique and the Dugdale's hypothesis are used to solve the problem.

JTAM, Sofia, vol. 26 Issue 4 pp. 82-92 (1996), [Full Article]

Comments of Two Papers Published in "Journal of Theoretical and Applied Mechanics’’

Al. Cheremensky
Institute of Mechanics, Bulg. Acad. Sci., Sofia

The main conclusions of two papers published in JTAM are disproved by these comments as some principal mistakes are found there.

JTAM, Sofia, vol. 26 Issue 4 pp. 93-96 (1996), [Full Article]