Issue 4

JTAM, Sofia, vol. 22 Issue 4 (1991)

Kinematics of a Three-Degrees-of-Free-dom Manipulator with Parallel Topology. Part I. Direct Kinematics

T. Tanev1, N.Shivarov1, M. Konstantinov2
1Institute of Engineering Cybernetics and Robotics, Bulg. Acad. Sci., Sofia
2University for Architecture, Civil Engineering and Geodesy, Sofia

This paper presents a complete kinematic analysis of a mechanism with parallel topology and three degrees of freedom of 3RRS type. The possible utilization of this mechanism as a link of manipulation systems is examined. Solutions of the direct and the inverse kinematic problems for this manipulation system are given. The restrictions on the parallel-topology robot motion are analysed.

JTAM, Sofia, vol. 22 Issue 4 pp. 009-016 (1991), [Full Article]

Double-Gradient Stabilization in Non Linear Thermoviscoplasticity

A. Rigatos, N. Charalambakis
Aristotle University, Thessaloniki, Greece

We consider a thermoviscoplastic solid exhibiting thermal softening and strain rate sensitivity, submitted to a steady shear force at the boundary. The stress function is assumed to depend on the higher order strain gradients. The non-linear analysis of the system of partial differential equations describing the process, as well as the numerical solution, indicate the stabilizing role of the corresponding coefficients. Moreover, a critical time for the "blow-up" of temperature is derived.

JTAM, Sofia, vol. 22 Issue 4 pp. 017-024 (1991), [Full Article]

On the Equilibrium of a High Flexure Manipulator

K. Mladenov
University for Architecture, Civil Engineering and Geodesy, Sofia

The equilibrium of a high flexure manipulator consisting of a tubed elastic column with constant cross-sectional area and bending stiffness is considered. The system is charged by a payload, parallel to the initial straight axis and by a stretching tangential follower force applied eccentrically in order to produce a bending moment at the extreme end. In contrast to the conclusions published previously it is shown that many equilibrium states may exist for the same loading conditions depending on the magnitudes of the forces.

JTAM, Sofia, vol. 22 Issue 4 pp. 025-034 (1991), [Full Article]

Carrying Capacity of Thin Inelastic Circular Plates

A. Baltov, O. Minchev
Institute of Mechanics and Biomechanics, Bulg. Acad. Sci., Sofia

We take into account thin flexible round plates, which deform elasto-visco-plastically under high intensity external loading and in which a volume damage can occur. The plates are submitted to an impulsive loading uniformly distributed in the middle plane of the plate. The plate carrying capacity is determined in accordance to deformation and strength criteria. Our purpose is to obtain the relation between the geometrical sizes and the material characteristics of the plate, on the one hand, and the dynamic impulse parameters (amplitude and duration), on the other, when the plates lose their carrying capacity.

JTAM, Sofia, vol. 22 Issue 4 pp. 035-040 (1991), [Full Article]

Transient Motion of a Drop in Boundless Liquid with Surfactants

L. Antanovskii
Institute of Hydrodynamics, Novosibirsk, USSR

The symmetrical form of capillary fluid mechanics equations involving the descripsion of heat and surfactant transfer processes is explored for solving the problem of slow trantient motion of a drop driven by both the buoyancy and the surface tension forces. The formula for the Laplace image of the drop mass centre position is obtained in the case of low-Reynolds-number flow in and around the drop, and the Le Chatelier principle implying the deceleration of the motion due to the capillarity is proved.

JTAM, Sofia, vol. 22 Issue 4 pp. 041-049 (1991), [Full Article]

Influence of the Surface Deformability on Marangoni Instability in an Insulating Liquid Layer

Z. Kozhoukharova
Institute of Mechanics and Biomechanics, Bulg. Acad. Sci., Sofia

The influence of the surface deformability on the Marangoni instability due to a physical absorption in an infinite liquid layer, placed at an insulating horizontal rigid wall and opened to the ambient gas, is studied. A linear stability theory is performed. The conditions of the overstability existence are established.

JTAM, Sofia, vol. 22 Issue 4 pp. 050-058 (1991), [Full Article]

Numerical Modelling of Three Dimensional Transitional Boundary Layer on Ellipsoid

D. Marinov1, Z. Zapryanov2
1Institute of Water Problems, Bulg. Acad. Sci., Sofia
2Sofia University, Sofia

The paper describes numerical results of three dimensional transitional boundary layer on prolate ellipsoid pasted by an uniform viscous flow with fixed initial turbulence level. The beginning of the laminar-turbulent transition is determined using an empirical criterion. The development of the disturbances produced by free stream turbulence is described by a turbulence model.

JTAM, Sofia, vol. 22 Issue 4 pp. 059-066 (1991), [Full Article]

Stability of Elastic Orthotropic Cylindrical Shells under Combined Static Axial Compression and Short Impulse of External Pressure

J. Ivanova, P. Pirgov
Institute of Mechanics and Biomechanics, Bulg. Acad. Sci., Sofia

An analytical study is made for elastic cylindrical shells with physical orthotropy subjected to static axial compression and external impulsive pressure. The geometrical method in the nonlinear theory of elastic shells is used. The critic relation between the axial compression and the impulsive external pressure is determined.

JTAM, Sofia, vol. 22 Issue 4 pp. 067-071 (1991), [Full Article]

Stability of Cylindrical Rigid-Plastic Shells under Impulse Loading

J. Ivanova, T. Dabova
Institute of Mechanics and Biomechanics, Bulg. Acad. Sci., Sofia

In the present paper an analytical method for solving dynamic equations of cylindrical rigid-plastic shells under impulse loading is used. The obtained analytical solution is in the class of special functions and together with a suitable criterion for a stability loss gives a possibility for practical uses. The stability loss, the critical time and the value of the radial deflection are all given in Fig. 1, 2.

JTAM, Sofia, vol. 22 Issue 4 pp. 072-081 (1991), [Full Article]

Rational Calculation of the Prying Forces Effects Arising in the Design of Some Steel Structures

A. Khairedin
Aristotle University, Thessaloniki, Greece

In this paper some kind of bolted connections of steel structures is studied, taking into account the prying forces due to unilateral contact phenomena arising between the adjacent members. Based on parametric investigations a simplified design method is proposed. It allows for consideration of the analysed phenomenon in the every-day engineering practice.

JTAM, Sofia, vol. 22 Issue 4 pp. 082-088 (1991), [Full Article]

Elastio-Plastic Constitutive Equations with Hardening Effects for Thin-Walled Beams

E. Stanoev
Centre of Electronics in Engineering and Building Industry

The general stress-strain relations, which govern the static elasto-plastic behaviour of isotropic material in the theory of plasticity are developed. A combined treatment of iso-tropic and kinematic hardening is considered by the introduction of an arbitrary parameter m (0 ≤ m ≤ l). The application of the developed constitutive relations to the theory of thin-walled beams is shown.

JTAM, Sofia, vol. 22 Issue 4 pp. 089-097 (1991), [Full Article]

Influence of Damage on Plastic Localization in Anisotropically Hardening Materials

N. Bontcheva
Institute of Mechanics and Biomechanics, Bulg. Acad. Sci., Sofia

The necessary conditions for plastic localization bands initiation are obtained. The considered material is rigid-plastic, strain rate sensitive, anisotropically hardening and thermally softening. Inside, the bands of the localized plastic deformation strains are much higher than in the rest of the material. This leads to a material damage. The development of microcracks and voids makes the material more deformable, which has an influence on the initiation of localization bands, due to the softening effects. The development of the damage is described by means of a scalar variable, according to the model proposed by Lemaitre. The influence of the different factors – plastic strain, temperature and damage on plastic localization is shown. It is seen that plastic localization can occur even in the isothermal case due to the structural changes in the material.

JTAM, Sofia, vol. 22 Issue 4 pp. 098-105 (1991), [Full Article]