JOURNAL OF THEORETICAL AND APPLIED MECHANICS
Volume 27 Number 4 1997
                                                                    CONTENTS

GENERAL

MECHANICS

V. Konoplev

A. Cheremensky
 

Institute for Problems of Mechanical Engineering, Russ. Ac. Sci., St. Petersburg
Institute of Mechanics, Bulg. Ac. Sci., Sofia

Linear Space of Equipollent Systems of Forces

This paper explores the mathematical nature of force systems in the multibody system mechanics. It yields computer-aided tools for using equipollent systems of forces.



P. Genova

V. Ivanova
 

IPF-Sliven

Synthesis and Optimization of Precise Planer Cam Mechanisms

In this paper are proposed the polynomial-harmonious motion laws with continuous 3rd and 4th transfer functions.
The planer cam mechanisms are systematized in eight varieties. General equations are worked out for their synthesis, based on the envelope theory. The synthesis of the cam profile is done with a precision up to the 4-th derivative of the motion law. To achieve this, we have derived the recurrent dependencies for calculation of the n-th derivative of the cam curve.An error analysis of the motion law and its derivatives up to the 2nd have been performed. There are two approaches suggested to increase the precision of the motion laws, realized by the cam.The optimization criteria of the basic parameters of cam mechanisms are complex and refer to the Efficiency Coefficient, the distortion, the stress and the size.


Al. Vatzkitchev
 

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

On the Using of Matrix Transformations in the Force and Deformation Analysis of Manipulation Systems with Built-in Force-Torque Sensors

This paper presents the development of some ideas about force/deformation analysis using force/torque sensors. It is based on matrix transformations between the robot links. The difference between the real loads and the sensor measurement is due to the different position of the contact zone and the sensor. The question of interdependence between the real parameters of load and the data from the sensor is posed. The formalisation of the tasks in force and deformation analysis matrices permits to simplify their decision. We can determine exactly the influence of each factor (joint coordinates, dimensions, gravitation and inertia) on the accorded analysed parameters. The use of such matrices decreases in great degree the calculation time for decision of the described tasks.



V. Abadjiev

D. Petrova
 

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

On the Singularity of Skew-Axes Gears with Straight-line Contact Between the Tooth Surfaces

What are considered are skew-axes gears generated in accordance with the second Oliver's principle and a straight-line contact between the tooth flanks. For this class of gears it is of essential interest the problem for non-existence of ordinary nodes and points of undercutting in the region of mesh. The scientific research is a basis of the computer test for a singularity. Thus the elimination of the undesired singular points is performed before the expensive technological design of the gear sets.



V. G. Petrov

S. G. Nikolov

I. Edissonov
 

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

Reconstruction of Polynomial Right-hand Sides of Two-dimensional Dynamical System on Given Integral Curve

In this paper is solved the problem for reconstruction of unknown right-hand sides of two-dimensional dynamical system on given integral curve. A method for determination of dynamical system with polynamial right-hand sides (with integral curve given parametrically) is elaborated. The proposed method is applied to concrete two-dimensional dynamical system.



CONTINUUM

MECHANICS

V. H. Vassilev

R. Flores-Berrones
 

Institute of Water Problems, Bulg. Ac. Sci., Sofia
Mexican Institute of Water Technology, Mexico

Finite Element Method Modelling of  Seismic Wave Propagation Effects on Buried Segmented Pipelines

Finite Element Method in two-dimensional formulation is used to investigate the dynamic behaviour of buried pipeline. Quasi-static analysis is adopted for assessment of the pipeline response. Buried pipeline is presented as a series of segmented elastic beams, connected longitudinally with joints. Soil is assumed to be homogeneous(or unhomogeneous) isotropic(or transversal isotropic) elastic media in plane strain conditions. Seismic forces at the each finite element are determined as mass forces depending on the ground acceleration. The possibility of slippage at the surface between the pipe and the soil is evaluated as well when the seismic ground strain becomes large. A computer program called SPLAN is developed under the above considerations including pre-processing and post- processing mechanisms and numerical experiments were done to study the influence of the geological and soil conditions on the stress strain state of the pipeline.


O. Santurjian

I. Etimova

L. Kolarov
 

Institute of Water Problems, Bulg. Ac. Sci., Sofia

Three-Dimensional Modelling of Thermal Regime and Stresses in Mass Concrete Structures - PC Program Thermostress

Studies of the thermal regime and the development of the thermal stresses in design are necessary for prevention of the blocks and other massive elements of mass concrete structures from crack formation. An important tool for such studies is the PC program ``Thermostress 5''. The program enables the numerical modelling of three-dimensional temperature and stress fields in concrete blocks during construction and operation and thus supports the thermal analyses of mass concrete structures as dams, foundations, thick walls etc. In the article are described the main features of the program, its logic and possibilities for solving of different problems and other information relevant for such scientific products.


St. Stoytchev

A. Rachev
 

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

Mechanical Criteria for Design and Use of Deformable Synthetic Grafts

Mathematical model of a graft/artery system is presented. Two criteria for mechanical compatibility, based on a minimization of the distance which represent a) the flow induced shear stress at the inner surface of the host artery and the graft, and b) compliamces of both vessels over a relevant physiological pressure range, are formulated. Numerical study was performed in the case of human common iliac and femoral arteries, replaced by fibrous polyurethane or Dacron grafts. The analysis of the results shows that the shear stress criterion provides better similarity of the pressure-radius curves of the host artery and the graft than the compliance criterion. We propose to select initial graft diameter and thickness by means of shear stress criterion.


SOLID

MECHANICS

I. Kolarov

V. Valeva
 

High Military Transport School, Sofia
Institute of Mechanics, Bulg. Ac. Sci., Sofia

On the Possibility for a Determination of the Stress State in Real Constructions of Press-fit Joints

The aim of the present paper is to investigate the possibility for a practical prediction of the mechanical stress concentrations in real constructions of the press-fit joints by the use both the numerical (BEM) and experimental (ultrasonic) methods.



G.Kolarov

Technical University - Sofia

Simple Error Estimators for Dimensional-Adaptivity: Applications for Beams

In this paper problems of dimensional adaptivity are studied. Possible transitions from a beam model to higher models and possible disturbances, that make the beam solution inaccurate, are presented. Several a-posteriori error estimators, which have a physically interpretation, for the model error of the 2D bending beam are proposed. All error estimators can be computed with a very small computational effort and with a little additional information from the solution of the beam model. The application of the error estimators to different examples shows the very good behaviour of the energy-based error estimators. The presented work can be used as a basis for the development of other error estimators for different disturbances and other models.



G. Stojchev

K. Tushtev
 

Technical University - Sofia

Variant  of  Plasticity  for  Cast  Iron

A material model for the elastoplastic deformation of cast iron is presented. It accounts for the different strength in tension and compression and the nonelastic volume change which is typical for cast iron.

The governing equations for the elastoplastic deformation, based on an non-associated flow rule, are derived. They are based on anisotropic hardening and hardening parameter determined  by  a modified plastic work. The hardening parameter is defined by a function dependent on the mean stress. The plastic potential is a function of the first invariant of the stress tensor and the second invariant of deviator tensor.

A non-linear procedure for the numerical implementation of the model for 2D proportional loading is presented. The agreement with experimental data is good.



FRACTURE

MECHANICS
 

L. Zolochevskaya
 

Kharkov State University, Ukraine

Anisotropic Creep Theory for Materials with Different Damage in Tension and Compression

A continuum damage mechanics model for creep response of initially transversely isotropic materials with parallel planar microcracks is presented. A complete polynomial expansion of the equivalent stress in the creep potential with respect to the unilateral damage is developed, and the general form of the creep constitutive equation for an initially transversely isotropic materials with parallel disk-like microcracks is derived. This model describes simultaneously initial anisotropy, damage induced anisotropy, different damage development in tension and compression, and different creep properties in tension and compression.



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