Issue 1

JTAM, Sofia, vol. 26 Issue 1 (1996)

Renewal and Rejuvenation of the Editorial Board

G. Brankov
Institute of Mechanics, Bulg. Acad. Sci., Sofia

JTAM, Sofia, vol. 26 Issue 1 pp. 11-12 (1996), [Full Article]

Within and Between Set Variances Modelled with Coupled Harmonic Oscillators

J. Boyd, P. Raychowdhury
Virginia Commonwealth University, USA

The total variance of two combined data sets is partitioned into "within set" and "between set" variance. The partition is modelled with harmonic oscillators coupled with ideal springs. The model depends upon the observation that the elastic potential energy of the oscilators and the total variation in the combined data sets have the same positive semidefinite quadratic forms. The equations of motion of the dynamical system are obtained from its Lagrangian function. Orthogonal and unitary transformations simplify the equations of motion.

JTAM, Sofia, vol. 26 Issue 1 pp. 13-21 (1996), [Full Article]

Synthesis of Skew-Axes Gears Pitch Circles

V. Abadjiev, D. Petrova
Institute of Mechanics, Bulg. Acad. Sci., Sofia

What is considered are pitch circles of skew-axes gears. The conditions for the existence of these circles have been found. The relations between the pitch circles parameters and the relevant externally or internally meshed gear pairs is pointed out.

JTAM, Sofia, vol. 26 Issue 1 pp. 22-29 (1996), [Full Article]

Identification of the Coefficients in Dynamic Models of Robot Manipulators

K. Delchev
Institute of Mechanics, Bulg. Acad. Sci., Sofia

A precise identification of dynamic parameters of robots is of utmost importance for the design of advanced, model based controllers. This paper presents an identification algorithm to estimate coefficients in the robot dynamic equations derived from the Lagrangian formulation. The algorithm is based on sequential, point-to-point, acceleration test motions in order to decrease measurement noise and computational effort. The coefficients estimated as nonparametric functions of the given joint positions can be directly used for synthesis of feedforward control on a given path.

JTAM, Sofia, vol. 26 Issue 1 pp. 30-38 (1996), [Full Article]

Computer-aided Optimization of the Thermomechanical Conditions in Rolling of Reinforcing Bars

G. Petzov, V. Markov
Technical University of Varna

Through analytical procedures, based on the relationships of thermoplasticity, a combined model for numerical simulating of the final passed in caliber rolling of round square bars from nonalloy steel and their thermomechanical hardening has been built. The model consists of solution for the stresses, strain rates and temperature variations in the deformation zone, as well as of a solution for the thermomechanical hardening. On the basis of this combined model a numerical experiment based on Simplex method for improving the thermomechanical conditions in caliber rolling has been put into effect. The model can be used for predicting as well as for optimization of the mechanical properties of reinforcing bars.

JTAM, Sofia, vol. 26 Issue 1 pp. 39-46 (1996), [Full Article]

Method of Variational Imbedding for Identification of Boundary Layer Thickness: a Numerical Implementation with Local Equation for Thickness

Ch. Marinov
Technical University of Varna

Method of Variational Imbedding for Identification of Boundary Layer Thickness: a Numerical Implementation with Local Equation for Thickness … 47 Identification of the thickness of boundary layer appears to be the most spoiled part of the numerical procedures for treating boundary layer flows because of the inverse nature of the problem. The natural way to derive somewhat more standard boundary value is to scale the normal independent coordinate by the unknown boundary-layer-thickness shape function. Thus arrives the problem of identifying an unknown coefficient in nonlinear parabolic set of equations from overspecified data measured on the boundary. In present paper a new approach is followed on the Method of Variational Imbedding (MVI) earlier developed for solving inverse problems for elliptic and parabolic equations. On its base we replacing the original inverse problem by the problem of minimization of quadratic functional of the problem. We prove the existing and uniqueness of a generalized solution of "imbedding" problem, construct a difference scheme and numerical algorithm for its implementation. Some results of numerical experiments are presented to illustrate the various results.

JTAM, Sofia, vol. 26 Issue 1 pp. 47-58 (1996), [Full Article]

On the Efficiency of Some Isoparametric Finite Elements in Plane Elasticity

Ivo Stevanov
University of Architecture, Civil Engineering and Geodesy, Sofia

Two isoparametric finite elements with central nodes are investigated in solving some plane elasticity problems. The results are compared with those obtained by by the well known 4-node and 8-node isoparametric finite elements (FE4 and FE8). A new interpolation function of higher degree is proposed for the 9-node "Lagrangian" element. Its application inproves the solution in most cases, provided that the Gaussian numerical integration order remains 3 (with 9 sampling points). An attempt is made to indicate the suitable type of finite element in some common cases.

JTAM, Sofia, vol. 26 Issue 1 pp. 59-66 (1996), [Full Article]

Modelling the Mechanical Bechaviour of a Polymer Membrane in an Oblique Impact

S. Karapetrova, M. Popova, V. Iliev
University of Chemical Technology and Metallurgy, Sofia

For the absolutely flexible membranes, several technical theories using numerical methods have been elaborated. In the cases a static load has been assumed. In the present work, using equations derived on the basis of the wave theory of the impact, the effect of an oblique impact of a cone upon the stress-strain state of a linearly elastic membrane has been investigated. The pattern of stress distribution in the plane area of the membrane has been obtained. A numerical example for hydrotechnical membrane made of rubberized synthetic fabric has been solved.

JTAM, Sofia, vol. 26 Issue 1 pp. 67-72 (1996), [Full Article]

Numerical Evaluation of the Possibilities of Ultrasonic Nondestructive Testing for Non-metal Inclusions and Corrosion

M. Mihovsky1, P. Raykov1, G. Dinev1, A. Popov2
1Institute of Mechanics, Bulg. Acad. Sci., Sofia

Numerical simulation experiment of reflection and transmission of ultrasonic plane wave through the thin layer between two materials for a range of acoustic frequencies and different attenuation coefficients of the materials is presented. On the base of these results the multi-frequency ultrasonic method for identification of layer's thickness is proposed.

JTAM, Sofia, vol. 26 Issue 1 pp. 73-78 (1996), [Full Article]

Determination of Dynamic Fracture Toughness of Some Steels

G. Kortenski
Institute of Metal Science, Bulg. Acad. Sci., Sofia

Method of the dynamic fracture toughness (KId) estimation using subsized specimens has been described. The method can be used in the case of small specimens testing only or when safety regulations require such specimens, e.g. for neutron irradiated or hydrogen treated materials. Results are given for non-treated steels and compared to KId values determined with other dynamic testing method, for example precracked Charpy test.

JTAM, Sofia, vol. 26 Issue 1 pp. 79-82 (1996), [Full Article]

Effect of Hydrogen on the Mechanical Properties and Notch Toughness of Newly Created Steel

G. Kortenski, Hr. Argirov
Institute of Metal Science, Bulg. Acad. Sci., Sofia

The effect of hydrogen on the mechanical properties of two modifications of newly created low alloyed steel with nitrogen (38XH3MAFA) and without nitrogen (38XH3MFA) were studied by means of tensile and three-point-bend tests. Specimens were electrolytically charged in .hydrogen and tested over a wide temperature range. Higher notch toughness (KCV) and lower brittle-to-ductile transition temperature for uncharged 38XH3MAFA steel specimens has been obtained experimentally compared to these for uncharged 38XH3MFA steel ones and KCV values obtained were 124.4 KJ/m2 and 99.2 KJ/m2 at +20°C respectively, and BDTT values were -75°C and -55°C respectively. The hydrogen embrittlement for both steels investigated were found to increase with temperature decrease and it is about 30% for the whole range investigated.

JTAM, Sofia, vol. 26 Issue 1 pp. 83-86 (1996), [Full Article]

Plastic Zone Size and Shape in Front of Fatigue Crack Tip for Nitrogen Stainless Steel

Youlian Davidov
Institute of Metal Science, Bulg. Acad. Sci., Sofia

The object of this study is experimental determination of the plastic zone size and shape in front of the crack tip for three modifications of Cr18NMn12 stainless steel with different nitrogen content under cyclic loading conditions. The increase of this content decreases the fatigue plastic deformation zone size, due to decrease of the cyclic ductility of this material.

JTAM, Sofia, vol. 26 Issue 1 pp. 87-91 (1996), [Full Article]