Issue 2

JTAM, Sofia, vol. 24 Issue 2 (1993)

Fracture Theory at Atomic Level

G. Brankov
Institute of Mechanics, Bulgarian Academy of Sciences, Sofia

The paper reviews the development of the investigations in the domain of Frac¬ture Mechanics at atomic and electronic level.

JTAM, Sofia, vol. 24 Issue 2 pp. 017-026 (1993), [Full Article]

On the Stability of Lyapunov's Periodic Solutions of a Mechanical System with Two Degrees of Freedom

Ogn. Christov
University of Sofia, Bulgaria

Consider the following mechanical sys¬tem. A particle, attached to a spring, is oscillating in a rigid body with a fixed point, along a line which coincides with a main inertial axis for the body. In the case when there are no external forces, the system is a Hamiltonian one with two degrees of freedom. According to the Lyapunov-Weinstain theorems, under certain conditions there exist at least two periodic solutions on any energy level. These solutions are usually called Lyapunov's periodic solutions. The stability of these two one paramertic families of periodic solutions is studied, using KAM-theory methods, namely Moser's twist theorem. The fact that the system is integrable when the particle oscillates along an axis of dynamical symmetry, is crucial.

JTAM, Sofia, vol. 24 Issue 2 pp. 027-032 (1993), [Full Article]

Modelling and Optimization of Nonlinear Friction Laws with Respect to Forced Vibrations Damping

A. Borissova1, N. Dakev2, S. Grozdev2
2Institute of Mechanics, Bulgarian Academy of Sciences, Sofia

Some aspects are considered to determine optimal friction laws in mechanical oscillation dissipative systems. A new criterion is proposed to evaluate the energy exchange processes. A problem is solved using the proposed energy performance index.

JTAM, Sofia, vol. 24 Issue 2 pp. 033-038 (1993), [Full Article]

Dynamic Loadings Due to Torsional and Bending Vibrations on the Links of Machines with Cardan Transmissions

A. Pissarev, B. Marinov
Technical University, Sofia

What are investigated in the paper are the dependences between the bending and the torsional vibrations in the links of machines with cardan transmissions. Expressions are obtained for the amplitude values of the bending and the torsional moments. An analysis of the derived results is done on the basis of the constructed amplitude-frequency characteristics.

JTAM, Sofia, vol. 24 Issue 2 pp. 039-045 (1993), [Full Article]

The Dimer Model as a Model of Self-Organized Criticality

J. Brankov, E. Litov
Institute of Mechanics, Bulgarian Academy of Sciences, Sofia

Recently, great interest is paid to the phenomenon of self-organized criticality. A simple model of the dissipative evolution of a pile of sand exhibits the connection between nonlinear dynamics and the appearance of spatial and temporal self-similarity. In the present work we establish the equivalence of the dimer model on a square lattice L with free boundaries and the Abelian sandpile model on another square lattice L' with mixed Dirichlet-Neumann conditions imposed on at least one pair of opposite boundary sides. This makes possible the numerical simulations with the Monte Carlo algorithm, devel¬oped by us for the generation of dimer configurations.

JTAM, Sofia, vol. 24 Issue 2 pp. 046-053 (1993), [Full Article]

A Linear Complementarity Approach to Seismic Frictional Interaction Between Adjacent Buildings Under Second-Order Geometric Effects

A. Liolios
Democritus University of Thrace, Greece

The paper deals with the numerical treatment of the so called inequality problem arising in Earthquake analysis of adjacent space structures. The influence of the seismic interaction and the modifica¬tions are quantitatively calculated in a civil engineering example.

JTAM, Sofia, vol. 24 Issue 2 pp. 054-062 (1993), [Full Article]

On the Nonstationary Random Vibrations of Three-Dimensional Shearwall Buildings

S. Ganchev
University of Architecture, Civil Engineering and Geodezy, Sofia

Three-dimensional shearwall buildings are analyzed subjected to earthquake excitations. The structure is discretized by the Finite Element Method and the Random vibrations theory is employed. A nonstationary seismic input is expressed by the stationary Kanai-Tajimi spectral density and a sinus time-modulating function. The earthquake direction is taken into account and most dangerous directions are recommended depending on the structure eigen modes. Response variances and the uperossing probability for a given barrier displacement are examined.

JTAM, Sofia, vol. 24 Issue 2 pp. 063-070 (1993), [Full Article]

Assessment of Seismic Excitation for Large Dam Sites

H. Boncheva, M. Kostov, G. Varbanov, A. Kaneva, D. Stefanov
Central Laboratory for Seismic Mechanics and Earthquake Engineering, Bulgarian Academy of Sciences, Sofia

The first step in the evaluation of seismic safety of large dams is specifying the seismic excitation – acceleration, frequency content and duration of expected ground motion at the site. It is done using seismic hazard analysis methods and different models for solving the soil-structure-water ineraction. Applying deterministic and probabilistic methods, the design seismic characteristics at large dams sites in Bulgaria have been obtained. The results of those projects illustrate the necessity of additional requirements to be included in the seismic code. Respective recommendations are drawn in the paper.

JTAM, Sofia, vol. 24 Issue 2 pp. 071-076 (1993), [Full Article]

Experimental Dynamic Analysis of Chaira Concrete Ggavity Dam

S. Simeonov, K. Hadjiyski, M. Haralanov
Central Laboratory for Seismic Mechanics and Earthquake Engineering, Bulgarian Acacemy of Sciences, Sofia

In-situ full scale dynamic experiment of the concrete wall of Chaira dam by low energy blast tests is conducted. The main objective of this experiment is to evaluate the dynamic characteristics of the structure and thus provide background data for correction and adjustment of mathematical models applied for assessment of its earthquake resistance capacity and for complying with the safety requirements. The in-situ testing procedure and data processing and analysis for identification of the modal parameters are discussed. In the end, the experimental results are compared with those obtained by different analytical models.

JTAM, Sofia, vol. 24 Issue 2 pp. 077-086 (1993), [Full Article]

Experimental and Numerical Study of a Horizontal Heated Round Jet

D. Djonova-Atanassova
Technical University, Sofia

Results from experimental study of horizontal heated round jets are presented. Two turbulence models are employed to predict such jets: a k – ε – t' model with an equation for the mean-square temperature fluctuations and a second order model with equations for the Reynolds stresses and the turbulent heat fluxes. The differential equations are solved using a finite-difference procedure. The predictions are compared to our own as well as to other existing measurements. The values of some of the constants in the models are changed in order to achieve better agreement with the experimental data.

JTAM, Sofia, vol. 24 Issue 2 pp. 087-093 (1993), [Full Article]

Numerical Simulation of Nitrogen Diffusion in a Solid Chromium Particle

V. Manolov1, O. Iliev2, I. Bijev1
1Institute of Metal Science, Bulgarian Academy of Sciences, Sofia
2Institute of Mathematics, Bulgarian Academy of Sciences, Sofia

A forming process of nitrogen-laden of heated, solid spherical chromium particle is considered. The chromium particle is supposed to be dipped in pure nitrogen gas medium. The unsteady flux of nitrogen atoms in the heated chromium particle is described by Fick's partial differential equation. As the nitrogen atoms interact with the metal matrix, phase layers are formed in the particle and each layer has specific width and nitrogen concentration values. The inter-layer boundary movement is described by a like-Stefan' equation. Two-layer system is numericaly investigated in the present paper. Inter-layer boundary movement and unsteady nitrogen concentration field are computed. The results from the numerical experiments allow the above mentioned technological process to be controlled and improved.

JTAM, Sofia, vol. 24 Issue 2 pp. 094-101 (1993), [Full Article]

Numerical Simulation of Density Distribution during the Compaction of Porous Materials

N. Bontcheva, R. Iankov
Institute of Mechanics, Bulgarian Academy of Sciences, Sofia

The present paper describes a FEM formulation for simulating powder metallurgy forming processes. The various constitutive equations, proposed by Shima and Oyane, and by Doraivelu et al., describing the plastic behaviour of porous materials are used. The P/M model has been implemented into a created FEM programme PORO, that solves 2D boundary value problems, taking into account the friction between the die and work-piece. The surface roughness of a preform depends on the mean particle size of the base material. The distribution of the surface roughness characteristics between work-piece and die is numerically determined.

JTAM, Sofia, vol. 24 Issue 2 pp. 102-108 (1993), [Full Article]

Relation between 3D and 2D Elastic Problems through the Finite Prism Method

Technical University, Sofia

The relation between the 3D elastic problem and the 2D elastic problems (plane stress, plane strain and anti plane problem) are presented. The Finite Prism Method allows for a solution of the 3D elastic problems for a prismatic body using a solution with trigonometric series along the prismatic body and a finite element solution (plane elements) in the cross section of the body. By using the finite prism method it is possible to obtain solutions for the 2D elastic problems. Examples which show the application of the finite prism method for solving 2D problems are presented.

JTAM, Sofia, vol. 24 Issue 2 pp. 109-114 (1993), [Full Article]

On Validity of Theoretical Models for Determination of Stress in Coatings

Chr. Kouyumdjiev, N. Stancheva
Technical University of Rousse

Bibliographic sources offer a number of theoretical models of a coating, plated on one side of an elastic flexible strip used as a test sample for coating's stress and strain state determination. The validity of the simplifying assumptions accepted at the modelling, as well as the reliability of the acquired results are being checked in the present work through the finite element method. The three-dimensional problems of the theory of elasticity solved correspond more accurately to the real test samples than the two-dimensional problems solved at validity checks in earlier works. The confirmation of the theoretical models and the relations derived on their basis give possibilities for their further application in laboratory practice.

JTAM, Sofia, vol. 24 Issue 2 pp. 115-121 (1993), [Full Article]

Peculiarities of the Optimal Design of Girder Structures Subjected to Bending Loading

V. Apostolov
Shipbuilding Institute, Varna

The peculiarities of the optimal design of girder structures are shown in the paper using different criterial functions and constants. Conditions for the so called dual optimization problem are discussed.

JTAM, Sofia, vol. 24 Issue 2 pp. 122-127 (1993), [Full Article]

Elastic-hydrodynamic Lubrication of Dynamically Loaded Finite Journal Bearings

V. Alexandrov, At. Popov, J. Javorova
University of Architecture, Civil Engineering and Geodesy, Sofia

The paper presents a solution to the problem of EHD lubrication for dynamically loaded finite journal bearings under isothermal and isoviscous condition. The shaft is covered with a resilient layer and its distortions are taken into consideration. Besides the contribution of the shaft rotation to the pressure distribution, another term due to the motion of the shaft centre is included in the governing equation. The generalized Reynolds equation is solved by iterative techniques and the influence of a number of factors on the load parameter (Sommerfeld number) are illustrated and discussed.

JTAM, Sofia, vol. 24 Issue 2 pp. 128-134 (1993), [Full Article]

Identification of Rheology Parameters of Rubber Using Digital Speckle Pattern Interferometry

A. Baltov, V. Balavessov, I. Rusev, P. Kulev, R. Kazandjiev
Institute of Mechanics, Bulgarian Academy of Sciences, Sofia

Rheology of technical rubber is studied using the Method of Digital Speckle Pattern Interferometry. Thin circular rubber plates are subjected to bending by applying a concentrated load at the plate center. The load is kept constant, so that material creep develops. A class of rheology models are employed in order to describe the material mechanical behaviour. The material rheology parameters are identified. The comparison between experimental data and model calculations shows good agreement.

JTAM, Sofia, vol. 24 Issue 2 pp. 135-142 (1993), [Full Article]

Applying the Electroinductive Method for the Investigation of Brittle Fracture in Metals

P. Barzakov1, M. Albrzawi2
1Technical University, Sofia
2Damascus University, Damascus

The experimental potentialities of the electroinductive method for the investigation of fracture not only in low-carbon steels, but also in medium-carbon ones are shown; one specimen being used for the simultaneous determination of initiation force Pi and critical force Pc, of the crack. It is proved that the above forces are always valid, notwithstanding specimens, tested without or with a crack created through fatigue or statically. An opportunity has been established to find out an express method for determining a correlation between the parameters Pc and Kc of the crack propagation.

JTAM, Sofia, vol. 24 Issue 2 pp. 143-149 (1993), [Full Article]

A Fast Method for Prediction of Reliability and Fatigue-life

K. Staevski
University of Sofia

A fast method of analysis of the engineering design according to a fatigue failure criterion on the basis of the explicit dependence between the parameters of the service load spectrum and the expected life is developed. The influence of the approximating spectrum, different cumulative damage hypotheses and factors, determining the design optimization, based on the obtained estimates are investigated. The proposed method is tested on real constructions.

JTAM, Sofia, vol. 24 Issue 2 pp. 150-156 (1993), [Full Article]

An Approach to the Estimation of Fatigue Life Under Block and Random Loading

M. Andonova
Technical University, Sofia

A simple life-prediction procedure was developed considering the mean stress effects on the inelastic strain energy dissipated per cycle and damage parameter concept. The method uses basic fatigue properties of material, energy based mean stress model, the hysteresis energy as damage parameter and the Palmgren¬Miner cumulative damage rule. Fatigue tests were carried out under a Gaussian type block and random loading of varying mean level. Theoretical and experimental lives are found to exhibit good agreement. Comparison is made between other mean stress models and the proposed energy mean stress model, best representing the data.

JTAM, Sofia, vol. 24 Issue 2 pp. 157-164 (1993), [Full Article]