Journal of Theoretical and Applied Mechanics

Volume 40, Number 2, 2010

 

Contents

 

E.Abadjieva, G.Boiadjiev, D.Petrova, V.Abadjiev

 

Institute of Mechanics, Bulgarian Academy of Sciences,

Acad. G. Bonchev St., Bl. 4, 1113 Sofia, Bulgaria,

e-mails: abadjiev@imbm.bas.bg, george@imbm.bas.bg, dochka@imbm.bas.bg

 

ON THE MESH REGION SYNTHESIS OF SPATIAL RACK DRIVES WITH ROTATING LINK HAVING CYLINDRICAL LINEAR HELICOIDS

 

Abstract. The paper deals with the basic geometric-kinematical characteristics of the process that transforms a rotation into a translation. The object of the research is a three-link spatial rack mechanism realizing this transformation by means of linearly contacting high kinematic joints.When geometric elements of the joints firmly connected with the rotating link are linear helicoids, a surface of action/mesh region is synthesized based on a worked-out mathematical model. The geometric elements connected with the link, transforming a translation, are kinematic envelops of the helicoids. The surface of action is obtained analytically and it is studied, and visualized for concrete spatial rack drives.

 

Key words: rack drive, region of mesh, helicoids, mathematical model, synthesis.

 

V. A. Chirikov, V. D. Georgiev

 

Technical University - Varna, Studentska St. 1, 9010 Varna, Bulgaria,

e-mails: chirikovva@yahoo.com, vg5076@gmail.com

 

APPLICATION OF EXACT GOVERNING DIFFERENTIAL EQUATION OF MOTION OF 6NVD-18 SHIP ENGINE FOR DETERMINATION OF ITS DYNAMIC BEHAVIOUR

 

Abstract. The general differential equation of motion for the mechanical part of an arbitrary combustion engine was presented In the previous paper [4]. The equation was derived by means of the offered direct method with respect to the crankshaft angle. In the present paper the governing equation of motion for 6NVD-18 four-stroke ship engine has been obtained. The engine is used in marine and river vessels as a main engine or as an auxiliary diesel-generator. The obtained governing equation of motion contains full kinematical and dynamical information on the closed kinematical loops of crankgears assembling the mechanical part of the engine. The general type of governing equation for a crankshaft of a combustion engine is confirmed by this example. A numerical solution of the equation utilizing the indicator pressure reference diagram of the engine has been used to determine the crankshaft rotation irregularity. The results of the calculation have been compared to the data obtained from the numerous measurements and tests carried out and studied in courses of Marine Engineering majors at TU-Varna.

 

Key words: combustion engine, dynamics, governing equation.

 

P. Gospodinov, D. Dankov, V. Roussinov, S. Stefanov

 

Institute of Mechanics, Bulgarian Academy of Science,

Acad. G. Bonchev St. Bl.4, 1113 Sofia,

e-mails: png@imbm.bas.bg, dankov@imbm.bas.bg,

vladimir@imbm.bas.bg, stefanov@imbm.bas.bg

 

Modeling of cylindrical Couette flow of rarefied gas. The case of rotating inner cylinder

 

Abstract. The cylindrical Couette flow of a rarefied gas is studied in the case when the inner cylinder is rotating while the outer cylinder is at rest. Velocity, density and temperature profiles are investigated by a Direct Monte Carlo Simulation method and a numerical solution of the Navier-Stokes equations for compressible flow is found. The results obtained by both methods are: in an excellent agreement at a small Knudsen number Kn=0.02; in a satisfactory agreement at Kn=0.1 and they vastly differs each other at a moderate Kn=0.5. The comparison shows that the continuum approach can be used successfully for calculations of non-isothermal rarefied gas flows at small Knudsen numbers Kn<0.1. These results are important for applications in non-planar microfluidic problems.

 

Key words: Kinetic theory, Rarefied gas, Microfluidics, DSMC.

 

Ludmila Parashkevova, Nikolina. Bontcheva

 

Institute of Mechanics, Bulgarian Academy of Sciences,

Acad. G. Bonchev St., Bl. 4, 1113 Sofia, Bulgaria,

e-mails: lusy@imbm.bas.bg, boncheva@imbm.bas.bg

 

MODELLING OF MULTIPHASE “IN SITU” COMPOSITE

 

Abstract. The elastic-plastic behaviour of rapidly solidified Al based (FeSi) -enriched alloys is considered. The material due to fast cooling the material can be regarded as “natural” (in situ) composite, containing intermetallic and non-intermetallic compounds of different shapes, mechanical properties and volume fractions. A new multilevel mechanical model for the "in situ" composite is proposed considering the aluminium matrix as a micropolar elastic plastic Cosserat material and the hardening phases as pure elastic ones. A two steps homogenization procedure is applied to obtain the overall properties of multiphase “in situ” composite. A variational approach is applied to evaluate the equivalent stress on macro level at the transition from micro to macro scale. The model is developed using information provided by microstructural investigations and EDX analysis. The multistage modelling of bulk material manufacturing process is simulated using the Finite Element Method (FEM). The model is implemented as user defined subroutine into the (FE) code MARC. The influence of the microstructural size parameters on the hardening behavior is discussed.

 

Key words: multiphase composites, multiscale modeling, rapidly solidified Al alloys.

 

E. E. Gdoutos

 

School of Engineering, Democritus University of Thrace,

GR-671 00 Xanthi, Greece,

e-mail: egdoutos@civil.duth.gr

 

M. Daniel

 

Departments of Civil and Mechanical Engineering,

Northwestern University, Evanston, IL 60208, USA

 

FAILURE OF SANDWICH STRUCTURES

 

Abstract. The failure behavior of composite sandwich beams subjected to a three- and four-point bending was studied. The beams were made of unidirectional carbon/epoxy facings and various core materials including PVC closed-cell foams, a polyurethane foam and an aluminum honeycomb. Various failure modes including facing wrinkling, indentation failure and core failure were observed and compared with analytical predictions. It was established that the initiation, propagation and interaction of failure modes depend on the type of loading, constituent material properties and geometrical dimensions.

 

Key words: Sandwich structures, facing wrinkling, indentation failure, core failure, closed-cell foams

 

Ana Yanakieva

 

Institute of Mechanics, Bulgarian Academy of Sciences,

Acad. G. Bonchev St., Bl. 4, 1113 Sofia, Bulgaria,

e-mail: aniyanakieva@imbm.bas.bg

 

Konstantin Kazakov

 

Structural Mechanics Department, Higher School of Civil Engineering,

175, Suhodolska St., 1373 Sofia, Bulgaria,

e-mail: kazakov@vsu.bg

 

A DYNAMIC COMPUTATIONAL MODEL OF CYCLIC LOADING OF STEEL BAR IN INELASTIC UNCONFINED CONCRETE SOLID

 

Abstract. This paper is devoted to the generation of a dynamic computational model of a cyclic loading of a steel bar in unconfined inelastic concrete solid. The contact between the steel bar and the concrete is modelled by 3D contact finite elements. Such elements overlaid the two surfaces. The steel bar is subjected to reversed, to tension and compression, cyclic axial force, typical case for seismic loading. The computational results are given in hysteretic-like form, concerning the bond stress-ultimate displacement and bond stress-strain relations. The reduction of the contact capacity is demonstrated and discussed as well.

 

Key words: FEM dynamic computational models, contact finite elements, cyclic reversed loading, hysteretic relation, capacity force, bond stress-slip relation.

 

B. Dimova, K. Vesselinov

 

Technical University-Sofia,

8, St. Kl. Ohridski Blvd, 1000 Sofia Bulgaria,

e-mails: bdimova@tu-sofia.bg, vesselinov@tu-sofia.bg

 

An energy-based approach for predicting fatigue life of magnesium alloys

 

             Abstract. A modified energy-based approach for predicting fatigue life of magnesium alloys is described. The approach is based on cyclic plastic strain energy density and takes into account the dynamics of the fatigue process, including the number of cycles to failure and total hysteresis energy. The modified coefficients in the proposed approach have been obtained from experimental tests under symmetrical loading. An experimental verification has been performed on smooth specimens for magnesium alloys AZ91, AM50 and AE42 subjected to cyclic tension/compression. A predictive capability of the approach is found to be in good agreement with the experimental results for the three types of Mg-alloys.

 

Key words: fatigue, fatigue life prediction, hysteresis energy, Mg-alloys.

 

R.Kazandjiev, P.Gospodinov

Institute of Mechanics, Bulgrian Academy of Sciences,

Acad. G.Bonchev St., 1113 Sofia, Bulgaria,

e-mails: Robert@imbm.bas.bg, png@imbm.bas.bg

 

M.Mironova

Central Laboratory of Physico-Chemical Mechanics, BAS

Acad. G.Bonchev St., Bl. 1, 1113 Sofia, Bulgaria,

e-mail: mirona@clphchm.bas.bg

 

Change of the pore structure of thin cement stone specimens subjected to sulfate attack

 

Abstract. The study proposes an assessment of pores structure modification of cement stone (hardened cement paste) as a result of sulfate attack. Thin cement stone specimens being immersed and kept in water-sulfate solution are considered and material porosity change is followed. The problem of diffusion of sulfate ions into the material is numerically solved. The duration of pore closure due to pore filling with ions and products of the chemical reactions is assessed. Good agreement with experimental evidence is found.

 

Key words: Cement stone, sulfate corrosion, diffusion, porosity

 

Y. Ivanova, M. Mihovski,

Institute of Mechanics, Bulgarian Academy of Sciences,

Acad. G. Bonchev St., Bl. 4, 1113 Sofia, Bulgaria,

e-mails: yonka@imbm.bas.bg, nntdd@imbm.bas.bg

 

T. Partalin

Faculty of Mathematics and Informatics, Sofia University “St .Kl. Ohridski”,

5, James Baucher Blvd, Sofia, Bulgaria,

e-mail: topart@fmi.uni-sofia.bg

 

EXPERIMENTAL STUDY OF THE ULTRASONIC WAVE PROPAGATION IN MATERIALS WITH MECHANICAL STRESSES

 

Abstract. The present paper concerns acoustoelasticity in stressed and elastically deformed materials during the bending loads. The dependence of ultrasonic wave parameters on the stress-state of materials is studied. An experimental evaluation of the velocity and the change of surface ultrasonic pulse as a function of bending loads is described.

 

Key words: ultrasonic surface wave, velocity, mechanical stress.

 

V. Kavardzhikov, D. Pashkouleva, G. Stoilov

Institute of Mechanics, Bulgarian Academy of Sciences,

Acad. G. Bonchev St., Bl. 4, Sofia1113, Bulgaria,

e-mails: kavarj@imbm.bas.bg, dessip@imbm.bas.bg, gstoilov@imbm.bas.bg

 

A Biaxial tensile test for meshes, fabrics and tissues

 

Abstract. A method for biaxial tensile testing of meshes, loaded at two orthogonal directions through a mechanical device, designed by authors and working together with a standard testing machine, is presented in this paper The mesh strain fields are revealed at different moments of biaxial loading process by a correlation analysis of deformed images related to respective F (ε) dependences, registered at orthogonal tensile directions.

A mesh, composed of simply cross, equidistant polyvinyl-chloride fibres, oriented at two orthogonal directions is studied to examine the method. Materials exhibiting large deformations (meshes knitted on different ways and intended for different purposes, as well as fabrics, tissues and other soft materials) can be examined using biaxial tensile tests by means of presented method and device.

 

Key words: biaxial tensile testing, cruciform PVC-mesh specimens, full field experimental technique.

 

A.Shulev*, I.Roussev, T. Tiankov, R. Oranski, D. Ignatova, K.Kostadinov

 

Institute of Mechanics – Bulgarian Academy of Sciences,

Bl. 4, Akad. G. Bonchev St., Sofia 1113, Bulgaria

Corresponding author e-mail: assen@imbm.bas.bg

 

VISION CONTROL OF A HYBRID ROBOTIC SYSTEM FOR CELL INJECTION

 

Abstract. In this contribution, an approach for two-dimensional vision control of the cell injection process realized by a hybrid-robotic system is presented. The hybrid system is combined of one large range robot and one low range manipulator with high-precision. A glass pipette mounted on the high-precision robot realizes cells penetration and injection of different substances. Suitable optical system has been designed to provide high-resolution imaging of the injection pipette over the working area defined by the cell holder dimensions. Numerical algorithms for pipette point detection, auto focusing and tracking during the working process are developed. The sub-pixel accuracy of these algorithms and high precision linear measuring system integrated in the large range robot allow precise calibration of the image space. In this way, the visual feedback controls the whole robot system and the pipette position with respect to the target cell. Once the cell’s position is detected and defined in the image space, the injection process could be completely automated.

 

Key words: 2-D vision control; hybrid-robotic system; biological cells injection; sub-pixel accuracy.

 

I.Jordanov

 

Institute of Mechanics, Bulgarian Academy of Sciences,

Acad. G.Bonchev St., Bl.4, 1113 Sofia, Bulgaria

e-mail: ivan@imbm.bas.bg

 

COUPLED KINK POPULATION WAVES

 

Abstract. We discuss the evolution of the density of members of two interacting populations that can migrate in the space. The migration can happen as nonlinear kink waves for the particular case of constant growth rates and competition coefficients. The kinks are coupled, i.e. the changes of the densities of the two populations are synchronous.

 

Key words: nonlinear dynamics, population dynamics, migration, solitary waves, coupled kinks.