Issue 4

JTAM, Sofia, vol. 34 Issue 4 (2004)

Investigation of the Flow of Dilute Disperse Systems Based on Epoxy Resin by Particle Image Velocimetry

V. Kavardzhikov, D. Pashkouleva, Ya. Ivanov
Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 4, 1113 Sofia, Bulgaria

The addition of short fibers to polymeric liquid systems is an effective approach to improving composite materials mechanical properties. That is why the study of disperse systems flows generates profound interest in rheology. In this work, for the first time, the flows of dilute disperse systems consisting of epoxy resin (disperse media) and glass fibers (disperse phase), which demonstrate Newtonian rheological behaviour, are studied by Particle Image Velocimetry (PIV). The velocity fields of fully developed flows in a tube with Reynolds number Re = 12.6×10-6 are measured with a spatial resolution of 100 μm. This allows deviations of investigated suspensions velocity profiles of theoretically obtained parabolic curves to be revealed and measured both at the tube wall and inside the flow.

JTAM, Sofia, vol. 34 Issue 4 pp. 01 (2004)

Interphase Model for a Multilayer Structures Applied in Heat Conduction Problems

J. Ivanova1, V. Valeva1, Z. Mroz2
1Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. Bl.4, 1113 Sofia, Bulgaria
2Institute of Fundamental Technological Research, Polish Academy of Sciences, 21, Swietokrzyska Str., 00-49 Warsaw, Poland

In the recent years the problem of studying the behaviour of advanced functionally graded materials (FGMs) subjected to high temperature, high temperature gradient and cyclic change of temperature has been investigated by many authors. In fact, FGMs are advanced high temperature materials capable of withstanding the extreme temperature environment, and their optimally composition profile allows for the reduction of thermal stresses in the structural layers along layer thickness. The interphase layer, involved in numerous applications of FGMs, has been an object of interest of numerous researches in the last decades. In fact the interphase layer (as a joint) can be regarded as a very thin physical layer, disposed between two layers with different thermal, geometric and material characteristics. The occurrence of cracks, damage, delamination alomg the interphase layer affects significantly the thermal and thermoelastic response of the whole structure. However in more cases the joint response depends also on internal stresses and strains within the interphase layer adjacent to joint interface. The usual assumption in thin-layer interface elements, that the response is governed by contact stress components may require a correction by introducing the effect of internal stresses into analysis. In the present paper we shall discuss the heat conduction response for the interphase layer embedded between two bodies, where the temperature discontinuity interphase may propagate on both sides of the layer and a crack or delamination may occur caused by the thermal stresses. We shall focus our attention to the heat transient problem of multilayer media employing the term interphase as a layer separated by two interfaces from neighbouring layers with varying properties and several interfaces. In section 2 the Biot variational principle, analogous to the principle of virtual work in elasticity will be applied. The basic assumptions are the linear and quadratic distribution of temperature within the interphase layers, as well as the perfect contact between the interphase layer and two bodies. The model is formulated in a general manner providing kinematics and equilibrium equations for the interphase layer.

JTAM, Sofia, vol. 34 Issue 4 pp. 02 (2004)

Modelling of Dynamic Plastic Strain, Water Pressure and Temperature Localizations in Unsaturated Soils

A. Baltov1, R. Charlier2, M. Datcheva1, F. Collin2
1Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 4, 1113-Sofia, Bulgaria
2Institute of Mechanics and Civil Engineering, University of Liege, B-4000 Liege, Belgium

The necessary condition for plastic strain, water pressure and temperature localization in unsaturated soils is proposed. The thermo-hydro-mechanical coupled model for unsaturated soils under dynamic loading is applied. The condition for the Adiabatic Shear Band (ASB) initiation is built on the base of stability conception and measure rate bifurcations.

JTAM, Sofia, vol. 34 Issue 4 pp. 03 (2004)

On the Uniqueness of the Surface Waves in Linear Viscoelasticity

Ts. P. Ivanov1, R. Savova2
1Faculty of Mathematics and Informatics, Sofia University St. Kl. Ohridski, 5, James Bourchier Blvd., 1164 Sofia, Bulgaria
2Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl.4, 1113 Sofia, Bulgaria

Propagation of surface waves of Rayleigh type on a viscoelastic half-space is considered. It is shown that there always exist a unique viscoelastic surface wave of an assigned frequency and a unique viscoelastic surface wave of an assigned wavelength, which satisfy the adopted criteria for behaviour at infinity. Each of these waves is interpreted as a superposition of two dispersive inhomogeneous plane waves. For each of them the plane of constant phase and the plane of constant amplitude are not orthogonal. The superposed waves have different directions of propagation, different phase velocities and attenuation coefficient values. Their directions of propagation are not parallel to the stress-free surface. Numerical computations referred to some typical values of the material constants and to some real materials are presented for both cases of surface waves.

JTAM, Sofia, vol. 34 Issue 4 pp. 04 (2004)

Method of Planes of Centers: Modelling Discrete Shaft Systems Containing Differential Gear

Petar K. Kolev
Higher School of Transport ”Todor Kableshkov”, 154, Geo Milev Str., 1574 Sofia, Bulgaria

The paper presents a method of branched shaft system with differential gear modelling, which is used to achieve separation of the differential equations describing the macro-dynamics processes without losing the interaction among them. Some new concepts have been introduced.

JTAM, Sofia, vol. 34 Issue 4 pp. 05 (2004)

An Experimental Estimation of Cast Iron Material Models

G. B. Stoychev1, P. V. Petrov1, K. Tushtev2
1Technical University of Sofia, 8, Kl. Ohridski Blvd., 1756 Sofia, Bulgaria
2Keramische Werkstoffe und Bauteile, University of Bremen, Am Biologischen Garten 2 / IW3, D-28359 Bremen, Germany

Elastoplastic deformation of grey cast iron is studied. Two-dimensional FE-analyses of axisymmetric model are performed in order to be compared with experimentally obtained results for thin-walled tube subjected to various stress states. The cast iron plasticity models, implemented into a commercial finite element code ANSYS and ABAQUS, are subjected to estimation by using experimental data obtained by bi-axial tests on tubular specimens. The plastic behaviour of the material is assumed to harden isotropically, and the models are restricted to monotonic loading only. On the other hand, the tubular specimens are subjected to monotonic proportional loading in tension, compression and bi-axial tests with ratio k=1; 0,5; -1 and -2 between the two principal stresses. The finite element model corresponds absolutely to the experimental specimen and gives the possibility of correct estimating the material models. One can trace out the subsequence of steps in finite element investigation of details made of grey cast iron. Experimental data needed for implementation in FE-code ANSYS and ABAQUS are given.

JTAM, Sofia, vol. 34 Issue 4 pp. 06 (2004)

Heat and Moisture Transport through Porous Material under Equilibrium Conditions

A. K. Haghi
The University of Guilan, P. O. Box 3756, Rasht, Iran

This article surveys the heat and moisture transfer characteristics of fabric systems under equilibrium conditions. The results are given for absorptive and non-absorptive systems measured under conditions in which no excess moisture secretion or recondensation takes place. It is shown that with an absorptive textile the after-exercise chill is large and persists for few hours but with a non-absorptive type it is negligible.

JTAM, Sofia, vol. 34 Issue 4 pp. 07 (2004)