Journal of Theoretical and Applied Mechanics

Volume 37, Number 1, 2007



St. Bachvarov


Technical University of Sofia,

8, Kliment Ohridski Str., Sofia 1000, Bulgaria


V. Zlatanov


University of Food Technologies – Plovdiv,

26, Maritsa Av., Plovdiv 4000, Bulgaria,



110th Birthday Anniversary of Prof. Arkadiy Stoyanov


On Prof. Arkadiy Stoyanov’s Contribution to the Kinematics of the Ideal Rigid Body


Abstract. Short biographical data of Prof. Arkadiy Stoyanov were given. Its major contributions in the field of Kinematics of the most general movement of a rigid body based on Prof. Arkadiy Stoyanov’s projective theory were presented.


Key words: kinematics, the most general movement of a rigid body, angular velocity, three points method.


S. P. Radev


Institute of Mechanics, Bulgarian Academy of Sciences,

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



L. Tadrist, F. Onofri


PolytechMarseille- IUSTI-UMR CNRS no 6595, Technopole de Chateau Gombert,

University of Provence, 5, rue Enrico Fermi, Marseille 13453, France,




Sinuous Instability of a Viscous Capillary jet into an immiscible non-viscous fluid.

Part I: 3d-equations of motion of a slender jet in respect to the moving trihedron of the jet axis


Abstract. The sinuous instability of a liquid capillary jet is studied by using the full Navier-Stokes equations. A linear instability analysis is performed in two steps. In the present (first) part of the paper the equations of motion and corresponding interface boundary conditions are written in respect to the moving trihedron of the jet axis. The resulting equations describe the general case of a simultaneous propagation of axisymmetrical and sinuous disturbances.

            In the second part of the paper these equations are applied to the analysis of the sinuous instability only. For the latter a dispersion equation is derived involving both jet viscosity and ambient fluid density. In some particular cases including non-viscous jet the dispersion equation is analysed both analytically and numerically.


Key words: capillary jet, sinuous instability.


B. E. Djakov


Institute of Electronics, Bulgarian Academy of Sciences,

72, Tsarigradsko Chaussee, BG-1784 Sofia, Bulgaria,



L. M. Shpanin, J. W. Spencer, G. R. Jones


Centre for Intelligent Monitoring Systems,

Dept. of Electrical Engineering and Electronics, University of Liverpool,

Brownlow Hill, Liverpool L69 3GJ, UK




Abstract. A new form of electromagnetic plasma propulsion has been observed in experiments with high current electric arcs interacting with a magnetic field and a solid wall of particular configuration. It is shown that the post arc plasma acceleration is likely to be due to the excessive pressure at the wall that during earlier stages of arcing counteracts the Ampere force. An attempt is made to explain formation and motion of the plasma cloud taking into account wall ablation on the basis of a self-similar Sedov’s model.


Key words: electric arc, electromagnetic interaction, ablation, gas dynamics, self-similarity.


R. Savova


Institute of Mechanics, Bulgarian Academy of Sciences,

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



Ts. P. Ivanov


Faculty of Mathematics and Informatics,

Sofia University St. Kl. Ohridski,

5, James Bourchier Blvd, 1164 Sofia, Bulgaria,



On the surface wave behaviour at infinity


Abstract. In the present paper we consider surface waves of an assigned frequency on a thermoviscoelastic half-space when appropriate criteria for behaviour at infinity are adopted. The detailed examination shows that a unique quasi-viscoelastic wave (surface wave of Rayleigh type) of an assigned frequency always exists. A unique quasi-thermal surface wave of an assigned frequency exists only in the case of small and middle values of the frequency, while for large values such a wave does not exist. It is also proved that for middle and large values of the frequency an additional wave appears. This wave satisfies only first two of the adopted four criteria for behaviour at infinity. It is quite different from the classical Rayleigh wave and we do not consider it as a surface wave of Rayleigh type.


Key words: thermoviscoelasticity, surface waves, frequency, wavelength.


S. Parvanova, G. Gospodinov


University of Architecture, Civil Engineering and Geodesy,

1, Christo Smirnenski Blvd, 1046 Sofia, Bulgaria,



A dual BOUNDARY ELEMENT procedure for

analysis of fracture in concrete


Abstract. The present paper is concerned with the numerical implementation of the Dual Boundary Element Method (DBEM) in the Linear Elastic Fracture Mechanics (LEFM) context for crack propagation in plain concrete. The main advantage of the DBEM is that mixed-mode crack problems can be solved successfully with a single region formulation, thus avoiding the ineffective sub-region approach. The method is combined with a previously developed Two-Step Subtraction Singularity Technique (TSSST) for evaluation of Stress Intensity Factors (SIFs), which is generalized here for the case of multiple cracks. The crack growth condition requires a fracture criterion, which forms a simple numerical procedure along with the DBEM and TSSST, capable of treating the general problem of mixed-mode fracture. Examples of deep, shear-critical beams are numerically solved and analyzed with the present approach, and highly accurate results are obtained. The accuracy and efficiency of the implementation, described herein make this formulation very suitable to study the crack growth problems in plain concrete under a general Mode I/II conditions.


Key words: boundary elements, fracture mechanics, plain concrete.


Anton Mitsev, Christo Betchev


University of Chemical Technology and Metallurgy,

8, St. Kl. Ohridski Blvd, 1756 Sofia, Bulgaria,



Rene Androsch, Hans-Joachim Radusch


Department of Engineering Sciences,

Martin Luther University Halle-Wittenberg, D-06099 Halle, Germany,






Abstract. Influence of a novel modifier based on polyamide/polycarbonate copolymer on the structure and properties of multicomponent systems consisting of PolyAmide 6 (PA 6), PolyAmide 66 (PA 66), glass fibres and Ethene-1-Butene-maleic anhydride Copolymer (EBC) has been investigated. Ternary PA 6/PA 66/elastomer blends show higher impact strength than binary PA 6/elastomer or PA 66/elastomer blends, caused by different phase-in-phase structures obtained by the different functionality of the polyamides. The stiffness of these systems is usually decreased.

The further improvement of the stiffness and the toughness of these ternary blends was obtained by adding glass fibres and by optimising the compatibility between PA 6 and PA 66 using new coupling agents. The experimental results obtained indicate an improved stiffness and toughness by adding up to 5% of the coupling agent. Conclusions have been drawn that the compatibiliser may promote specific interactions between the different components of the system.


Key words: polyamides, glass fibres, mechanical properties, toughness, stiffness, process-ability.