Issue 2
JTAM, Sofia, vol. 31 Issue 2 (2001)

Frequencies of Vibration for a Circular Array of Masses Coupled Through Their Displacements and Velocities

J. N. Boyd, R. G. Hudepohl, P. N. Raychowdhury
Department of Mathematical Sciences, Virginia Commonwealth University, Richmond, VA 23284-2014, U.S.A.


In this paper, we obtain in closed form the natural frequencies of vibration for a symmetric arrangement of pairs of masses, m and M, on a fixed circle. Each mass is attached to its two nearest neighbours by identical, ideal springs which possess extended mass μ. The springs couple the motions of the vibrating masses through their displacements on the circle. The effect of attributing mass to the springs is to couple the motions of the masses through their velocities as well. We write the Lagrangian for the array and then transform the Lagrangian with a unitary transformation derived from the rotations of a regular polygon. The purpose of the transformation is to decouple the equations of motion to the maximum extent possible from the symmetry of the array. We consider in some detail the case for four pairs of interconnected masses and then present results for the general case with N pairs of masses. We have employed Mathematica to perform many of the straightforward but lengthy calculations.

JTAM, Sofia, vol. 31 Issue 2 pp. 01 (2001)

Rotating of the Satellite-Gyrostat Around an Axis Fixed in the Orbital Frame

P. V. Atanasova
Institute of Mechanics, Bulgarian Academy of Sciences,Acad. G. Bonchev Str. Bl. 4, 1113 Sofia, Bulgaria

This paper describes the state of a satellite-gyrostat rotating around an axis fixed in the orbital frame. It is assumed that the gyrostat is on a circular equatorial orbit and it is under the influence of gravitational and magnetic torques as well as time-constant internal and external ones.

JTAM, Sofia, vol. 31 Issue 2 pp. 02 (2001)
Ultrafiltration of Oil Emulsions by Pulsating Pressure in a Round Channel

N. D. Nikolov1, J. D. Nikolova1, L. P. Bojinov2
1University “Prof. Dr. Assen Zlatarov” – Bourgas, 1, Prof. Yakimov Str., 8010 Bourgas, Bulgaria
2Technical University – Varna, 1, Studentska Str., 9010 Varna, Bulgaria

A process of ultrafiltration with a continuous withdrawal of superconcentrate from the existing concentration zone at a round cross section of the channel at pulsating flow in laminar region is carried out. Acetatecellulose tubular membranes with inside diameter of 12,5 mm are used. An increased relative productivity is established versus the case of utilization without superconcentrate receptacle.

JTAM, Sofia, vol. 31 Issue 2 pp. 03 (2001)
Localization of Plastic Deformation and of Continuum Damage: Different Points of View

A. Baltov1, N. Bontcheva1, V. Kafka2
1Institute of Mechanics, Bulgarian academy of sciences, Acad. G. Bonchev Str., Bl. 4, 1113 Sofia, Bulgaria
2Institute of Theoretical and Applied Mechanics, Academy of Sciences of The Czech Republic, 76, Prosecka Str., 19000 Praha – 9, Czech Republic

A parallel description of two approaches to determining localization of plastic deformation is given. Approaches on macrolevel and on mesolevel are considered. The macroscopic approach is based on the rigid plastic model and on bifurcation of the process parameters. The method is applied to metals, taking into account influence of temperature, damage and porosity. The mesoscopic approach is based on a two-component elastic-plastic model. The material properties on mesolevel are derived from the microscopic observations and analyses. The criterion for localization bands initiation is given on mesolevel.

JTAM, Sofia, vol. 31 Issue 2 pp. 04 (2001)
Constitutive Modelling of the Creep Behaviour of Chalk

M. Datcheva
Institute of Mechanics, Bulgarian Academy of Sciences Acad. G. Bonchev Str., Bl. 4, 1113 Sofia, Bulgaria

Constitutive equations for chalk time dependent behaviour are determined from available experimental data by finding the failure surface and the evolution of irreversible stress work and using the procedure proposed by Cristescu [5] In the present paper the transient creep is only studied, since in the time intervals considered and experimentally investigated it was the only one exhibited by the chalk. The different failure properties under compression and extension are taken into account. The loading function determination general procedure is adopted for conventional tests data.

JTAM, Sofia, vol. 31 Issue 2 pp. 05 (2001)
A Combined Critical Plane and Strain-Based Approach with Two Families of Microcracks to Multiaxial Low Cycle Fatigue Damage

Al. Zolochevsky, Y. Obataya, T. Itoh
Fukui University, Department of Mechanical Engineering, Faculty of Engineering, 9-1, Bunkyo 3-chome, Fukui, 910-8507, Japan

This paper proposes a combined equivalent strain and critical plane concept for multiaxial low cycle fatigue damage that considers different physical mechanisms for a shear and tensile types of failure. In general, each potential failure plane (critical plane) contains two families of parallel microcracks. The expression for the equivalent strain in the damage growth equation includes three material parameters as well as the strain intensity for the amplitudes, and joint invariants for the strain amplitudes and for the two unit vectors associated with the directions of microcracks. The determination of material parameters required in the proposed approach, from a series of basic experiments, is shown. Theoretical predictions of multiaxial low cycle fatigue lives are compared with those obtained from fatigue tests using cruciform specimens.

JTAM, Sofia, vol. 31 Issue 2 pp. 06 (2001)
Mathematical Models of Thin Interlayer between Contacting Solids

B. Kouyumdjiev1, I. Ivanov1, A. Baltov2
1“An. Kanchev” University of Rousse, 8, Studentska Str., 7017 Rousse, Bulgaria
2Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 4, 1113 Sofia, Bulgaria

A number of works are dedicated to determination of the stress-strain state of solids interacting by sliding friction in the presence of thin layers formed on them. During modelling a linear distribution of displacements along the layer thickness is usually assumed. A method of assessing applicability of this linear model as well as of a quadratic model is proposed in the present work. The results show the influence on the models errors of two parameters: the relative thickness of the layer and the ratio of mechanical characteristics of layer and solid. They allow a better-grounded choice of one of the models when developing new specialized finite elements for solving the above problems.

JTAM, Sofia, vol. 31 Issue 2 pp. 07 (2001)
Minimum Energy Principles and Generalized Variational Principles of Nonlinear Piezoelectricity

Ji-Huan He
Shanghai Institute of Applied Mathematics and Mechanics, Shanghai University, Shanghai 200072, P.R. China

Variational theory is the base of the finite element methods. By the semi-inverse method proposed by He, Hu-Washizu-like principle and Hellinger-Reissner-like principle of nonlinear piezoelectricity are derived from the field equations and boundary conditions without using the Lagrange multiplier method. By constraining the obtained generalized functionals, two minimal potential functionals are obtained, which reduce to the minimum potential energy principle and the minimum complementary energy principle of elasticity when the effect of electricity is ignored.

JTAM, Sofia, vol. 31 Issue 2 pp. 08 (2001)