Journal of Theoretical and Applied Mechanics
Volume 38, Numbers 3, 2008
Contents
Clementina
D. Mladenova
Acad.
G. Bonchev Str., Bl.4,
e-mail: clem@imbm.bas.bg
Ivaïlo
M. Mladenov
Acad.
G. Bonchev Str., Bl. 21,
e-mail: mladenov@obzor.bio21.bas.bg
Spacecraft Dynamics
Under the Influence of Gravity Torques
Abstract. On the basis of the general
theory of kinematics and dynamics of a rigid body, this paper studies the
dynamics of a rigid spacecraft under the influence of the gravity torques. The
dynamics equations are obtained in a first-order form with a coefficient matrix
which allows its eigenvalues to be obtained in an analytical form. An
appropriate parameterization of SO(3) group is used, which has nice and clear
geometrical properties and it is quite efficient in computational aspect. An
example is given, which demonstrates the stability of the model.
Key words: rigid body, kinematics and
dynamics, parameterizations of rotation group, eigenvalue problem.
G. Rosenhouse, F.
Kirzhner, Y. Zimmels
Faculty of Civil and Environmental
Israel Institute of Technology,
e-mails: GioraRosenhouse@hotmail.com, fwamtech@bezeqint.net,
e-mail: kfelix@techunix.technion.ac.il
Soil replacement for isolation of structures against underground
vibrations
Abstract. Strong Soil vibrations can be
intruding and leading on the extreme to damages. They are a reason for defects
in delicate products of machinery and adverse effect on vibration sensitive
instruments. In order to avoid such results, vibration amplitudes should be
restricted to the limits specified in standards. Vibrations that exceed or are
expected to exceed standard limits call for protective measures. In this work
we consider design configurations that provide effective isolation against
vibrations and in certain cases reduced differential settlements, using ground
replacement by an elastic layer. Numerical examples using elastic layers, for
example of 0.5 m thickness, result in effective vibration isolation within wide
frequency range.
Key words:
vibrations, isolation, ground replacement, structures.
Al. B .Kazakoff
Acad.
G. Bonchev Str., Bl. 4, 1113
e-mail: alex_kazakoff@yahoo.co.uk
D. P. Karaivanov
8, Kliment Ohridski Blvd, 1756 Sofia, Bulgaria,
e-mail: dipekabg@yahoo.com
S. Troha
58,
Vukovarska Str.,
e-mail: stroha@riteh.hr
COMPUTER
ANALYSIS OF A THREE MASS CRANE DYNAMIC SYSTEM
Abstract. Vibration characteristics of a generalized model of a
robocrane – the model itself is an abstraction combination of a
transmanipulator and a robocrane, are investigated in a vertical base plane
with simultaneous linear travelling and lifting load motion. The investigation
is based on the developed three mass dynamic model of a robocrane considering
the variable load mass according to a certain time function. The system non-linear
differential equations of motion are analyzed in different case studies of
formation of reactive force with a definite crane control. The objective of the
work presented in this paper is a derivation of the system motion differential
equations, considering the accelerating and decelerating working modes and the
equilibrium at linear variation of the brake torque. The stoppage time is
defined at which the vibration amplitudes of the robocrane construction are
minimal and the necessary gradient of the variation of the brake torque as a
function of the system parameters is defined. Computer code MATLAB is used in
the dynamic system computer modelling.
Key words: robocrane vibrations, crane vibrations, crane
dynamics, ordinary differential equations of second order, computer modelling,
frequency analysis, brake torque characteristics.
N. Petrov
Acad.
G. Bonchev Str., Bl. 4, 1113 Sofia, Bulgaria,
e-mail: petrov333@gmail.com
A. Szekeres
Department of Applied Mechanics,
Muegyetem 1-3,
e-mail: szekeres@mm.bme.hu
NEW APPROACH TO
THE NON-CLASSICAL HEAT CONDUCTION
Abstract. The aim of the present study is to
offer a non-classical model able to solve the problem connected with the
paradox of the infinite speed of propagation of the thermal perturbation. The
principle assumption is that the momentary dependence between the entropy and
heat fluxes in Clausius–Duhem inequality does not take into account the
relaxation phenomena due to the micro-structural formation and degradation
processes. We reformulate this dependence as memory type. As a result a
hyperbolic partial differential equation for the caloric balance is obtained
instead of parabolic one, by which the above mention paradox is eliminated.
Also we obtain a new heat conduction equation which is a generalization of the
equations offered by Maxwell, and Green
and Lindsay.
Key words: non classical, heat,
conduction, claussius, duhem, thermodynamics, caloric, hyperbolic, parabolic.
Yongbin Zhang
e-mail: hammelrinau4@126.com
When should the Boundary
Lubrication Be Considered in a Hydrodynamic Contact?
Abstract. The mixed
lubrication with mixed boundary lubrication and conventional hydrodynamic
lubrication films in the whole contact in a one-dimensional problem is studied.
The boundary lubrication is formed by the physical adsorbing of the fluid
molecules to the contact surfaces. It is modelled by the flow factor approach
suggested recently. The conventional hydrodynamic lubrication is modelled by
the conventional fluid mechanics equation. Theoretical analysis is derived and
analytical solution is obtained for the studied contact for different critical
boundary lubrication film thickness, different surface patterns and different
boundary lubrication film thickness. The results show that the critical
boundary lubrication film thickness for the boundary lubrication should be
considered in the modelling of a mixed lubrication may practically be on the
0.001
scale. The model as well as the previous model for studying
the similar contact developed by the author indicates that 0.005 can often be regarded as the critical boundary lubrication
film thickness for the boundary lubrication to be considered in a practical
mixed lubrication modelling.
Key words: boundary lubrication, hydrodynamic contact, model.
Mohamed Elajrami, Mohamed Benguediab
Laboratory
of Material and Reactive Systems,
Department
of Mechanical Engineering,
Ben
M’Hidi City B.P 89 22000,
Sidi Bel Abbes,
e-mail: eladjrami_mohamed@yahoo.fr
Guillèn Ronald
Effects of various
drilling procedures and of the cold expansion on the fatigue life of rivet
holes
Abstract. The use of riveting as assembly technique,
especially in the aeronautical construction, requires the implementation of
several holes in aluminium alloy sheets, which leads to an inhomogeneous stress
and strain field distribution and to stress localization in the drilled zones
which will affect the fatigue life of 2024-T3 aluminium alloy.
In addition, the machining process used for
drilling can increase or decrease the fatigue life of materials. This paper
presents the results of an experimental study whose main objective is firstly
to determine which drilling procedure least affects the fatigue life and to
show the role of the residual stresses introduced by each process on the
fatigue behaviour of the material, and secondly to evaluate the effects of the
residual stress field caused by the cold expansion of the hole on the fatigue
life improving and on the crack initiation and propagation in the 2024-T3
aluminium alloy.
For that four drilling procedures are compared in
fatigue: directly drilling bit of the wanted diameter, drilling a small
diameter hole called a pilot hole in the rivet hole location prior to drilling
the final diameter using a reamer, a water jet and finally punching.
The hole quality is
compared in two parameters: conicity and surface quality, which are observed using a Scanning Electron
Microscopy (SEM).
X-ray diffraction is used to determine the
residual stress profile resulting of each drilling procedure and to measure the
residual stresses resulting of the cold expansion on the hole edge.
Key words: drilling
procedures, cold expansion, rivet holes, residual stresses, fatigue life.