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BULGARIAN ACADEMY OF SCIENCES NATIONAL COMMITTEE OF THEORETICAL AND APPLIED MECHANICS Journal of Theoretical and Applied Mechanics
Print ISSN: 0861-6663 Online ISSN: 1314-8710
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JTAM, Sofia, vol. 50 Issue 2 (2020) |
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TRIBOLOGICAL INVESTIGATION OF PLA-BASED NANOCOMPOSITES BY SCRATCH AND WEAR EXPERIMENTS Todor Batakliev OLEM, Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, Sofia 1113, Bulgaria
Nanocomposites of multiwall carbon nanotubes (MWCNTs) and
graphene nanoplatelets (GNPs) incorporated in polylactic acid were studied.
Characterization and understanding of tribological behavior requires the
exact definition of the scratch and wear conditions and look on the
morphological features of the composite materials. The materials have been
prepared by melt extrusion technique as a first step of the process. The
received pellets having nanofiller loading up to 12 wt% were used to
produce filaments suitable for 3D-printing. Scratch tests were performed on
extremely smooth surface of 3D-printed samples applying constant normal
force of 2N with micro-cutting diamond blade. The scratch resistance of
12 wt% GNP/PLA nanocomposite was found to be twice higher than the neat
PLA used as reference.
JTAM, Sofia, vol. 50 Issue 2 pp. 105-113 (2020), [Full Article]
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DYNAMICS OF CHAINS CONTAINING BI-STABLE ELASTIC ELEMENT AS STABLE NEGATIVE STIFFNESS Akintoye Olumide Oyelade Department of Civil and Environmental Engineering, University of Lagos, Akoka, Nigeria
The instability of negative stiffness materials or
systems is rooted in their thermodynamic state, which is not in equilibrium
with its environment. In this paper, a model of a chain of masses joined by
springs with a non-monotone dependence of the spring force versus spring
displacement (negative stiffness) relation is made stable by constraining it
in positive matrix. The stability of the system is tested using energy
function. Numerical experiments are conducted to test the dynamics of two
mass model system under small external excitation. The numerical results are
validated for a range of parameters by comparing the predictions with
calculations from analytical approximation using Newton Harmonic Balance
(NHB) method. The presented results highlight prospects in the design of
mechanical metamaterials based on negative-stiffness elements.
JTAM, Sofia, vol. 50 Issue 2 pp. 114-129 (2020), [Full Article]
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FREQUENCY~CHARACTERISTICS OF SEISMIC PIEZOELECTRIC SENSORS UNDER ONE-DIMENSIONAL MECHANICAL ACTION Nikolay Georgiev1, Yavor Boichev1, Konstantina Belotelova2, Ivan Ivanov1 1Institute of Metal Science, Equipment and Technologies with Hydro- and Aerodynamics Centre "Acad. A. Balevski", Bulgarian Academy of Sciences (IMSETHC-BAS), Sofia, Bulgaria 2R-sensors, Moskow, Russia
In this study, a mathematical model that describes the
frequency response of piezoelectric sensors under one-dimensional mechanical
stress is presented. In laboratory conditions, experimental studies of
sensors designed and manufactured by IMSTCHA-BAS and NSF were conducted.
These studies confirm the adequacy of the presented mathematical model.
JTAM, Sofia, vol. 50 Issue 2 pp. 130-141 (2020), [Full Article]
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HUMAN-INDUCED VIBRATIONS ON FOOTBRIDGES. CURRENT CODES OF PRACTICE – OVERVIEW Krumka Kasapova, Dobromir Dinev University of Architecture, Civil Engineering and Geodesy; 1, Smirnenski Blvd., 1046, Sofia, Bulgaria
Considering the trend in the last decades of using lightweight materials and, in some sense extraordinary structural configurations, structures have become more and more slender and therefore more susceptible to dynamic excitation. This holds especially for long-span and slender footbridges, since such structures commonly have natural frequencies that fall within the respective frequency range of human excitation.
JTAM, Sofia, vol. 50 Issue 2 pp. 142-157 (2020), [Full Article]
The present paper regards in details the procedures of the current codes of practice. It may serve as a background for subsequent assessment of the dynamic behaviour of footbridges. |
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SYNTHESIS OF CAM MECHANISMS WITH INPUT TRANSLATION, FLAT AND PROFILED FOLLOWER Blagoyka Ilieva Paleva-Kadiyska Department Mechanical Engineering and Technologies, Faculty of Engineering, Neofit Rilski South-West University, Blagoevgrad 2700, Bulgaria
A mathematical model is derived for synthesis of
CAM mechanisms with input translation, flat and profiled follower.
Considering the pressure angle the basic parameters of the mechanism are
determined. Several equations are derived: of the curve of centers of the
processing tools and the pitch curves of the roll, the CAM profiles, and
their common evolute. An example is given illustrating the
application of the mathematical model under three different possible
synthesis requirements.
JTAM, Sofia, vol. 50 Issue 2 pp. 158-165 (2020), [Full Article]
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EFFECTS OF THE VARIATIONS IN THE CUMULATIVE LINER WALL THICKNESS ON THE PARAMETERS OF CUMULATIVE JET FORMATION PROCESS Stancho P. Petkov, Petko S. Petkov, Adelina D. Tumbarska Acad. Angel Balevski Institute of Metal Science, Equipment and Technologies with Hydro- and Aerodynamics Centre, Bulgarian Academy of Sciences, 67, Shipchenski prohod St., 1574 Sofia, Bulgaria
A review of the impact of various geometric,
technological, and other imperfections in the shaped charges design on the
parameters and characteristics of the cumulative jet formation process is
made. A physical-mathematical model is proposed, on the basis of which
numerical simulation is performed, allowing quantifying the influence of
variations in the cumulative liner wall thickness on the deviation from the
symmetry of the jet. The results of the calculations are presented in a
graphical form, allowing for their use in further theoretical studies of the
shaped charges functioning as well as in the design of ammunitions with
predefined cumulative jet parameters.
JTAM, Sofia, vol. 50 Issue 2 pp. 166-175 (2020), [Full Article]
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ESTIMATION OF THE PORES AGGLOMERATION EFFECT ON THE COMPRESSIVE BEHAVIOR OF METALLIC FOAMS: INFILTRATION AND POWDER METALLURGY STUDY CASES Juan C. Carranza1, Berhta Y. Casas1, Ismeli Alfonso1, Luis Pérez2, Robin A.L. Drew3, Jorge A. Verduzco4, Ignacio A. Figueroa5 1Instituto de Investigaciones en Materiales, Unidad Morelia, Universidad Nacional Autónoma de México, Campus Morelia UNAM, Antigua Carretera a Pátzcuaro No. 8701, Col. Ex-Hacienda de San José de la Huerta. C.P. 58190, Morelia, Michoacán, México 2Department of Mechanical Engineering, Universidad Técnica Federico Santa MarÃa, Av. España 1680, Casilla 110-V, ValparaÃso, Chile 3Department of Mechanical, Industrial and Aerospace Engineering, Concordia University, 1455 Maisonneuve Blvd West, Montreal, QC, Canada H3G 1M8 4Instituto de Investigaciуn en Metalurgia y Materiales, Universidad Michoacana de San Nicolás de Hidalgo. C.P. 58190, Morelia, Michoacán, México 5Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, C.P. 58190, Ciudad de México, México
The present work investigates the effect of the
pores arrangement on the elastic compressive behavior of metallic foams.
Models with porosities ranging from 45 to 65% were obtained using Finite
Element Analysis (FEA), with two arrangements: agglomeration similar to that
obtained in the powder metallurgy process (PM), and a distribution without
agglomeration obtained using infiltration. Experimental foams were obtained
for validating purposes. Results show that for both PM and infiltration
cases the estimated Young's moduli (E) decrease with the increase in
porosity. Moreover, the degree of agglomeration (m) was introduced as a
measure of the pore distribution, which presented an important effect on E.
The magnitude of E, which was obtained by means of numerical simulation of
uniaxial compression, was significantly lower (close to 70%) for
agglomerated pores (high m values) than that for the non-agglomerated
condition. Besides, Young's moduli obtained by simulating the foams
manufactured by PM and infiltration were in excellent agreement with the
experimentally manufactured foam values. These results remark the importance
of using models according to the real topology defined by the manufacturing
process.
JTAM, Sofia, vol. 50 Issue 2 pp. 176-189 (2020), [Full Article]
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NUMERICAL SIMULATION OF THE 3-D FLOW IN A CYLINDRICAL DUCT WITH TWO DIAPHRAGMS AT LOW MACH NUMBERS Yaroslav Trotsenko1, Igor Vovk2 1Taras Shevchenko National University of Kyiv, 03680 Kyiv, Ukraine 2Institute of Hydromechanics, National Academy of Sciences of Ukraine, 03057 Kyiv, Ukraine
The three-dimensional flow of a viscous incompressible
fluid in a cylindrical duct with two serial diaphragms is studied by the
numerical solution of the non-stationary Navier--Stokes equations. The
solution algorithm is based on the finite volume method using second-order
accurate in both space and time difference schemes. It is shown that under
certain conditions a sequential series of ring vortices is formed in the
shear layer between the diaphragms. This causes self-oscillations of the
pressure field in the neighbourhood of the second diaphragm orifice, as well
as pressure fluctuations in the whole medium between the baffles. The flow
structure has an azimuthal asymmetry that influences local features of the
flow, but practically does not affect its integral characteristics.
JTAM, Sofia, vol. 50 Issue 2 pp. 190-201 (2020), [Full Article]
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