Journal of Theoretical and Applied Mechanics, Sofia, vol. 50 Issue 2 (2020)

Table of contents



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]




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]




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]




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.

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.

JTAM, Sofia, vol. 50 Issue 2 pp. 142-157 (2020), [Full Article]




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]




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]




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 María, Av. España 1680, Casilla 110-V, Valparaíso, 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]




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]