Journal of Theoretical and Applied Mechanics, Sofia, vol. 51 Issue 3 (2021)

Table of contents



CLOSED FORM SOLUTION OF VERTICAL CONCENTRATION DISTRIBUTION EQUATION: REVISITED WITH HOMOTOPY PERTURBATION METHOD

Punit Jain, Koeli Ghoshal
Department of Mathematics, Indian Institute of Technology, Kharagpur-721302, India


The present study revisits the solution of Hunt equation to determine the vertical concentration distribution in a turbulent flow carrying sediment using Homotopy Perturbation Method (HPM). It presents a closed (compact) form series solution for sediment concentration and convergence of series solution is also stated briefly. Finally, the solution obtained by HPM is validated by comparing it with the implicit solution and the numerical solution as well as with existing experimental data.

JTAM, Sofia, vol. 51 Issue 3 pp. 277-300 (2021), [Full Article]




HYDRODYNAMIC STABILITY OF A PERIODIC CHANNEL FLOW OF VISCOELASTIC FLUID

Abdessamade Rafiki1, Ahmed Hifdi2, Youssef Haddout1, Youssef Belkassmi1
1Research Team ERMAM, Polydisciplinary Faculty of Ouarzazate Box 638, Ibn Zohr University, Morocco
2Laboratory of Mechanics, University Hassan II-Casablanca, Morocco


The local linear stability approaches and the combined effects of a periodic channel modulation and fluid elasticity are investigated on the stability of plane Poiseuille flow of a second-order and second-grade fluids. The fourth order modified Orr-Sommerfeld equation is solved using a spectral method. The critical values of a Reynolds number and a wavenumber are computed for several values of an elasticity number, K and at different section x. According to the channel section, we find that the channel amplitude has a stabilizing or destabilizing effect on the fluids flow. The Y-shaped structure of spectrum of Newtonian fluid flow is remarquabely modified by the elasticity of these fluids. This investigation proves the existence of a dangerous station of the flow corresponding to xc = 3π/2n whith n = 0.1.

JTAM, Sofia, vol. 51 Issue 3 pp. 301-316 (2021), [Full Article]




INFLUENCE OF GRAPHENE NANOPLATES AND MULTIWALL CARBON NANOTUBES ON RHEOLOGY, STRUCTURE, AND PROPERTIES RELATIONSHIP OF POLY (LACTIC ACID)

Radost Ivanova, Rumiana Kotsilkova
Institute of Mechanics, OLEM, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 4, Sofia, Bulgaria


In this study, rheology, structure, and mechanical properties are investigated in point of their enhancement by incorporation of graphene nanoplates (GNP) and multiwall carbon nanotubes (MWCNTs) in poly (lactic acid) matrix. PLA-based nanocomposites with 6% total amount of GNP and MWCNT in different combination, produced by melt extrusion method, are investigated. Incorporating of GNP to PLA at the binary nanocomposites and GNP and MWCNT (i.e. increasing of GNP concentration and an analogous decrease of MWCNT concentration) to PLA at the ternary nanocomposites leads to reduction of the linear viscoelastic range and critical strain. For pure molten polymer, a terminal-like behavior has been observed in low frequencies and the slope n is proportional to 2 (G' ∼ ωn). From the results, we can observe that as the GNP concentration increases, n decreases gradually. All samples are characteristic as a "gel" type structure, which is usually associated with reaching the percolation threshold. We demonstrated that XRD can be used as a quick and unambiguous method to determine the homogeneity of the nanocomposites in terms of carbon filler dispersion in a polymer matrix. GNP and MWCNT addition improved the impact strength of PLA composites, which confirm the successful reinforced effect on polymer matrix of both fillers.

JTAM, Sofia, vol. 51 Issue 3 pp. 317-334 (2021), [Full Article]




EFFECTS OF POROSITY AND PORE SIZE DISTRIBUTION ON THE COMPRESSIVE BEHAVIOUR OF Pd BASED GLASS FOAMS

Ludmila Parashkevova1, Ludmil Drenchev2
1Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Block 4, 1113 Sofia, Bulgaria
2Institute of Metal Science, Equipment, and Technologies with Center for Hydro- and Aerodynamics "Acad. A. Balevski", Bulgarian Academy of Sciences, 67 Shipchenski prohod St., 1574 Sofia, Bulgaria


The present work is devoted to the mechanical properties investigation of new materials with high porosity and an amorphous matrix. The technological parameters of obtaining 3D samples based on a palladium alloy and the results of uniaxial compression tests are presented. The synthesis of porous alloy Pd48Cu22Ni8Sb22 is realized by a two-step procedure with the boron acid as a foaming agent. An analytical homogenization for porous media is proposed taking into account the pore size distribution. The matrix is considered as a micropolar medium with an internal length which is assumed to be proportional to the molar volume of the alloy frozen in a glassy state. Numerical simulations are performed to clarify the role of pore size in predicting elastic moduli and onset of plastic flow.

JTAM, Sofia, vol. 51 Issue 3 pp. 335-351 (2021), [Full Article]




FIELD MEASUREMENTS AND NUMERICAL MODELING OF A FLEXIBLE PYLON: A PILOT STUDY ON STRUCTURAL HEALTH MONITORING

George D. Manolis, Triantafyllos Makarios, Konstantinos Katakalos, Georgios Dadoulis, Aggeliki Tziotziou, Eleni Charalambidou
Laboratory for Experimental Mechanics, Department of Civil Engineering, Aristotle University, GR-54124 Thessaloniki, Greece


In this work, we report on field measurements of a flexible pylon under the influence of external loads for the purpose of developing a structural health monitoring system capable of operating continuously and in remote mode. The basic idea behind such a system is to use the minimum number of sensors, enhanced with structural analysis capabilities, so that the continuously generated streams of data can be processed locally. These data streams could contain kinematic and/or stress variables as functions of time and would be remotely transmitted to a central processing unit for further evaluation. This will happen only if a preliminary evaluation of the data indicates that there is a strong possibility that the monitored structure is showing signs of overloading resulting in localized damage. In that case, a maintenance crew would be sent to inspect the structure in question more thoroughly. The entire procedure can be placed within an artificial intelligence environment in the form of an expert system that could outline a repair and rehabilitation protocol. Among the loads under consideration could be wind, temperature and ambient vibrations due to traffic. It is believed that in the near future, structural health monitoring will play a pivotal role in helping ensure the safe and continuous operation of infrastructure systems such electric power lines, telecommunication antennas, natural gas pipelines and bridges.

JTAM, Sofia, vol. 51 Issue 3 pp. 352-367 (2021), [Full Article]




DYNAMIC MULTIPLE NANOCAVITIES INTERACTION IN ELASTIC ANISOTROPIC SOLIDS

Sonia Parvanova1, Petia Dineva2
1Department of Structural Engineering, University of Architecture, Civil Engineering and Geodesy, Sofia, Bulgaria
2Institute of Mechanics, Bulgarian Academy of Sciences, Sofia, Bulgaria


An efficient mechanical model describing wave scattering and dynamic stress concentration effects in finite elastic anisotropic solids with multiple nanocavities under dynamic transient load is developed. The model and accompanied numerical scheme is based on the direct and inverse Fourier transform, BEM (boundary element method) for anisotropic materials and Gurtin and Murdoch theory taking into account the surface stress within the formalism of continuum mechanics. Accuracy of the numerical simulations is proved by comparisons with finite element method solutions. A detailed parametric study for the transient wave field sensitivity to the surface elasticity phenomenon and dynamic interaction of multiple nanocavities is presented.

JTAM, Sofia, vol. 51 Issue 3 pp. 368-390 (2021), [Full Article]




CONTACT PROBLEM FOR A RIGID FLAT STAMP AND A LINEAR ELASTIC STRIP BONDED TO POROUS HALF-PLANE

Mikhail Chebakov1, Elena Kolosova1, Roumen Iankov2, Maria Datcheva2
1Vorovich Institute of Mathematics, Mechanics and Computer Sciences, Southern Federal University, Milchakova St., 8a, Rostov-on-Don, 344090, Russian Federation
2Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., block 4, 1113 Sofia, Bulgaria


In this article, we consider the plane contact problem for indentation into a system of elastic layer deposited on a half-plane composed of a linear elastic material with voids. The Cowin-Nunziato's micro-dilatation theory is used to explain the linear elastic response of the material with voids occupying the half-plane. It is assumed that the punch is rigid with a flat base and the contact is frictionless. Using the integral Fourier transform, the problem for determining the contact pressure is reduced to a singular integral equation for the unknown contact stress, whose approximate solution is found using the collocation method. The values of the contact stresses and the deformation of the strip's free surface are analyzed by varying the thickness of the elastic strip and the material parameters for the elastic layer and the material with voids. The numerical results are presented in the form of graphs.

JTAM, Sofia, vol. 51 Issue 3 pp. 391-404 (2021), [Full Article]