Abstracts
INVITED LECTURES
Opening Plenary Lecture: Bifurcations and Stability in Machining Processes
Marian Wiercigroch
Comprehensive understanding of chatter, bifurcations and stability is essential for improving productivity, quality, and efficiency of manufacturing operations. Chatter manifests itself as undesired large amplitude self-excited vibration, whilst bifurcation and stability analysis hold the keys to its effective control and suppression. Bifurcations in manufacturing occur when processing and system parameters cross certain values and(or) they are results of complex interactions resulting predominantly in creating or disappearance of limits cycles. Bifurcations in manufacturing processes belong to the most complex ones, where nonlinearities are very strong and play the dominant role e.g. [1]. Process stability is vital to ensure high-quality surface finish, dimensional accuracy, and extended machine and tool life.
In this lecture, first I will define nonlinearity, chatter, bifurcations and stability in manufacturing processes with the focus on metal cutting. Then I will discuss on the frictional chatter, which was introduced and investigated with my group and collaborators (see e.g. [2,3,4]). Chatter in precision grinding has been a subject of many our investigations, where low dimensional strongly nonlinear models were used to undertaken in depth analytical and numerical studies, see for example [5,6]. Finally, examples of advanced nonlinear dynamics techniques such as path following bifurcation analysis, basins of attraction [7], Poincare maps and Lyapunov exponents will be discussed, which have been used to examine stability and determine a practically important measure so-called cutting safety [8,9].
- Wiercigroch, M., Budak, E. Nonlinearities, chatter generation and suppression in metal cutting. Philosophical Transactions of the Royal Society of London: Part A 359, 663-693, 2001.
- Wiercigroch, M., Krivtsov, A.M. Frictional chatter in orthogonal metal cutting. Philosophical Transactions of the Royal Society of London: Part A 359, 713-738, 2001.
- Rusinek, R., Wiercigroch, M., Wahi, P. Influence of tool flank forces on complex dynamics of cutting system process. International Journal of Bifurcation and Chaos 24(9) 1450115, 2014.
- Yan, Y., Xu. J, Wiercigroch, M. Modelling of regenerative and frictional cutting dynamics. International Journal of Mechanical Sciences 156, 86-93, 2019.
- Yan, Y., Xu, J., Wiercigroch, M. Chatter in transverse grinding process. Journal of Sound and Vibration 333, 937-953, 2014.
- Yan, Y., Xu, J., Wiercigroch, M. Regenerative and frictional chatter in plunge grinding. Nonlinear Dynamics 86, 283-307, 2016.
- Yan, Y., Xu, J., Wiercigroch, M. Basins of attraction of the bistable region of time-delayed cutting dynamics. Physical Review E 96(3), 032205, 2017.
- Yan, Y., Xu. J, Wiercigroch, M. Estimation and improvement of cutting safety. Nonlinear Dynamics 98(4), 2975-2988, 2019.
- Yan, Y., Liu, G., Wiercigroch, M., Xu, J. Safety estimation for a new model of regenerative and frictional cutting dynamics. International Journal of Mechanical Sciences 201, 106468, 2021.
Plenary Lecture: Arterial Solid Mechanics. A Brief Review and the “Bulgarian Imprint”
Alexander Rachev
The talk is addressed mainly to this part of the audience that has never been engaged in research in arterial solid mechanics. The basic physiological function of arteries is to convey and distribute oxygenated blood to organs and tissues according to metabolic demands. A brief review of mechanical and biological processes that govern arterial performance is given. Special attention is paid to the constitutive formulation of arterial tissue in the framework of nonlinear continuum mechanics. Mathematical models that describe the passive, active, and remodeling response of arteries are presented. Some critical remarks express only the author’s personal opinion.
Each good research is unique and extends the boundaries of our knowledge. The “Bulgarian imprint” in the title refers to the results of Bulgarian scientists, in most cases in collaboration with foreign partners, that gained well-documented recognition and have a notable impact on the further development of specific topics of arterial biomechanics.
Plenary Lecture: Nonlinear Vibrations of Multi-Stable Mechanical Systems
Jerzy Warminski
A specific future of nonlinear systems is an existence a few solutions for the same set of structural parameters. A classical Duffing’s oscillator may serve as an example where three different solutions, two stable and one unstable, may exist in a certain frequency domain. However, if the Duffing oscillator is excited parametrically and in the same time externally, a number of solutions may increase up to five, as presented in [1]. The additional self-excitation creates quasi-periodic solutions which occur after Neimark-Sacker bifurcation [2].
The models govern by classical Duffing’s equation have just one equilibrium position, which is stable and then its potential function has just one minimum. Systems with more than one equilibrium belong to another group of the multi-stable structures. The multi-stability can be created by specific devices, for example by repulsing magnetic force. Such nonlinear force enables to modify a shape of a potential function and to get two or more potential wells. Another option is to add axial force to the structure which moves the vibrating system close to a buckling point with two equilibria (two potential wells). Such axial compression may be caused by axial mechanical loading or by increased temperature for example [3].
The composite technology offers new possibilities to create multi-stable structures [4]. The bistability, with an associated rapid jump from one to another equilibrium (so called snap-through effect) is attractive for a design of efficient energy harvesters [5]. Most of the published papers on bistable laminates are devoted to symmetric or almost symmetric configurations with free boundaries. A special design of a laminate shell based on an unique pseudo-conical shape, with asymmetric configuration of lamina is proposed in [4,5]. Depending on the assumed geometry two or even five stable equilibria can be obtained. In the present paper the unique nonlinear properties of the shell are investigated for local in-well oscillations and as well as for large oscillations with global cross-well dynamics, with the snap-through effect.
The main goal of this paper is to present the untypical nonlinear effects of multi-stable systems and their application to energy harvesting [5], morphing and control.
Acknowledgment
This research was funded in part by National Science Centre, Poland 2021/41/B/ST8/03190.
References
- WARMINSKI J: Frequency locking in a nonlinear MEMS oscillator driven by harmonic force and time delay International Journal of Dynamics and Control 3 (2), 122-136, 2015.
- Warminski J.: Nonlinear dynamics of self-, parametric, and externally excited oscillator with time delay: van der Pol versus Rayleigh models. Nonlinear Dynamics 99, 35-56, 2020.
- Manoach E., Warminski J., Kloda L., Warminska A., Doneva S., Nonlinear vibrations of a bi-material beam under thermal and mechanical loadings, Mechanical Systems and Signal Processing, 177, 1-21, 2022.
- Brunetti M., Mitura A., Romeo F., Warminski, J.: Nonlinear dynamics of bistable composite cantilever shells: an experimental and modelling study. Journal of Sound and Vibration 526 (2), 116779, 2022.
- Mitura, A., Brunetti, M., Kloda, L., Romeo, F., Warminski, J.: Experimental nonlinear dynamic regimes for energy harvesting from cantilever bistable shells. Mechanical Systems and Signal Processing 206 (9), 110890, 2024.
Plenary Lecture: Energy absorption of lightweight materials and structures under dynamic loading
Dora Karagiozova
Man-made cellular materials with a large range of different topologies - honeycombs, open and closed cell foams, metal hollow spheres, micro-lattices, stacked origami, etc. - are widely used in various engineering applications, especially in the automotive, aerospace and defense industries. While the cellular materials would not be a competitor for load-bearing structural parts on their own due to their relatively low strength, they offer significant advantages when combined with other materials into a sandwich construction as the cellular materials they possess good energy absorption properties for very little weight penalty and relatively small force transfer through the structure. In particular, the dynamic compressive behavior of cellular materials is crucial to their applications in energy absorption and blast/impact protection.
In the lecture, the essential features of the response of metal-based cellular materials to impact loading based on an advanced theoretical description of the response of cellular materials with different configurations when using a continuum mechanics approach are highlighted. This approach relies on the one-dimensional stress wave propagation when using the actual experimentally derived stress-strain curve, characterizing a strain-hardening material, together with its Hugoniot representation. Examples are shown for the stress wave propagation in cellular materials with uniform density and gradient density when highlighting the difference between the continuous stress wave (a simple wave) and a wave of a strong discontinuity (shock wave). The application of the proposed theoretical approach is demonstrated on an experimentally tested structure under blast loading. The deformation mechanisms of materials which exhibit structural softening, such as out-of-plane loaded honeycomb, are discussed to reveal the importance of the materials topology for their dynamic compaction. It is shown that the dynamic out-of-plane compaction of honeycombs does not obey the law of shock wave propagation as the underpinning deformation mechanisms at the cell level govern the macroscopic response of the cellular materials.
Closing Plenary Lecture: Global Dynamics Perspective for the Analysis, Control and Safe Design in Macro- to Nano-Mechanics
Giuseppe Rega
Global nonlinear dynamics has been evolving in a revolutionary way in the last two decades, with development of sophisticated techniques employing concepts/tools of dynamical systems, bifurcation, and chaos theory, and applications to a wide variety of mechanical/structural systems. Relevant achievements entail a substantial change of perspective in dealing with vibration problems, and are ready to affect meaningfully the analysis, control, and design of systems at different scales. After properly framing the subject within some main stages of developments of nonlinear dynamics in solid/structural mechanics, the lecture will focus on highlighting the role played by global analysis in unveiling the nonlinear response and actual safety of engineering systems in diverse environments. Reduced order models of macro/micro-structures will be considered, to address the following items.
- Detecting effects induced in long-term dynamics by the coupling of mechanical (fast) and thermal (slow) fields.
- Characterizing and quantifying main topological phenomena of global response in phase and parameter spaces.
- Dynamical integrity to evaluate robustness and overall stability in mechanical/structural applications.
- Exploiting global concepts/phenomena for control purposes.
- Interpreting theoretical vs practical mismatches of global safety ensuing from real system disturbances.
- Non-deterministic global framework: operator approaches to stochastic attractors and basins of attraction.
- Enhanced load carrying capacity of systems/structures via a novel, global dynamics-informed, paradigm.
- How facing the enormous complications of global dynamics for multidimensional systems.
SOLID MECHANICS
Preparation of Epoxy Composites with Organoclays and Study of their Mechanical Properties
Verislav Angelov
The present research aimed to obtain epoxy composites with organoclay filler and to investigate some of their mechanical properties.
Epoxy composites were obtained by the method of "in situ" polymerization in the Laboratory "OLEM" at Institute of Mechanics-BAS and after that is used a modern vacuum installation in IOMT-BAS in order to remove air bubbles from the epoxy resin before curing the composites.
The mechanical characteristics (hardness and Young's modulus) were tested on a TIRA TEST machine at Institute of Mechanics-BAS. Epoxy composites with fillers - Cl-20A, Cl-30B, 1.44 P, 1.28 E, 1.31 PS have worse mechanical properties than neat epoxy resin. Composites with organoclay 1.34 TCN show slightly better mechanical properties, compared to pure epoxy resin.
The X-ray diffraction (XRD) measurements were performed on the organo-clay epoxy composites. The X-ray diffractograms were obtained using Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 0.15418 nm) and LynxEye detector. Using the peak position (2θ) in the XRD patterns, the inter layer space was calculated through the Bragg’s law: nλ = 2d sin θ, where λ is the wavelength of the incident wave (λ = 0.15418 nm), d is the spacing between the layers of organo-clay in the composites.
Constitutive Modeling of Elastomers by Deep Symbolic Regression
Mikhail Itskov, Rasul Abdusalamov
In this contribution we apply deep symbolic regression to generate constitutive models for elastomers on the basis of experimental or artificially created stress-strain data. Symbolic regression represents an evolutionary algorithm searching for a mathematical model in the form of an algebraic expression which consists of a set of initially defined mathematical operations and functions and satisfies fitness and simplicity criteria. First, we illustrate an application of this algorithm to hyperelastic material models. For the validation of the proposed approach, benchmark tests with the generalized Mooney-Rivlin model are presented. In all these tests, the proposed algorithm can find the predefined models. Additionally, a data set for a temperature-dependent thermoplastic polyester elastomer is evaluated. In the latter case, good agreement with the experimental data is obtained.
Finally, the procedure is applied to describe the Mullins effect in filled rubbers. This effect is characterized by a considerable stress softening after the very first loading up to the maximal stretch applied before. For the training of the symbolic regressor both experimental and artificially generated data can be used. The later ones are obtained by a simple available isotropic model of the idealized Mullins effect. The free energy function constructed by symbolic regression depends in this case not only on the strain invariants but also on the maximal stretch previously reached in the loading history. In the next steps, the model should be generalized to anisotropic Mullins effect and take also into account accompanying permanent set.Vibration problems in the linear theory of Moore-Gibson-Thompson thermoporoelasticity
Merab Svanadze
In this work, the linear model of Moore-Gibson-Thompson (MGT) thermoporoelasticity is proposed in which the combination of the deformation of elastic material, Darcy's law for fluid flow in pore network and the MGT law of heat conduction is considered. The basic boundary value problems (BVP) of steady vibrations of this model are investigated. Indeed, the fundamental solution of the system of steady vibration equations is constructed. Green’s identities are established and the uniqueness theorems for the classical solutions of the BVPs are proved. On the basis of the surface and volume potentials the BVPs of steady vibrations are reduced to the always solvable singular integral equations. Finally, the existence theorems for classical solutions of the BVPs are proved by virtue of the potential method and the theory of singular integral equations.
Acknowledgements: This work was supported by Shota Rustaveli National Science Foundation of Georgia (SRNSFG) [Grant # FR-23-4905].
Two-parametric analysis of a semi-infinite three-layered high-contrast elastic strip under antiplane shear deformation
Illia Chernomorets, Julius Kaplunov, Danila Prikazchikov
Antiplane shear of a symmetric three-layered elastic semi-strip under the self-equilibrated edge load is studied. The arbitrary ratios of thickness and stiffness are considered, with emphasis on high-contrast scenario; in doing so, two aforementioned dimensionless problem parameters are assumed mutually independent. A simple explicit condition, supporting a slowly decaying solution of the equilibrium equations, is derived. Leading order estimates for the decay rate and amplitude of this solution are presented, highlighting its dominant contribution to displacement field. The derivation involves an asymptotic formula ensuring elimination of the boundary layer localised near the edge of the semi-strip. The peculiarities of the latter are also addressed. Numerical results, including comparisons of the derived shortened and full expressions for the sought for slowly decaying solutions, are presented.
Low-frequency vibrations of aerogel-based sandwich structures
Danila Prikazchikov, Julius Kaplunov, Ludmila Prikazchikova, Ameya Rege
It is known that high contrast in layered elastic structures can lead to emergence of extra low-frequency vibration modes, see [1], presenting the dispersive behaviour of a three-layer high-contrast plate, and also [2, 3] developing asymptotic models for the case of antiplane shear motion, as well as formulating correct boundary conditions relying on an extension of the Saint-Venant’s principle. Recently, one of the setups considered in [1] was adapted to sandwich plates containing a soft aerogel core layer, see [4].
In the current contribution, the problem formulated in [4] is further developed. The two-mode approximations are derived for both low-frequency propagating and associated evanescent modes. A nontrivial composite nature of these asymptotic expansions is discussed, along with their various shortened forms and the corresponding domains of validity. Numerical illustrations for comparisons of asymptotic and exact results are presented.
Acknowledgement
JK, DP and AR acknowledge support from the DFG-TUBITAK grant № 520432297 “Multiscale Dynamics of Aerogel and Aerogel Composite Matching Layers”.
References
[1] J. Kaplunov, D. Prikazchikov, L. Prikazchikova. Dispersion of elastic waves in a strongly inhomogeneous three-layered plate. International Journal of Solids and Structures 113 (2017), 169-179.
[2] J. Kaplunov, L. Prikazchikova, M. Alkinidri. Antiplane shear of an asymmetric sandwich plate. Continuum Mechanics and Thermodynamics 33 (2021), 1247-1262.
[3] L. Prikazchikova, Y. Ece Aydın, B. Erbaş, J. Kaplunov. Asymptotic analysis of an anti-plane dynamic problem for a three-layered strongly inhomogeneous laminate. Mathematics and Mechanics of Solids, 25(1) (2020), 3-16.
[4] S. Aney, M. Schestakow, L. Prikazchikova, B. Milow, D. Prikazchikov, J. Kaplunov, H. Voggenreiter, A. Rege. Cellulose aerogels as multifunctional and sustainable alternatives for aircraft cabin components. Deutscher Luft- und Raumfahrtkongress (2023), 7p., doi: 10.25967/570245.
Determination of temperature fields in energy-efficient composite elements intended for use in building construction
Ana Yanakieva, Anguel Baltov, Gergana Nikolova
A theoretical model of temperature dispersion in composite bodies has been developed. The temperature distribution of chosen reinforced structural elements built of new ecological and/or recycled composites was calculated using the ambient temperature. The collected results have been analysed, and appropriate recommendations for building construction have been provided.
Numerical modelling and analytical solution of Euler-Bernoulli beam on elastic Winkler foundation subjected to dynamic time-harmonic load
Sonia Parvanova, German Stoyanov, Petar Shishkov
The object of investigation of the current study is the dynamic response of an Euler-Bernoulli beam, resting on elastic foundation. A straight beam with prescribed bending stiffness of the cross section, supported along its entire length by an elastic medium, and subjected to time-harmonic loads, acting in the plane of symmetry of the cross section, is considered. Rigid beams, beams of medium length, as well as infinitely long beams are investigated. The analytical solution of the ordinary differential equation in the frequency domain is performed by the initial parameters’ method employing Krilov’s functions in the case of beams of medium length. For infinitely long beams the analytical solution is accomplished taking advantage of the Zimmerman functions. The solutions of the beams falling in the transition between both categories are confirmed by the two approaches.
Additionally, the authors have developed software code in MATLAB based on the Finite Element Method (FEM) for solution of beams on elastic foundation, subjected to constant time-harmonic loads. Both the analytical and numerical solutions are augmented to cover the case of moving point loads with prescribed velocity. Comparisons between both sets of solutions are made and some conclusions and recommendations about their application are summarized.
Application of different modelling techniques for soil-structure interaction problems
Sonia Parvanova, Georgi Vasilev, Amar Pashov
The current study investigates wave propagation from embedded sources in semi-infinite geological media with surface relief, underground structures, and foundations. Our focus lies in modeling and analysing the seismic behavior of tunnels, particularly considering the dynamic interaction effects between tunnels and the surrounding soil medium. We employ various techniques, including the classical boundary element method (BEM) for two-dimensional elastodynamics, the finite element method (FEM) combined with perfectly matched layers (PMLs), and the built-in options of the commercial software package ANSYS for modeling semi-infinite regions. Our analysis is concentrated on time harmonic input as the dynamic load.
The direct BEM is applied in its classical form using fundamental solution for elastic isotropic medium presented via Bessel’s functions. Additionally, it is based on the frequency-dependent fundamental solution derived by Radon transform for general anisotropic continua. As the most accurate technique for modelling the infinite extension of the soil region, ensuring completion of Sommerfeld’s radiation condition, the BEM solutions serve as reference points.
For longitudinally invariant geometries, a computationally efficient two-and-a-half (2.5D) method can be used to calculate the three-dimensional wave field in the soil. At the boundaries of the finite element mesh, perfectly matched layers (PMLs) are added to prevent spurious wave reflections. The 2.5D PML methodology is employed for isotropic and transversely isotropic semi-infinite half-spaces. A comparison with BEM solutions is conducted.
The FEM solutions, facilitated by the built-in options of ANSYS, are utilized to address both 2D and 3D soil-structure interaction problems. These solutions employ the concept of PML for 3D problems, for both isotropic and anisotropic mediums, and infinite finite elements for 2D and 3D isotropic continua.
Comparisons among the three adopted techniques are conducted, and conclusions regarding their respective applications are drawn. In essence, the primary objective of the proposed research is to provide the engineering community with valuable and calibrated results from simulations derived through extensive and detailed parametric studies.
Dynamic and Stability of Circular Plates
Simona Doneva
Comparative Analysis of the Results of Surveying Building Stock with a Thermal Vision Camera
Tsvetomir Borisov, Yordan Mirchev, Ana Yanakieva, Dessislava Pashkouleva
This study focuses on thermal imaging research of the building stock using a thermal imaging camera. The building of the Institute of Mechanics at the Bulgarian Academy of Sciences was selected for the purposes of the study. A comparative analysis was conducted on the results obtained before and after the replacement of the windows in the selected building. The tests were performed with the E6T198547 thermal camera. Initially, the part of the building with old wooden window frames was examined using the thermal camera. Subsequently, the same examination using the same thermal camera was conducted on the part of the building with new PVC window frames, which had already been replaced in the past. The comparative analysis is based on the temperature difference in selected structural elements from the facade of the building obtained under the same test conditions.
Optimal versus Suboptimal Tuned Mass Dampers: A Comparison Study based on Energy Transfer between Primary and Secondary Structures
Georgios Dadoulis, George Manolis, Konstantinos Katakalos
In order to limit vibrations in bridges caused by the passage of heavy traffic that could potentially cause damage, a passive device configuration, namely the tuned mass damper (TMD), has been widely used in engineering practice. In this work we investigate the vibratory motion caused by a heavy mass sliding on a simply supported bridge with a TMD attached at center span. Specifically, the TMD is modelled as a single-degree-of-freedom (SDOF) unit comprising a mass, spring and damper placed on the span to act as a secondary system for absorbing vibrations from the primary system, i.e., the bridge itself. The sub-optimal TMD without a damper was previously analyzed using a Lagrangian energy balance formulation to derive the governing equations of motion, followed by an analytical solution using the Laplace transform. This allowed for a detailed investigation of the transmission of vibratory energy between primary and secondary systems with results given in terms of time histories, power spectral density functions and spectrograms. These results were validated by separate experimental measurements conducted on a simply supported steel beam traversed by a heavy sliding mass. Subsequent numerical simulations have gauged the performance of a number of TMD configurations with optimized material parameters, starting with the aforementioned sub-optimal TMD where damping is provided by the spring element only.
Supporting Agenies: Mercator Fellowship through German Research Foundation (DFG) under grant SM 281/20-1 and the Hellenic Foundation for Research and Innovation (HFRI) under grant 6255.Drop Test Validation of a Virtual Prototype of EV Battery Pack
Georgi Todorov, Konstantin Dimitrov, Konstantin Kamberov
Semi-Automatic Phased Array Ultrasonic Testing in Aluminum Plate on Airbus Wing for Delamination or Corrosion Damage
Yordan Mirchev, Tsvetomir Borisov, Pavel Chukachev
Semi-Automatic Phased Array Ultrasonic Testing for Composite Material CFRP in Immersion Mode
Tsvetomir Borisоv, Yordan Mirchev
Evaluation and analysis of microseismic records on the territory of the Sofia Basin
Emil Oynakov, Radan Ivanov, Lilia Dimitrova, Alexander Radulov
The increased growth of the population and building stock, the concentration of administrative, commercial, industrial and residential buildings, interconnected infrastructure networks (underground and above ground), etc., mean that modern earthquakes can be more damaging to the city than those in the past, even if their magnitudes do not exceed the highest observed so far. The likely secondary effects are also relevant, especially in densely populated areas.
The study of microseisms in solving the problems of seismic microzoning is widely used as a detailed and inexpensive method that provides fairly accurate results. This technique investigates the resonance properties of the upper part of the geological section by measuring microseismic oscillations.
Currently, around the globe, research is being conducted to study microseismic noise as carrier of information about man-made impacts on the geological environment or modern tectonic processes.
The content of the microseismic wave field continuously recorded at any measurement point is closely related to the structure and geology of the Earth's crust. All registered oscillations have individual amplitude-frequency characteristics, temporal mode of existence and variations (permanent, periodic, spontaneous) and, what is particularly important, characteristic spatial distribution in the geological environment.
Seismic noise records were analyzed to study the structure of the microseismic wavefield and the spatial variability of soil dynamic characteristics for the Sofia Basin area. The natural frequencies of the soil were determined by two methods: the standard spectral ratio (SSR) method and the horizontal to vertical spectral ratio (HVSR) method (Nakamura's method).
On the basis of the recorded extensive experimental material, the possibility of using the natural oscillations of the soil (microseisms) for differentiating the ground in a seismic sense has been proven.Dynamic fracture of graded cracked magnetoelectroelastic nanosheet with surface topography
Tsviatko Rangelov, Petia Dineva, Yonko Stoynov
The anti-plane dynamic problem for a graded cracked half-plane with nanotopography under time-harmonic wave is studied. The computational tool is boundary integral equation method (BIEM) based on analytically derived Green’s function for a graded in depth half-plane. The mechanical model is based on a combination of classical continuum mechanics theory for the magnetoelectroelastic (MEE) bulk half-plane under non-classical boundary conditions along the relief and crack surface, supplemented with a localized constitutive equation in the frame of the Gurtin–Murdoch model. The simulations reveal the behaviour of the local zones of generalized stress concentration factors (GSCF). It is shown the sensitivity of the mechanical stress, electric and magnetic field concentration to the material gradient, surface elasticity, nanorelief and nanocrack existence, coupled material properties and finally to the type and characteristics of the applied load. The discussed parametric analysis is with application in mathematical physics, computational nanomechanics, surface mechanics and thin film mechanics.
Keywords: Functionally graded МЕЕ half-plane, nanotopography, nanocrack, Gurtin-Murdoch model, incident SH wave, BIEM via graded half-plane Green’s function, GSCF.
Acknowledgement. This work is supported by the Bulgarian National Science Fund, contract No KП-06-H57/3/15.11.2021.
BEM modelling and parametric analyses of anisotropic soil-structure interaction
Sonia Parvanova, Petia Dineva
The subject of the present study is elastodynamic analysis of anisotropic half-plane with surface relief containing foundation and embedded lined or unlined tunnels. The aim is to take into account the different key factors as: (a) the soil topography; (b) the soil anisotropy; (c) the type and characteristics of the source of dynamic excitation; (d) the whole path of the elastic wave radiated by a source at a prescribed point, propagating through the anisotropic geological region containing tunnels and foundation till its free surface. The computational tool is the direct boundary element method based on the frequency-dependent fundamental solution for 2D anisotropic elastodynamics derived by the Radon transform. Numerical scheme verification and parametric studies are discussed. The obtained results clearly illustrate the wave field sensitivity to the type and characteristics of the dynamic source, to the relief peculiarities, to the material anisotropy and to the complex soil–foundation-lined tunnels dynamic interaction.
Keywords In-plane elastodynamics · Anisotropic soil · Soil–foundation-lined tunnels interaction · Scattered wave field · Stress concentration filed · BEM
Green’s Functions for the Viscoelastic Half-space: A Convolutional Neural Network Approach
George Manolis, Georgios Dadoulis, Petia Dineva, Tsviatko Rangelov
A basic identification problem in elastodynamics is addressed in this work, whereby it becomes possible through the use of machine learning to identify the presence of a free surface in a three-dimensional viscoelastic continuum by measuring the dynamic response (displacements, tractions) at a receiver node due to an impulse at a source node. Specifically, a convolutional neural network (CNN) is constructed based on numerical solutions in the form of spectrograms furnished by the fundamental solutions for the full-space in the frequency domain. Next, numerical studies based on the half-space Green’s functions for the viscoelastic continuum are conducted to furnish data that is evaluated by the CNN, thus yielding a ‘confusion’ matrix quantifying the probability that the frequency-dependent signals have/have not been scattered. This in turn indicates the presence/absence of a free surface. It is also possible to re-train the CNN to furnish an estimate of the depth of the receiver point from the free surface, if such a surface exists. The present work also identifies which components of the displacement and of the traction Green’s function are the most reliable for use in the identification procedure. In closing, applications of this CNN could be in the areas of geophysical prospecting and material science.
Supporting agencies: Grant No BG05M2OP001-1.001-0003, financed by the Bulgarian Science and Education for Smart Growth Operational Program (2014-2020) and co-financed by the European Union through the European Structural and Investment Funds.
Thermoelastic Large Amplitude Vibrations of Two-Layered Beams
Emil Manoach, J. Warminski, Simona Doneva
The goal of this work is to extend the developed by the authors of this study the theoretical model of the vibration of bi-material beams subjected the combined action of mechanical and thermal loads. The geometrically nonlinear version of the Timoshenko beam theory is used to describe the theoretical model of the problem. Starting from the geometrical, constitutive and equilibrium equations governing equations of each layer of the bi-material beam are derived with respect to one coordinate system. In a contrast with the previouly developed model, where a generalized properties of the beam were used, here the properties of each layer were taken into the equations of motion. Then reduced order models for the equations of each layer were developed using the normal forms of vibration of the layers . The relation between the generalized coordinates in time of the layers were obtained which was used in the solution of the equations of motion. Numerical results for the response of the beam in time and frequency domain were obtained and stability and bifurcation of the beam due to elevated temperature and loading parameters were studied.
Comparing а different cohesive zone model elements in finite elements simulation of mechanical behavior of reinforced concrete corbel strengthened by CFRP.
Veselin Stankov, Ivelina Ivanova, Jules Assih, Dimitar Dontchev
FEM for quasistatic simulation of mechanical behavior of reinforced concrete beam strengthened by composite materials.
Veselin Stankov, Ghiwa Bou Abdallah, Ivelina Ivanova, Jules Assih, Dimitar Dontchev
FLUID MECHANICS
Lotus Effect on surfaces with randomly distributed micro-pillars
Nina Pesheva, Stanimir Iliev, Pavel Iliev
We present a numerical study of the receding apparent contact angle for a liquid meniscus in contact with an ultra-hydrophobic surface with randomly distributed isolated micro-pillars in Cassie’s regime. We use the full capillary model to obtain this angle for the Wilhelmy plate geometry in the framework of the heterogeneous approximation model. A broad interval of values of pillar concentration is studied for micro-pillars of both square and circular shapes of the pillar cross section. Comparisons with experimental results for the receding contact angles for a surfaces with periodically arranged pillars is carried out. The results of the numerical simulations showed that the receding apparent contact angle for a plate surface with randomly located defects is very close to the value of the receding contact angle on a surfaces with periodically spaced pillars.
Mathematical Modelling of Nonlinear Waves
Angela Slavova
The study of water waves involves various disciplines such as mathematics, physics and engineering and within this there are many specific areas of direct or associated interest such as pure mathematics, applied mathematics, modelling, numerical simulation, laboratory experiments, data collection in the field, the design and construction of ships, harbours, the prediction of natural disasters, climate studies and so on.
In this talk we shall present travelling wave solutions of shallow water waves. Camassa-Holm considered a third order nonlinear PDE of two variables modelling the propagation of unidirectional irrotational shallow water waves over a flat bed, as well as water waves moving over an underlying shear flow. In the special case of the motion of a shallow water over a flat bottom the corresponding system was simplified by Green and Naghdi and related to an appropriate two component first order Camassa-Holm system. Another interesting system of nonlinear PDE is the viscoelastic generalization of Burger's equation. In the above mentioned systems we are looking for travelling wave solutions and we are studying their profiles. To do this we use several results from the classical Analysis of ODE that enable us to give the geometrical picture and in several cases to express the solutions by the inverse of Legendre's elliptic functions.
As an application we shall present propagation of tsunami waves from their small disturbance at the sea level to the size they reach approaching the coast. Even with the aid of the most advanced computers it is not possible to find the exact solutions to the nonlinear governing equations for water waves. For this purpose we introduce Cellular Nonlinear Network (CNN) approach.
Strategic Stochastic Techniques for Improved Sensitivity Analysis in Air Pollution Modeling
Velichka Traneva, Venelin Todorov, Stoyan Tranev
In this paper, we present an investigation into advanced stochastic methods for performing sensitivity analysis on the Unified Danish Eulerian Model (UNI-DEM), a comprehensive air quality model used extensively for simulating the long-range transport of air pollutants across Europe. Our study specifically focuses on enhancing the reliability and precision of sensitivity analyses conducted on this model, which is crucial for addressing environmental policy-making and management strategies effectively.
To tackle the inherent complexities of UNI-DEM, which includes numerous input parameters and high-dimensional data, we employ a range of sophisticated stochastic techniques. These include advanced Monte Carlo methods and quasi-Monte Carlo methods. The application of these methods allows for a more efficient exploration of the model's input parameter space and helps identify key factors influencing model outputs with greater accuracy.
We detail the implementation of these stochastic approaches and discuss their advantages over traditional sensitivity analysis methods, particularly in terms of computational efficiency and the ability to handle complex, multidimensional environmental systems. Additionally, the paper presents a comparative analysis of these advanced methods against standard approaches using several case studies that highlight significant improvements in sensitivity detection and parameter impact delineation.
The results from our analysis demonstrate that the advanced stochastic methods significantly enhance the understanding of the sensitivity behavior of the UNI-DEM, providing deeper insights into the dynamic interactions within the model. This, in turn, aids in the more accurate forecasting of pollution levels and informs more targeted environmental policy decisions. Through this work, we contribute to the ongoing development and optimization of sensitivity analysis techniques in environmental modeling, offering robust tools that can adapt to the challenges posed by complex ecological simulations.
Mathematical modeling of thermal stratification in underground long-term hot water storage
Milan Rashevski, Slavtcho Slavtchev
Thermal stratification due to incompressible laminar flows in underground long-term hot water storage is simulated numerically. The mathematical model is based on the unsteady two-dimensional Navier-Stokes and energy equations. Dynamic temperature boundary conditions are applied on the cavern walls. Finite difference method (FDM) is used to solve the problem in vorticity-stream function formulation. Different cavern proportions are simulated and compared. The evolution of vortices and its influence on the temperature and velocity fields is presented through graphics at given moments of time. The influence of the geometrical proportion on the thermal stratification in the storage is discussed. Thermal losses due to fluid convection is evaluated quantitatively, showing high seasonal thermal efficiency of the insulated hot water storage.
An advanced Computational Mechanics approach of optimized design for preparing of Darrieus-Ugrinsky Helical Wind Turbine for Four-Dimensional Printing
Nikolay Zlatov
An advanced Computational Mechanics approach of optimized design for preparing of Darrieus-Ugrinsky Helical Wind Turbine for Four-Dimensional Printing
Prof. Dr. Nikolay Zlatov, Ass. Prof. Dr. Krastimir Popov, Prof. Georgi Hristov , Prof. Plamen Zahariev, Ass. Prof. Chi Hieu Le, Prof. Ivan Beloev
AbstractThe article discusses the potential of computational mechanics (CFD) in improving the understanding of unsteady aerodynamics in Darrieus-Ugrinsky Helical Wind Turbine. With advancements in computing resources, CFD is expected to become a cost-effective and accurate tool in this field. Darrieus-Ugrinsky Helical Wind Turbines are gaining popularity, particularly for small-scale applications and unconventional installation sites. The study utilizes unique experimental parametric model using the parameterization methods in SolidWorks' Flow Simulation module. This gives a better way to optimize a combined rotor to validate different CFD approaches, focusing on parametric simulations. The article examines the correct definition of the computational domain, selection of parameters of the models, and correction of simulated data. The results indicate that well-configured CFD simulations can provide an accurate estimation of turbine performance and effectively describe the flow field, creating a lower pressure outlet, and improving power and torque coefficients. This research is done using wind data for Bulgarian environment from Meteorological Archive of NIMH (National Institute of Meteorology and Hydrology)
Mathematical modelling of laminar convection in illuminated vertical rectangular channels
Milan Rashevski, Slavtcho Slavtchev
the Navier-Stokes equations with the Boussinesq approximation and the energy
equation. The channel is illuminated from the side in the near infrared spectrum
which induces a non-uniform internal volumetric heat source with exponential decay
governed by the Beer-Lambert law. Three different wall temperature boundary
conditions and various aspect ratios are considered. Analytical solutions for the
velocity and temperature fields are derived and the corresponding profiles are
presented for the Reynolds numbers: Re=0 and Re=100. Some important
hydrodynamic and thermal characteristics are determined, such as: the condition for
flow reversal, Fanning friction factor, bulk liquid temperature and Nusselt numbers
at the walls. It is shown that all other conditions being equal, the existence of a
reverse flow depends mainly on the aspect ratio.
Keywords: liquid viscous convection, vertical rectangular channel, non-uniform
volumetric heat source, analytical solutions, flow reversal condition
Simulation and control of lifting body autonomous vehicle with visual tracking
Valentin Penev, Nikolay Zlatov, Rumen Ivanov, Gary Rowlands
The report outlines research, simulation and control of a lifting body autonomous vehicle with visual tracking system. The lifting body aerodynamic configuration has some advantages and disadvantages. The flight mechanics is highly non-linear, complex and requires contemporary control laws to compensate non-minimal phase behavior of а platform. High demands are put on the numerical methods and solvers to compensate the disadvantages of flight mechanics of a lifting body. The main disadvantage is a generation of oscillations from flight mechanics during the flight and that needs a special treatment by flight control system. Some elements of self-stabilization control surfaces have been used as well as model based control and improvements of attitude heading reference system to reduce the oscillations.
KEY WORDS: Lifting body, Guidance and navigation, Non-linear control
Modernization in Hydropower Plants through upgrades of existing infrastructure and building new capacities
Georgi Todorov, Tsvetan Tsalov, Konstantin Kamberov, Rossen Iliev, Yavor Sofronov, Blagovest Zlatev
Acoustic waves modeling flow in rarefied gas between stationary cylinder at inhomogeneous cylinder wall temperature
Dobri Dankov, Peter Gospodinov, Mirona Mironova
THE AQUATIC VEGETATION DISTRIBUTION AND ABUNDANCE IMPACT ON THE RIVER DISCHARGE DETERMINATION BY VELOCITY-AREA METHOD
Mila Chilikova-Lubomirova, Elena Nikolova, Daniel Diaconu
Open flow discharge measurements are carried out according to clearly defined and ISO certified methods. One of them is the velocity-area method, based on the well-grounded principles of hydrodynamics. It is widely used method that serves also for river discharge determination. But in practice some obstacles impact and may interrupt the obtained results. One of them is connected to the aquatic and benthic vegetation development, a process depending on many factors, connected to the local conditions. To clarify the topic this material is presenting the results of a study observing this method implementation in a real measurement site in the Bulgarian Blato river. Obtained results clearly define and describe the aquatic vegetation and abundance impact on the measured river discharge.
Exact analytical solutions of some fractional partial differential equations in fluid mechanics
Elena Nikolova, Mila Chilikova-Lubomirova
analytical solutions of nonlinear partial differential equations with fractional
derivative order. The fractional derivatives are defined in the sense of
conformable fractional derivatives.The adapted methodology is applied to
solve several evolution equations describing the spatio-temporal dynamics of
different fluid systems. The analytical solutions obtained are supported by
numerical simulations and are interpreted in terms of fluid dynamics.
Recent developments and applications of the Simple equations method for obtaining exact solutions of nonlinear differential equations
Nikolay Vitanov
We discuss the Simple equations method (SEsM) for obtaining exact soltions of nonlinear differential equations. Several recent devopments connected to the elements of the methodology (such as advances in the area of the use of appropriate transformations) are mentioned. Applications of the methodology for obtaining exact solutions of model equations from the area of biology and epidemiology are reported.
Snow water equivalent estimation based on hydrology model simulations and Copernicus satellite observations for the South Central Region of Bulgaria
Polya Dobreva, Olga Nitcheva, Donka Shopova, Zoya Mateeva
Seasonal snow is a key parameter in hydrology and soil-plant research. The assessment of the snow water equivalent (SWE), i.e. water stored in a snow pack, is important for the sustainable water management and climate-smart agriculture. This study presents two approaches in SWE monthly values generation: (i) detailed simulations of water cycle by the Community Land Model (CLM 3) and (ii) using archived and ready for application data from the Copernicus- CGLOPS-2 tool (obtained through a comprehensive methodology integrating snow depth field measurements with satellite observations). The study is carried out on the South Central Region (SCR) of Bulgaria for the 2020 year. The comparison between both approaches shows that the satellite-based SWE data are not applicable for mountain terrains, which are 40 % of the studied territory. For the rest of the SCR territory the values of the Copernicus- CGLOPS-2 SWE database have good convergence with the results from the CLM calculations.
When it happens, how do we know where pollutants are spreading in the Bulgarian Black Sea waters ?
Emil Bournaski, Ekaterina Batchvarova, Yavor Chapanov
Among the pollutants, oil and petroleum products are the most common and dangerous for the Black Sea. These are oil spills at sea during seaport operations and possible accidents at oil terminals and ships. Sea water pollutants are also rivers flowing into the sea carrying oil products, untreated industrial and domestic sewage and other pollutants. To assess the risk of pollution of Bulgarian coastal waters together with their monitoring, it is necessary to model and predict the distribution of these pollutants in the sea. This study uses operational data from the recently established Black Sea Monitoring and Forecasting Center for temperature, sea level, salinity, water currents and mixed layer. The Center is part of the Marine thematic services of the European Copernicus Programme, with the aim of developing and constantly improving an operational forecast and climate reanalysis of the Black Sea state on these parameters. In the present work, the characteristic spatial and temporal circulation of the surface and deeper waters of the Bulgarian Black Sea coast with the corresponding temperature and salinity is expressed. It is possible to provide a 10-day forecast of marine hydrodynamics based on climate reanalysis and regular verification of model results by comparison with observational data. The work supports the assessment of the risk of pollution and safety of the Black Sea from natural factors and from man-made or terrorist acts.
Vortex-Induced Vibrations of an Elastic Geometrically Non-Linear Micro-Beam with Gas Modeled by DSMC
Kiril Shterev, Emil Manoach, Simona Doneva
The fluid–structure interaction is one of the most important coupled problems in mechanics. The topic is crucial for many high-technology areas. This work considers the interaction between an elastic obstacle and rarefied gas flow, seeking specifics that arise during this interaction. The Direct Simulation Monte Carlo method was used to model the rarefied gas flow and the geometrically non-linear Euler–Bernoulli beam theory was used to describe the motion of the elastic obstacle. The fully coupled system of rarefied gas flow and vibrating micro beam was solved numerically. The obtained results were compared with the results obtained considering the small deflection beam theory. The most notable differences between linear and non-linear models were obtained in the resonance area when the frequency of vortex shedding of the flow was close to the natural frequency of the beam. The resonance phenomenon could be used in certain high-technology applications and the correct modeling is important.
Acknowledgments: This work was accomplished with financial support from the Bulgarian research fund, grant KP-06-N72/7, 2023, and the research that led to these results was carried out with the help of the infrastructure purchased under the National Roadmap for Scientific Infrastructure, financially coordinated by the Ministry of Education and Science of the Republic of Bulgaria" (Grant No. D01-325/01.12.2023).
Investigation of Melt Flow and Its Impact on Defects in the Physical Model
Todor Todorov, Yavor Sofronov, Todor Gavrilov, Blagovest Zlatev, Borislav Romanov
The work presented explores the application of a development validation approach through physical testing to enhance injection molding processes for complex plastic parts. The study aims to optimize the manufacturing process by modelling and reducing filling defects as well as reducing production challenges and manufacturing costs. The analysis focuses on minimizing warpage and temperature after cooling. The process includes creating a soft mold tool to validate ergonomics and functionality, as well as to assess the accuracy of virtual simulations. This approach aims to preemptive avoid undesirable defects early in the production stage, ensuring the reliability of simulations and improving overall manufacturing efficiency.
Acknowledgement: The support under project No BG-RRP-2.004-0005 is gratefully acknowledged. The research and analysis for the study are performed by the support of project “Research of the opportunities of developing “active” safety goggles with UV-light face mask implementing functions for fast sanitizing breathing air – ACTIVE PRO UV” - КП-06-Н47/9 and National program "Young scientists and postdoctoral students - 2"
BIOMECHANICS, BIORHEOLOGY AND BIOMATHEMATICS
Stress relaxation of articular cartilage: contribution of flow-dependent and flow-independent viscoelasticity
Stoyan Stoytchev, Svetoslav Nikolov
Stress relaxation of articular cartilage: contribution of flow-dependent and flow-independent viscoelasticity
Stoyan Stoytchev1, Svetoslav Nikolov1,2
1Institute of Mechanics, Bulgarian Academy of Sciences, Sofia, Bulgaria
2University of Transport, Sofia, Bulgaria
stoyan@imbm.bas.bg; s.nikolov@imbm.bas.bg
The articular cartilage consists of two main phases– the solid phase and the fluid phase. The solid phase is chiefly composed of complex macromolecules including collagen and proteoglycans. The fluid phase is presented by interstitial fluid filling in the solid phase’s pores and comprises up to 85 percent of the tissue by weight. The rheological behavior of such poroviscoelastic material during compression depends upon the intrinsic interaction between the solid matrix’s deformation and the interstitial fluid’s motion. The proposed mathematical model is based on the biphasic poroelastic (BPE) theory [1] which couples the interstitial fluid flow and matrix deformation. Additionally, Fung’s viscoelastic quasilinear theory was included. The model equations resulted in partial differential equations for the solid and fluid phases separately, which were solved analytically and numerically. For some practical applications, such as confined cartilage compression, the boundary value problem was solved, and solutions for the solid matrix deformation, fluid flow fields, and stress relaxation were obtained. Further an optimization procedure, using available literature experimental results, was elaborated to estimate the model parameters (hydraulic permeability and short-time and long-time relaxation). Recently it has been shown [2] that the BPE model failed in predicting stress relaxation, that is, the flow-dependent viscoelastic mechanism is not able solely (coincidence 41.4 % of the theoretical and experimental data) to cover the stress relaxation mechanism. In the current presentation, we discuss possible ways to combine fluid flow-dependent and fluid flow-independent viscoelastic mechanisms to account for the true mechanical behavior of articular cartilage under compression.
Keywords: articular cartilage, stress relaxation, viscoelastic mechanisms, partial, differential equation
References
[1] V.C. Mow, S.C. Kuei, W.M. Lai, C.G. Armstrong, Biphasic creep and stress relaxation of articular cartilage in compression: theory and experiments, J. Biomech. Eng., 102:73-84, 1980.
[2] St. Stoytchev, S. Nikolov, Effects of flow-dependent and flow-independent viscoelastic mechanisms on the stress relaxation of articular cartilage, Series on Biomechanics, 37(1):43-50, 2023.
Acknowledgments. This work was supported by the Bulgarian National Science Fund – grant KΠ-06-H57/18.
3D model study of age related changes of main geometric and mass-inertial parameters of Bulgarian females
Vladimir Kotev, Gergana Nikolova, Daniel Dantchev, Mihail Tsveov
Mathematical model for NETosis in the initial stage of systemic lupus erythematosus
Vladimira Suvandjieva, Peter Rashkov
Their role in innate immunity is to neutralize pathogens and to initiate the adaptive immune response. Neutrophils can act in several ways: degranulation (release of cytotoxic molecules), phagocytosis (removal by engulfment and ingestion), and NETosis.
NETosis is a regulated form of neutrophil death, which causes extrusion of nucleus contents (chromatin, nucleosome, cytoplasmic and granular material), proinflammatory cytokines, and antimicrobial peptides from the cell, resulting in its death and the formation of neutrophil extracellular traps (NETs). These traps have a web-like structure which prevent the pathogen from spreading in the organism. NETosis has been reported in infectious diseases such as RSV, HIV, Chikungunya virus, SARS-Cov-2, but it may a sign of an inadequate immune response in chronic diseases such as systemic lupus erythematosus (SLE).
In SLE patients, exposed contents of nucleosome in NETs serve as immunogenic autoantigen, leading over time to an immune reaction directed towards the host itself. Furthermore, microparticles derived from apoptotic cells in the case of SLE have been found to enhance the formation of NETs, leading to a feed-forward effect on the autoimmune response.
We present a mechanistic mathematical model for the network of interactions between neutrophils, macrophages, apoptotic matter, autoantigen resulting from necrotic residue and NETosis. The model uses the approach of models of consumers with multiple resources known from ecology. Bifurcation analysis is used to characterise the effect of several parameters on the asymptotic behaviour of the model, including yield of autoantigen, macrophage recruitment, rate of production of apoptotic matter and transition rate of apoptotic into necrotic matter.
This work is partially supported by Contract No. KP-06-KOST/13 of the Bulgarian Fund for Scientific Research (FNI).
Modeling Humanoid Robot Mouth Expressions Reflecting Different Emotional States via Elliptic Intuitionistic Fuzzy Sets
Velichka Traneva, Stoyan Tranev, Venelin Todorov
keywords: Elliptic Intuitionistic Fuzzy Sets, Humanoid Robot, Index Matrix, Mimics.
Acknowledgments:
Work is supported by the Assen Zlatarov University through project Ref. No. NIX-486/2023 "Modeling Management Decisions with New Analysis Tools in a Fuzzy Environment''.
In vitro investigation of long-term properties of hernia meshes
Miglena Doneva, Dessislava Pashkouleva
Hernia repair is a common surgical procedure in abdominal surgery. The implanted meshes stay in the body many years and is necessary to know how long they retain its properties. The long-term performance in mesh materials can be characterized by in vivo or in vitro experiments. In vitro tests of hernia meshes are: investigation of explanted meshes, or samples placed in the chamber of the thermostatic bath full of buffer solution, to mimic a biological environment similar to human physiology, or comparison of the mechanical behaviour of meshes before and after their expiration date (ED). The study aims to investigate the influence of aging on the mechanical properties of surgical meshes comparing their mechanical properties before and after the expiration date.
Seven types of meshes made of polypropylene and polyester were characterized and compared before and after their expiring date. Long term mesh mechanical properties were determined using quasi-static tensile test. For each specimen maximum tensile stress Tmax, stretch at maximum stress λmax and elastic secant modulus of mesh E5 at 5% strain were calculated from derived stress-stretch curves.
The results indicate age-related changes in the mechanical characteristics of investigated surgical meshes which correlate with the polymeric material.
Gender dependence of body composition characteristics in young Bulgarian tennis players
Gergana Nikolova, Albena Dimitrova, Daniel Dantchev
Understanding the human body's composition is crucial to improving athletic performance. In the current work, a total of 78 tennis players between the ages of 14 and 17 have their body composition characteristics measured. The athletes are divided into two groups: 34 girls tennis players and 44 boys tennis players. Body composition components were determined by means of multi-frequency bioelectrical impedance measurements, with the use of InBody (model: 170) analyzer. We present the mean values and standard deviations of the height (cm), weight (kg), body mass index (kg/m2), muscle mass (kg, %), fat mass (kg), total body water (l), fat free mass (kg) as well as the corresponding probability distributions of these characteristics for each group. The differences in body composition components between boys and girls tennis players are well expressed. The most considerable inter-group differences are observed in terms of muscle mass and body fat mass (%, kg), which have significantly higher values in the male and female tennis players, respectively. The body composition characteristics of tennis players must be continuously studied in order to give athletes and coaches knowledge that will help them create effective and fruitful programs for better tennis performance and injury prevention.
Delay Differential Equations Modelling of Population and Food Storage Dynamics in a Honeybee Colony
Atanas Atanasov, Slavi Georgiev, Lubin Vulkov
the parameters which are not directly observable. The computational results of the sensitivity analysis and the inverse problem study are compared.
Advanced approaches to finding RMSD in bioinformatics studies
Fatima Sapundzhi, Metodi Popstoilov, Meglena Lazarova, Slavi Georgiev
Bioinformatics researchers widely use root mean square deviation (RMSD) to determine the similarity between two data sets. We present an advanced approach to finding this metric to improve the accuracy of RMSD calculations in Manhattan distances.
The results for the similarity between the two structures are compared with previously used methods, showing better results by applying the new approach. This research presents a simple procedure to calculate the RMSD between pairs of 3D structures and to align them to find the minimal value of RMSD in Manhattan distance.
The RMSD calculation web service is developed in C# programming language and is available for bioinformatics researchers who can use it for their research.
Three-Dimensional Analysis of Tooth Inlay Systems under Mechanical Stress: Micro-CT Imaging and Finite Element Simulations
Miryana Raykovska, Roumen Iankov, Ivan Georgiev, Nikoleta Nikolova, Angela Gusiyska, Maria Datcheva
The integration of numerical simulations and micro-CT visualization and reconstruction of the tooth-inlay systems represents a transformative approach to dental science, offering unprecedented insights into the complex dynamics of the oral environment. Micro-CT scans of tooth-inlay systems provide high-resolution, three-dimensional representations of the dental anatomy. These images are transformed into geometrical objects for numerical modelling and simulations, which allow dental experts to explore different phenomena digitally. The performed numerical simulations enable to predict the mechanical behavior and stress distribution of the considered dental structure under various loading conditions and restorative materials. The goal is to assess the performance, durability, and structural integrity of tooth- inlay systems under realistic conditions, providing valuable insights for dental practitioners and researchers in the field of restorative dentistry. The proposed approach significantly streamlines and accelerates the analysis of various restorative materials and cavity forms, saving time and resources, and enabling the selection of models for further research.
On the generalized TASEP on finite open tracks
Alexander Povolotsky, N. Bunzarova, Nina Pesheva
Development of a device and methodology to study blood coagulation by measuring the AC phase shift response in parallel with blood viscosity
Nadia Antonova
Antonova N.* 1, Zlatev R.2, Ramos R.2, Stoytcheva M.2
1 Dept. Biomechanics, Institute of Mechanics to the Bulgarian Academy of Sciences, Akad. G.Bonchev str., Bl.4, 1113 Sofia, Bulgaria, antonova@imbm.bas.bg
2 Instituto de Ingeniería, Universidad Autónoma de Baja California, Mexicali, B.C. México
There are various methods for assessing blood flow properties. The electrical methods used so far electrical conductivity and impedance for their evaluation.
A new approach to studying blood coagulation together with the dynamic viscosity (shear stresses) is proposed and applied. A virtual instrument based on the LabVIEW platform was developed to measure the AC versus phase voltage shift caused by a blood sample at 100 mV p-p AC voltage application in the frequency range between 1 Hz and 10 KHz.
Blood samples from healthy individuals, collected in heparinized tubes, were analyzed following ethical guidelines. Their rheological properties were investigated with the rotational viscometers Low Shear 30 Contraves and LS300 proRheo. Thrombus formation induced in vitro by 0.1 ml of 2% aqueous CaCl2 was studied at constant shear flow ranging from 0.0237 s-1 to 94.5 s-1.
Coagulation kinetics is characterized by an initial gradual increase in blood viscosity, followed by an exponential increase corresponding to intense blood clotting. In parallel, a decrease in blood conductivity is observed.
Families of Bode diagrams in the frequency range of 100 to 1000 Hz recorded over time after the addition of the coagulation agent to the blood sample are obtained. The initial phase shift of the still liquid sample is negative, which correlates very well with theoretical models and the configuration of the equivalent electrical circuit at an electrode-electrolyte interface.
The combination of AC/voltage phase shift with rheological parameters, primarily blood viscosity, allows us to improve the understanding and interpretation of hemorheological disorders to blood circulation.
Keywords: AC current phase shift, blood coagulation, blood viscosity
Disrupting Viral Sabotage: Computational Insights into SARS-CoV-2's Manipulation of Cellular Protein Synthesis and Strategies for Intervention
Nevena Ilieva, Peicho Petkov, Elena Lilkova, Leandar Litov
The idea that coding is a crucial characteristic distinguishing living organisms from non-living entities suggests that information flow – primarily occurring through the processes of transcription and translation in DNA – and its potential obstructions are vital components of living systems. Viral infections can disrupt the cell's normal information flow, hindering regular functions and potentially leading to cell death. This underscores the delicate nature of the cellular information system and the dynamic interplay between living beings and various environmental factors and agents.
By providing computational insights into SARS-CoV-2's manipulation of cellular protein synthesis, we aim to explore the possibilities of computational biology for the intelligent design of intervention strategies against this viral action. Our focus is on peptide aptamers – combinatorial sequences of 5-20 amino acids that belong to the fastest-growing class of therapeutics, the protein-based bioactive agents. Addressing the challenges of intracellular drug delivery, this pilot study may reveal new perspectives for the development of biological agents with the potential for therapeutic applications and practical solutions in medicine and pharmaceutics.
Acknowledgments This research is partly supported by the Bulgarian National Science Fund under grant KP-06-COST-20/2023. We acknowledge the inspiring discussions on the research topic within the COST Action CA21169 DYNALIFE, supported by COST (European Cooperation in Science and Technology).
The impact of changes to fascicle length and to pennation on range of motion
Ines Subashka
Changes in muscle architecture are accompanied by changes in muscle function. The aim of this study was to assess the relationship between changes in rectus femoris and biceps femoris architecture, joint range of motion and the progress in the active and passive front split. The research was conducted in the period between February 2023 – August 2023. The participants followed a 6-month training protocol which included eccentric and isometric muscle efforts performed respectively at long muscle length (LML) and short muscle length (SML). The control group did static stretching. Both groups were involved in the same strength and conditioning workouts. The study was done among 10 men and 34 women, aged 35,91± 6,84. Architectural parameters were measured using ultrasonography imagining. Changes in range of motion were measured using a goniometer and functional parameters were measured through sport tests (results in active and passive front split). The progress in the active front split was bigger in the experimental group - 25,5% more for the left leg and 23,47% more for the right leg, compared to the control group. The fascicle length in the experimental group increased as follows: for rectus femoris destra (RFD) Lf=35,40% and 22,94% for rectus femoris sinistra (RFS). While for the control group the Lf of RFD decreased by 9,68% and by 10,99% for RFS. The results indicate that performing eccentric and concentric muscle efforts, respectively at LML and SML would lead to significant changes in muscle architecture. This will lead to increased range of motion and better results in the active and passive front splits.
Key words: muscle architecture, fascicle length, pennation angle, range of motion
Local carotid stiffness, hemodynamic forces and blood viscosity
Irena Velcheva, Nadia Antonova, Tsocho Kmetski, Galina Tsonevska, Katerina Stambolieva, Blagovest Bechev, Anika Alexandrova
OBJECTIVE: The carotid stiffness is an important factor in the pathogenesis of cerebrovascular small vessel disease. Our study aimed to evaluate the relation of the local arterial stiffness of the common carotid artery (CCA) to the hemodynamic forces and blood viscosity in patients with cerebral lacunar infarctions (LI).
METHODS: Twenty-two patients with chronic LI and 15 age-matched controls were examined. An ultrasound examination of the CCA intima-media thickness (IMT), the parameters of local CCA stiffness: distensibility (DC) and compliance coefficients (CC), α and β stiffness indices and pulse wave velocity (PWV) were performed. The local hemodynamic forces were calculated: circumferential wall tension (CWT) and wall shear stress (WSS). Whole blood viscosity (WBV) and shear stresses at shear rates of 0.277 s-1 to 94.5 s-1 were measured in patients and controls.
RESULTS. Higher values of IMT, a significant decrease of DC and CC and an increase of α and β stiffness indices and PWV in the LI patients compared to the controls were obtained. A parallel significant increase in CWT and a decrease in WSS was found. An increase in WBV and a significant increase in shear stresses were detected. In the LI patients, the increased stiffness indices were associated with an increase in age, cholesterol and WBV at higher shear rates in the left CCA. In the controls, the IMT and stiffness indices correlated significantly with the hemodynamic factors and WBV in both CCAs, while the stiffness indices correlated with the hemodynamic forces in the left CCA.
CONCLUSION. The results of the present study demonstrate different associations of the local carotid stiffness indices with the hemodynamic forces and WBV in patients with LI and controls.
Keywords: lacunar infarctions, common carotid artery, intima-media thickness, local carotid stiffness, hemodynamic forces, whole blood viscosity
Mechanistic models of growth and decay: effect of peptidases on the action of enkephalin analogues
Fatima Sapundzhi, Tatyana Dzimbova, Meglena Lazarova, Slavi Georgiev
Mechanistic models are an important tool in modelling biological processes because they store knowledge about them and enable planning, monitoring and control by accurately describing their in vivo analogues.
Using mechanistic models of growth and decay, the present study examines the effects of peptidases on enkephalin analogue action. Enkephalins act as neurotransmitters and neuromodulators. Additionally, it was found that met-enkephalin has an essential role in cell proliferation and tissue organization during development. A large number of synthetic analogues of these peptides have been created, which have a strong effect due to their binding to specific receptors. Most of the analogues have been tested in vivo, but they also need to be stable once inside the body to have an effect. The main problem with this type of compounds is the fact that they are subject to the enzymatic action of peptidases.
Docking was used to study the action of neprilysin on enkephalin analogues. As the receptor binding is stronger and the peptidase recognition is less powerful, the action of the corresponding analogue will be longer and more effective. A combined approach using mechanistic models of growth and decay and docking can prove useful in the design of biologically active analogues.
Application of machine learning techniques in computational modeling and drug design
Fatima Sapundzhi, Tatyana Dzimbova, Meglena Lazarova, Slavi Georgiev, Metodi Popstoilov
Integrating computational modeling, docking experiments, and artificial intelligence algorithms improves the drug design process's prediction and analysis of interactions between drug molecules and biological targets.
The rapid identification of potential drug candidates has been accelerated by integrating datasets and predicting molecular interactions. A combination of artificial intelligence and machine learning approaches is used in the present study to investigate the relationship between opioid ligand structure and biological activity.
Several machine learning algorithms were used, which enabled more accurate predictions to be made on the data.
Based on the established relationship between docking results and biological activity, we can predict biological effects for newly synthesized analogs compared to other compounds in the tested series. The analysis verifies the correspondence between the models of the biological macromolecules and their actual 3D structures.
GENERAL TOPICS
Modelling And Development Of Vegetables Picking Tool For Agriculture Robots
Vladimir Kotev, Mina Tzoneva, Radoslav Rusinov, George Komitov
In the resent years’ research on the application of robots in agriculture increases rapidly. The present paper shows a research on design, dynamics, and control of a universal picking tool for peppers which will be attached on the standard robot arms. A 3 degree of freedom (DoF) mechanism for grasping and cutting of peppers is proposed. Then its kinematics and dynamics is studied. Based on the results of kinematics and dynamics all motors are selected and 3D CAD model is made. Moreover, a prototype of the vegetables picking tool is developed. It is intended to be attached on robot arms. Taking into account of the kinematics and dynamics a control of the picking tool is developed. Finally, to test performance of the prototype experiments on grasping and cutting peppers is done.
Numerical Solution of Direct and Inverse Problems for Multilayer Diffusion
Miglena Koleva, Lubin Vulkov
A large number of mathematical models describing diffusion processes across layered materials are used in solving of environmental, biological and heat-mass transfer problems. This study focusses on numerical solution of direct and inverse problems of multilayer diffusion. We consider multispecies parabolic models with four types Dirichlet-Neumann-Robin internal boundary conditions that apply at the interfaces between adjacent layers. Instead of external boundary conditions of the first and the last layers, energy integral specifications are posed. First, we perform a Rothe semidiscretization combined with analytical solution of the elliptic problems on each time layer to develop an efficient numerical approach. Second, we solve numerically the inverse problem for identification of the Dirichlrt boundary conditions. Numerical test examples are discussed.
Control Strategies for Adaptive Structures
Spasena Dakova, Michael Böhm, Oliver Sawodny
The increasing demand for living space due to urbanization and the shortage of building materials calls for alternative ways of construction. One possible solution is the development of adaptive high-rise buildings. By actively influencing the structural properties through actuators integrated into the structure, the load case of the building, as measured by sensors, can be influenced. Thus, a significantly lighter structure can be realized, while maintaining the particularly high stress and deformation criteria. However, the optimal operation of adaptive high-rise buildings requires an advanced control concept.
This work describes the controller design for adaptive structures using the example of the worldwide first adaptive high-rise building, built at the Campus of the University of Stuttgart. Hereby, there are two main control objectives – compensation of stationary loads and damping of vibrations. As a prerequisite for the model-based controller design, mathematical models are derived that approximate the structural behavior with sufficient accuracy. The structure is actuated by hydraulic cylinders. The design of a force tracking controller allows the application of predefined forces to achieve the primal control objectives.
Building upon the force tracking controller, an optimization-based method for static load compensation is developed and experimentally validated, taking into account the limited actuator forces. In addition, a model predictive controller for vibration damping is implemented, which also considers the limited actuator forces in the controller design. First experimental results on the vibration damping of adaptive high-rise buildings show the potential of the control strategies.
Lee-Yang theorem and the one-dimensional Ising model: an alternative view and some new results
Nicholay Tonchev, Daniel Dantchev
FOR PRECISION-DERIVATIVE FOURTH-ORDER APPROXIMATION AND SPECIFIED PRESSURE ANGLE
Blagoyka Paleva-Kadiyska, Iliya Andonov, Roumen Roussev, Vitan Galabov
A new computer-applicable mathematical model for the synthesis of slider-crank mechanisms (SCM) has been derived, in which the Burmester curves for infinitely close relative positions are generated indirectly by means of the Carter-Hall circle. Characteristic of the model is that it is linear, easy to implement with any computer program for engineering calculations (eg. MathCAD, Mathlab, etc.) and would greatly ease the work of engineers in synthesis of mechanisms. The mathematical model is composed only of equations of straight, written in Cartesian coordinates, including when determining the kinematic diagrams of slider-crank mechanisms. Introdused an additional condition for achieving a certain value of the pressure angle, which unambiguously determines the kinematic diagrams of the mechanism.
The ease of working with the model and its adequacy with the graphical constructs is illustrated with an example of the synthesis of a SCM that generates a function with а four-order derivative approximation to the given function.
SYNTHESIS OF SLIDER-CRANK MECHANISMS WITH APPLICATION OF THE THEORY OF STATIONARY CURVATURE
Blagoyka Paleva-Kadiyska, Iliya Andonov, Roumen Roussev, Vitan Galabov
This paper validates the developed new computer-applicable linear mathematical model for the synthesis of grippers with slider-crank mechanisms by applying the cubic of stationary curvature theory and in particular the Carter-Hall circle to generate Burmester curves. The model also includes a condition for achieving a certain value of the pressure angle is taken into account, in which the kinematic diagram of the mechanism is unambiguously determined. Through the synthesis of the mechanisms of two grippers, which have a practical application, the adequacy of the mathematical model has been demonstrated. The first is with variable gripping force and the second is with constant gripping force
The proposed new model significantly eases the work of engineers in the synthesis of slider-crank mechanisms, which are very often found in modern devices, such as e.g. some grippers for robots in microelectronics. The new method avoids the shortcoming of known methods for synthesis by infinitely close positions, namely the nonlinear mathematical models for synthesis that are reached with them. The developed linear model is easy to implement with any computer program for engineering calculations (eg. MathCAD, Mathlab, etc.)
Influence of technological parameters of milling on the machinability and defect formation in polymer composites
Krzysztof Ciecieląg, Agnieszka Skoczylas, Jakub Matuszak
Polymer composites are materials with very good strength properties. The strength of a composite material made up of fibers and a matrix is greater than that of steel or aluminum alloys. Composites also have the ability to dampen vibration, they can easily be formed into any shape, have high corrosion resistance and good electrical insulating properties. Combined with their low specific weight, polymer composites constitute a group of materials that are widely used in industry. This creates a need for polymer composites for various applications. Despite various methods of producing polymer composites, finished products are obtained by machining, as it ensures achieving the required shapes and dimensional tolerances. The machining of polymer composites differs from operations performed on steel and aluminum alloys due to the anisotropicity and heterogeneity of their structure. Polymer composites are difficult-to-machine materials and their processing requires the use of appropriate tools and machining conditions. Conventional machining methods such as turning, drilling and milling are widely used to machine polymer composites, which proves the need for investigating phenomena occurring in these processes.
The machining of polymer composites requires the use of appropriate parameters. Variable technological parameters of machining affect the cutting force and surface layer. Previous studies on this problem differ in their results due to the difficulty with machining composite materials. The aim of this study was to determine the influence of cutting speed, feed rate and cutting depth on selected machinability indicators. Obtained results made it possible to define mechanisms affecting cutting forces and surface roughness. Microscopic observations were also made. Since composite machining produces defects in the form of chipping, tearing and cracking of composite components, the observation of the surface after processing made it possible to classify defects and their location.
Cutting forces were determined using Kistler’s 3D dynamometer (type 9257B) in combination with a Kistler amplifier (type 5070). Signals acquired via a Dynoware data acquisition card (type 5697A) were recorded using the Dynowave software (type 2825A). Surface layer examination was carried out using the T8000RC 120-140 device from Hommel-Etamic. The surface after machining was observed with the VHX-5000 digital microscope.A Ginzburg-Landau Ising type mean-field model regarded as a dynamical system
Vassil Vassilev, Daniel Dantchev, Svetoslav Nikolov
In this work, we analyse a finite thermodynamic system described by the mean-field Ginzburg-Landau Ising model. The Euler-Lagrange equation associated with the corresponding φ4 Ginzburg-Landau Hamiltonian is regarded as a conservative dynamical system with one degree of freedom describing the motion of a fictitious particle in a quartic potential. The order parameter profiles, which characterised the state of the considered thermodynamic system depending on its temperature and the external ordering field applied, are associated with the trajectories of the particles in the configuration space of the respective dynamical system. We derive explicit parametrization of the foregoing trajectories in terms of Weierstrass elliptic functions. The order parameter profiles, which minimize the energy functional of the thermodynamic system under consideration are related to orbits in the phase space of the corresponding dynamical system passing near one of its equilibrium points.
Dynamic response of a shallow-draft floating wind turbine concept
Marcin Kapitaniak
This study presents a comprehensive analysis of a novel shallow-draft
Floating Offshore Wind Turbine (FOWT) concept, designed by T-Omega Wind, which
promises to revolutionize wind energy harvesting in deep and harsh sea waters.
The research, conducted at the National Decommissioning Centre, University of
Aberdeen, and the Kelvin Hydrodynamics Laboratory, University of Strathclyde,
delves into the nonlinear dynamics of the FOWT under various sea wave
excitations. The primary focus is on evaluating the system’s heave and pitch
displacements.The FOWT’s innovative design features a lightweight
structure with 80% less structural steel than conventional models, supported by
four stiff interconnected floats. This design not only allows the system to
glide over sea waves but also incorporates a single mooring line to facilitate
a weathervane effect, significantly reducing mooring costs. The system’s
capacity is 10 MW, and its high natural frequency is conducive to operation in
harsh marine environments. Simulations using a real-time multiphysics Marine
Simulator [1] integrated with the Algoryx Hydrodynamics module have been performed
to predict the system’s behaviour. These simulations are validated against
experimental results from a 1:60 scaled prototype tested in a state-of-the-art
water tank facility [2].
The dynamic responses of the system are quantified through
Response Amplitude Operators (RAOs), with phase planes, Poincaré sections, and
FFT analysis employed to identify resonant frequencies and system response
periodicity. The study identifies optimal operating conditions for energy
harvesting and confirms the system’s dynamical behaviour, even under ‘High Sea
State’ conditions. Experimental studies corroborate the simulation results,
demonstrating the system’s reliability and resilience. The findings reveal that
the FOWT concept can float over steep high waves and is economically viable in
deep water. The calibrated model provides an ad hoc damping expression that
accurately predicts system dynamics for ‘High’ and ‘Low’ Sea States. The study
concludes by proposing analytical expressions for RAO responses and offering
validated damping parameters, which confirm the Marine Simulator as an
effective tool for predicting FOWT dynamic responses with reduced computation
time.
References:[1] Martinez R., Arnau S., Scullion C., Collins P., Neilson
R.D., Kapitaniak M., Variable buoyancy anchor deployment analysis for floating
wind applications using a Marine Simulator, Ocean Eng., Vol. 285 (2023), 115417.
[2] Terrero-Gonzalez A., Dai S., Neilson R.D., Papadopoulos
J., Kapitaniak M., Dynamic response of a shallow-draft floating wind turbine
concept: Experiments and modelling, Renewable Energy, Vol. 226, (2024), 120454
DEFORMATION CHECKS ON THE SHAFTS OF BIG BAND SAW MACHINES - CASE A
Boycho Marinov
DEFORMATION CHECKS ON THE SHAFTS OF BIG BAND SAW MACHINES - CASE B
Boycho Marinov
In this study, the influence of the spatial deformations on the upper shaft of big band saw machines is investigated. Deformation checks for the most endangered cross sections of the shaft are performed. For this purpose, an algorithm that includes analytical and numerical part has been developed. Theoretical expressions for calculating the spatial deformations are derived. These expressions are used to calculate the maximum deflections in the most endangered cross sections. The biggest values are compared with the admissible values for the respective material. Normal operation of the shaft, and therefore of the band saw machine is guaranteed if the actual values of the deformations are equal or less than the admissible values.
Modelling of Multiple-Impact Jarring
Idin Nazzari, Ekaterina Pavlovskaia, Marian Wiercigroch
Downhole stuck tools are a significant source of downtime during drilling, and jarring tools can remedy such problems. In a typical jarring operation, axial impact releases a stuck tool during drilling. Innovative jarring tool designs generate multiple and high-frequency impacts but the existing models of jarring operation focus on conventional jarring operation with a single impact. In this work, behaviour of a typical multiple impact jarring tool is examined to understand the effect of design and operational parameters on the tool’s performance. Low-dimensional models were developed comprising lump masses, springs, and viscous dampers, which can mimic the tool’s main function of generating high-amplitude impact forces from the energy stored in the pre-compressed spring. The magnitude and frequency of these impacts are controlled by an overpull and cam mechanisms, respectively. Numerical analysis is performed by direct numerical integration and path-following methods, showing that the system can exhibit co-existing attractors and chaotic behaviour. Parametric studies are carried out to determine the range of design and operational parameters that would enhance the jarring tool performance by generating preferable impact force patterns. It has been shown that external excitation frequency has a major effect on system operation, and it can be used to gain better tool performance.
Modeling musical systems using MATLAB
Ivan Yanakiev
Musical systems for organizing tonal space are the foundation of music. Since Pythagoras, the topic has been a fruitful field for experiments for composers and music theorists. This is possible due to the correlation between human auditory perception specifics and how two or more frequencies relate to one another. The current approach towards modeling musical systems is focused on digitalizing the process for establishing the notes’ frequencies and applying additive synthesis for designing timbre for the tone sets. Moreover a possible application of the digital modelling process of the timbre for devising musical systems with a higher degree of intrinsic consonance in relation to the tonal system specificities.
Keywords: musical systems, tonal systems, intrinsic dissonance, musical modeling.
Dynamic mechanical characteristics of acoustic composites based on epoxy resin and complex structural modifiers
Sergei Bukharov, Nodira Abed, Alexander Alexiev, Roumen Iankov, Maria Datcheva
Nonlinear behaviour of mechanically loaded composite structures with stress concentrators
Ivana Atanasovska, Dejan Momcilovic, Milica Milic, Stepa Paunovic
The nonlinear behaviour of complex real composite structures is still very challenging and covers a wide range of research topics, such as aerospace engineering applications. One of the main problems in this framework is the behaviour of composite structures with stress concentrators, which can originate from the structure's shapes intended for connection or access to other parts of the structures. In this paper, the composite thin plates with stress concentrators in the form of holes with a few different shapes are investigated as an introduction consideration. Also, the real complex composite structure of the UAV is analysed with the developed approach. For assessment of the influence of stress concentrators at the considered composite structures, the Theory of Critical Distances (TCD) is used. The Finite Element Analysis of the modelled cases given pre-calculation of stress gradients of modelled cases. The potential and efficiency of the presented approach for analysing of nonlinear behaviour of composite structures with stress concentrators are discussed.
NUMERICAL SIMULATION OF PLASMA NITRIDING PROCESS FOR STAINLESS STEEL GEARS
Yavor Sofronov
Correlation between the Workspace and the Dynamic Bandwidth of a Production Machine - Practical Guideline for Use in a Design Department
Gerhard Kehl
In order to achieve high productivity, production machines must perform precise and fast movements. On the other hand, static and dynamic process forces also occur, which place high demands on the compliance behavior. Ultimately, the NC program, which includes productive main times and unproductive secondary times (e.g. tool and workpiece changes), can be broken down into individual movements between two points in the workspace. The positioning time between two points is mainly influenced by the dynamic bandwidth of the production machine. To predict the machining time of an NC program, it is necessary to know the adjustable kinematic and control parameters already during the machine design phase, at a time when a prototype of the machine does not yet exist in real terms, but only as a digital twin. An essential component of the digital twin is a finite element model, which in particular represents the vibration behavior. The above kinematic and control parameters depend, among other things, on the dominating mechanical natural frequency and damping of the movement axes of the machine. While axis speed and acceleration can be influenced by the choice of suitable drive elements, axis jerk and control parameters are strictly limited by the vibration behavior of the machine, especially the overshoot at the tool center point. The higher the natural frequency and damping, the higher the axis jerks and controller gains can be set without overshooting beyond the target position. However, when developing machines across series, the question arises to what extent the natural frequency changes when the machine is scaled to a changed workspace. Experience shows that high dynamic bandwidth of a machine with a small workspace (e.g. for milling of metallic housings for luxurious watches) cannot be achieved by a machine with a large workspace (e.g. for milling engine blocks for truck combustion engines), even if a comparable drive design (e.g. with servo motors and ball screws) is used. The paper provides explanations for this and presents a usable scaling rule to predict the dynamic bandwidth as a function of the machine workspace. The validity is shown on basis of the wide product range of a German machine tool manufacturer.
MANUFACTURING INDUCED DAMAGE AND ITS METROLOGICAL IDENTIFICATION
Mechanical damage to biological objects: causes, methods of identification, consequences
Serhii Kharchenko, Sylwester Samborski, Farida Kharchenko
Technological processes in agricultural, food and pharmaceutical industries involve mechanical impact of working parts of the equipment on biological objects. As a result of the action of working parts, such as mixers, sifters, dispensers, conveyors, etc., or impacts against elements of their structure, external force is exerted on the surface of the biological object. When the restraining internal stresses of the biological object are exceeded, damage occurs.
The magnitudes of external impact and dynamic (shock) load on the biological object depend on the structural-kinematic parameters, properties of materials, and coatings of equipment parts. Internal stresses are determined by the properties of the material, as well as the size and shape of the biological object.
The consequences of damage are reflected in: intensive development of microflora in damaged areas, reduction of quality and duration of storage of final products, as well as loss of reproductive properties of biological objects.
The common classification of mechanical injuries is based on the aspects of damage magnitude and is grouped into micro- and macrodamage. It should be noted that this classification does not provide an understanding of damage localization relative to the structure (elements) of a biological object. The conducted studies confirmed the influence of coordinates of damage location relative to the embryo of biological objects — seeds of agricultural crops on their reproductive properties.
For a comprehensive analysis of damage, the following parameters are proposed: the damage coefficient — the ratio of the volumes of all damages to the volume of the biological object, the damage intensity coefficient — characterizes the concentration of cracks in a specific area and is equal to the ratio of the volumes of damages to an individual structural element of the biological object; the damage localization coefficient - characterizing the location of damage relative to the reproductive element of the object.
To identify the damage parameters, the high-resolution 3D X-ray system of the Zeiss Xradia 510 Versa tomograph was used, which allows to identify the damage inside and outside the objects. The obtained images were analysed and the relevant parameters were determined.
The research algorithm consisted of the following stages: static and dynamic loading of objects, tomography of objects, analysis and identification of damage parameters, and the establishment of patterns in the development of damage.
The use of the proposed methodology has allowed for the identification of patterns in the occurrence and propagation of damage in biological objects, depending on their properties, point of application, type and magnitude of external load. The obtained data serve as the basis for the development of methods to reduce the level of mechanical damage to biological objects in the technological processes of various industries.
Acknowledgments. This equipment is part of a project No. 2022/45/P/ST8/02312 co-funded by the National Science Centre and the European Union Framework Programme for Research and Innovation Horizon 2020 under the Marie Skłodowska-Curie grant agreement No. 945339.
Tracking delamination front orientation with equipollent vector
Sylwester Samborski
The study shows application of Python scripting to harvest encapsulated data from the Abaqus *.odb files containing results of finite element simulation of interlaminar cleavage in elastically coupled CFRP beams. The concept of equipollent vector is an analytical tool for predicting the direction of delamination front. A set of results coming from the finite element models of beams exhibiting crack onset and propagation, exploiting the virtual crack closure technique at different composite layups proves usability of the formulated vector approach in numerical analyses of laminated structures with defects.
Application of the Acoustic Emission Technique Supported with Machine Learning Algorithms to Damage Assessment in the Elastically Coupled Composite Laminates
Sylwester Samborski, Jakub Rzeczkowski, Jakub Skoczylas, Aleksander Czajka
Keywords: acoustic emission, DCB test, composite laminates, machine learning
Acoustic emission (AE) is a non-destructive testing technique, belonging to the group of structural health monitoring (SHM) methods proved to be highly effective in detecting and monitoring of damage and failure in different structures. In current research the mode I double cantilever beam (DCB) test [1,2] was conducted on carbon/epoxy laminate with the bending-twisting elastic couplings according to the ASTM D5528 Standard. All experiments were supported with visual high resolution registration of propagated crack tip, as well as with the measurements of elastic waves generated by developing internal damages by using the acoustic emission set Vallen AMSY-5 equipped with two piezoelectric sensors. In addition, different damage mechanisms occurring during crack propagation process were quantitatively evaluated by using the k-means and the Gaussian mixture model machine learning algorithms.
ACKNOWLEDGEMENTS
The grant was financed in the framework of the pro-quality program of Lublin University of Technology „Grants for grants” 2/GnG/2023
[1] Rzeczkowski J, Samborski S, Prokopek K. Experimental Determination of the Mode I Fracture Toughness in FRP Laminates with Hybrid Delamination Interfaces. Adv. Sci. Technol. Res. J. 2022;16(5):129–35.
[2] Rzeczkowski J, Samborski S. Experimental and Numerical Research of Delamination Process in CFRP Laminates with Bending-Twisting Elastic Couplings. Materials (Basel) 2022;15(21).
The influence of surface preparation and thermal shocks on the strength of the adhesive joint
Mariusz Kłonica
Nowadays, a geometric structure of surface is quite important because of the following significant reasons: functional, exploitative and esthetic. Recently, an intensive development of various metrology techniques for a surface layer gives the opportunity to predict the functional and the exploitative attributes of the surface. Selected 2D and 3D surface roughness parameters were analyzed after various methods of preparing the surface layer in terms of adhesive joints.
The bonding technology, thanks to a progress in the adhesive materials chemistry is continuously developing. The effectiveness of bonding depends largely on proper selection of adhesive joint, as well as an adequate technique of surface treatment. Many factors have effect on this, first of all increasing the bonding effectiveness thanks to the modern, fast curing adhesive joint, an ease of tailoring the adhesive joint properties, through a simple and cheap chemical or physical modification, as well as the growing trust to the discussed joining technology. Adhesive bonding is increasingly being applied as structural joining technique in highly reliable machines and appliances operating in the circumstances of variable thermomechanical loads.
The manufactured specimens of the epoxy material had been subjected to cyclic thermal loading, with respect to a defined program, in a thermal shock chamber within a temperature range of ‑40°C to +60°C in a different number of cycles. The strength of the adhesive connection after various methods of preparing the surface layer of construction materials as well as after thermal load was analyzed.
Selected machining indicators in machining magnesium alloys
Ireneusz Zagórski, Mariusz Kłonica
3D & 4D PRINTING: FROM MATERIALS DESIGN TO STRUCTURAL AND FUNCTIONAL APPLICATIONS
Nanoindentation and nanoscratch testing of new composite materials based on high density polyethylene
Todor Batakliev, Vladimir Georgiev, Evgeni Ivanov, Rumiana Kotsilkova
Recently, the interdisciplinary field of hybrid polymer nanocomposites has gained significant attention due to the rising demand for high-performance materials in various industries such as aerospace, automotive, electronics, and energy. In many cases the goal of the scientific research is to harness the synergistic effects of at least two types of unique nanoparticles blended in polymer matrix in order to overcome the constraints of using individual nanomaterials. The current study was focused on the nanomechanical characterization of high density polyethylene (HDPE) based composite samples modified with equal loading of graphene nanoplatelets (GNPs) and/or multiwall carbon nanotubes (MWCNTs). Quasi-static nanoindentation analysis demonstrated the impact of the carbon nanofillers on the receiving of nanocomposites with higher mechanical parameters as hardness and reduced modulus of elasticity. The appearance of indentation size effect in case of elastic polymer matrix has been assessed by applying three distinct peak forces in the trapezoidal load function. Nanoscratch experiments outlined the tribological behavior of the nanocomposite samples and inferred the influence of the carbon nanofillers on the values of the coefficient of friction (COF). It seems that the incorporation of 4wt% GNPs in the polymer structure improved the scratch resistance of the material resulting in higher value of the exerted lateral force and therefore leading to detection of higher coefficient of friction at scratch. Considerable pile–up response of the scratched polymer specimens was observed by means of in–situ SPM imaging of the tested surface sample area. The role of the carbon nanoparticles in the composite pile–up behavior and the effect of the pile–up on the measured friction coefficients have been explored.
Acknowledgements: This work was supported by the Project No. BG-RRP-2.011-0001-C01/, funded by the European Union through the NextGenerationEU instrument (NextGenerationEU).
Comprehensive Investigation of Thermal Characteristics in Lightweight 3D Printed Graphene-Based Discs Featuring Customized Air Cavities: Experimental, Theoretical and Numerical Approaches
Evgeni Ivanov, Giovanni Spinelli, Rosella Guarini, Rumiana Kotsilkova, Vittorio Romano
Efforts to reduce mass and optimize material usage are main considerations during structural
design phases, especially in industries like aerospace, where lightweight yet robust structures are particularly required. Sandwiched composite structures, often employing honeycomb panels, have gained traction due to their ability to minimize weight while maintaining structural integrity. Despite their effectiveness under heavy loads, these structures often face challenges in terms of thermal efficiency. Contemporary design endeavours aim to create multifunctional structures that exhibit superior heat transfer properties while upholding structural integrity. This goal is particularly relevant in the electronic industry, where emerging devices operating at higher frequencies necessitate effective heat dissipation strategies. Traditionally, polymers have been overlooked in thermal management due to their inherent low electrical and thermal conductivity. However, their favourable attributes, including lightweight nature, cost-effectiveness, ease of processing, corrosion resistance, and high strength-to-weight ratio, have sparked renewed interest in their application. Recognizing the potential of polymer composites in thermal management, there is a growing consensus that developing advanced composites with enhanced thermal conductivity could offer a viable solution. Recent advancements in this field have leveraged nanotechnology, particularly carbon-based fillers, to improve the thermal conductivity of polymer matrices. Integrating these highly thermally conductive fillers into polymer matrices has shown promise in improving the overall thermal performance of composite materials. Moreover, the recent additive manufacturing technology, also known as 3D printing, due to its extreme versatility, enables the easy creation of complex and intricately designed shapes of parts to enhance their thermal transport properties. The present work presents a comprehensive exploration of the thermal behaviour of 3D printed discs, highlighting recent advancements in material science. Filaments composed of pure poly(lactic acid) (PLA) and PLA filled with 6 wt.% of graphene nanoplatelets (GNPs) were utilized as raw materials. Experimental, theoretical, and simulation results indicate a notable enhancement in thermal conductivity, with values increasing from 0.167 [W/mK] for unfilled PLA to 0.335 [W/mK] for reinforced PLA, representing a significant improvement of 101%. Employing the capabilities of 3D printing technology, various air cavities were intentionally designed to create lightweight and cost-effective materials without compromising thermal performance. Furthermore, cavities of equal volume but different geometries were explored to understand their impact on thermal behaviour compared to air-free specimens. Additionally, the influence of air volume was investigated. The findings, supported by theoretical analyses and simulation studies using the finite element method, serve as a valuable reference for the design and optimization of advanced lightweight materials.
Acknowledgments: This work was supported by the Project No. BG-RRP-2.011-0001-C01/, funded by the European Union through the NextGenerationEU instrument (NextGenerationEU). The authors would like to acknowledge the EU Horizon 2020 RISE Project ReACTIVE Too (Grant Agreement No 871163) and the Bulgarian National Science Found (BNSF) under Grant No. KP-06-H77/4.
On the interplay of electrical, thermal and mechanical behaviour of PLA-carbon nanocomposites for 3D and 4D printing applications
Rumiana Kotsilkova, Stiliyana Stoyanova, Vladimir Georgiev, Todor Batakliev, Evgeni Ivanov
Four-dimensional printing (4DP) technology is a next-generation additive manufacturing that enables printed objects to further change their shapes, functionalities, or properties upon exposure to external stimuli. Thus, thermo-responsive shape-memory (t-ShM) polymers, which possess the ability to change shape by exposure to heat, has attracted particular interest in recent years. ShM behavior has been observed in thermoplastics such as polylactic acid (PLA) available as 3D printing (FDM) filaments. Heating ShM thermoplastics above the glass transition temperature (Tg) accelerates their stress relaxation process leading
to self-deformation and self-bending. Conductive nanofillers can introduce additional functions and improve the properties and 3D-printability of thermally stimulated ShM polymers, to make them electroactive and to activate their ShM function indirectly, by Joule resistive heating. Multicomponent approaches help also to improve the mechanical properties of materials and to introduce new actuation properties of such 4DP objects.
The present work is focused on the mechanical, electrical and thermal behaviours of multicomponent composites of polylactic acid (PLA) filled with graphene and carbon nanotubes in order to investigate their effectiveness for 3D and 4D printing applications. The incorporation of the conductive carbon particles within the polymer matrix allows formation of programmable conduction paths, whose electric properties are intimately coupled to thermo‐mechanical processes. Different types of multifunctional characterisation have been made: (i) structural, mechanical, electrical and thermal characterization; (ii) thermo‐electrical tests, analysing the influence of temperature on the DC resistance of the samples; and (iii) electro‐thermal tests, evaluating the influence of electrical conductivity on the sample temperature due to Joule heating. The results show that PLA-carbon nanocomposites demonstrate high electrical and thermal conductivity and good mechanical properties suitable for 3D printing (FDM) applications. The observed strong thermo‐electrical and electro‐thermal interplays confirm the potential of the biodegradable PLA polymer with graphene and carbon nanotubes for 4Dprinting application with thermosensitive ShM functionality induced by controlled Joule heating.
Key words: electroactive nanocomposites, Joule heating, Shape memory, 3D and 4D printing
Acknowledgements: This work was supported by the Project No. BG-RRP-2.011-0001-C01/, funded by the European Union through the NextGenerationEU instrument (NextGenerationEU).
Exploring the Potential of PVDF/CNT/GNP Composites for Self-Regulating Thermal Management
Vladimir Georgiev, Todor Batakliev, Evgeni Ivanov, Rumiana Kotsilkova
The introduction of nanoparticles, such as multi walled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs), can improve the thermal and electrical properties of PVDF. These materials are suitable and promising additives because they have high thermal and electrical conductivity, which can contribute to obtaining composite materials with applications in electronics and automotive as temperature sensors or flexible heating elements.
The aim of this study is to investigate the Joule effect and temperature coefficient of resistance (TCR) of 3D printed composites on the base of PVDF.
Composite PVDF/MWCNT, PVDF/GNP and PVDF/MWCNT/GNP materials were fabricated by melt mixing in a twin-screw corotating extruder. Thermal conductivity and specific heat capacity were measured by LFA analyzer. The thermal properties of the composites were investigated by differential scanning calorimetry (DSC). The Joule effect was investigated by measuring the temperature when a constant voltage was applied. TCR is defined by the variation of electrical resistance with temperature. It was found that at elevated temperatures, the composites exhibited a slight increase in electrical resistivity, which is mainly attributed to breakdown of interconnected MWCNT or GNP networks resulting from volumetric expansion of PVDF matrix in the heating process. The Joule effect of the composites varies according to the type of fillers. It was found out that the composite with content 6% GNP/PVDF showed highest Joule effect. This behavior can be attributed mainly to the superior thermal and electrical conductivity of GNPs, optimal filler dispersion, and the effective formation of conductive networks within the PVDF matrix. When embedded in PVDF, they create pathways for easier electron flow, leading to higher overall conductivity of the composite. Further, inclusion of the filler improves the material's ability to conduct and distribute heat generated by the Joule effect throughout its volume. This efficient heat dissipation prevents localized overheating, allowing the composite to handle higher Joule heating power without detrimental effects.
The authors gratefully acknowledge the support of the Bulgarian National Science Fund (BNSF) under Grant No. KP-06-H77/4.
COMPLEXITY IN ENGINEERING SYSTEMS
Buckling load of columns with variable cross section and flexible end restraints
Alexandre Wahrhaftig, Moshe Eisenberger
Mitigating Drillstring Stick-Slip: Effects of Delay and Constraints
Vahid Vaziri
RECENT TRENDS IN VIBRATION INDUCED ENERGY HARVESTING
Performance of a Nonlinear Energy Sink in Energy Harvesting
Rahul Das, Anil Bajaj, Sayan Gupta
highly effective across various applications. Typically, a traditional vibration-
based energy harvesting device includes a harvesting component, often com-
posed of smart materials, attached to a primary oscillator (PO), functioning
efficiently within a narrow frequency band around the PO’s natural frequency.
The operational bandwidth can be enhanced through the introduction of non-
linearities in the system. One such approach involves the use of a Nonlinear
Energy Sink (NES), comprising of an essentially nonlinear oscillator attached
to a PO, that generally behaves as a linear oscillator. The frequency energy
dependence resulting from nonlinearity offers the potential for the system to
reach resonance conditions across wide frequency bands, which can be exploited
for unidirectional energy transfer.
The present study investigates the energy flow characteristics of a nonlinear
energy sink, where the PO also behaves nonlinearly. The primary system con-
sists of an oscillator with cubic stiffness coupled with an essentially nonlinear
oscillator also featuring cubic stiffness. The tuning is carefully adjusted to en-
sure unidirectional energy flow towards the nonlinear attachment. Analytical
investigations are conducted to understand the mechanisms and conditions for
achieving unidirectional energy transfer, aiming to enhance efficiency through
proper tuning of system parameters. Computational studies complement the
analysis, exploring resonance capture in the system in comparison with the
modal response of the underlying Hamiltonian system.
Nonlinear energy sink with energy harvester
Mohammad Dalroti
Fractionally damped multi-beam system for efficient piezoelectric energy harvesting
Stepa Paunovic, Ivana Atanasovska
Nonlinear Energy Harvester in Rotating Beam Design – Theoretical and Experimental Study
Jarosław Latalski, J. Warminski
Piezoelectric materials exhibit the mutually reversible electro and mechanical coupling effects. They undergo mechanical deformations when placed in an electric field and become electrically polarized under mechanical stress. These effects are referred to as direct and converse piezoelectric effects, respectively. While the later phenomenon is commonly used for structural control and vibrations mitigation, the direct effect can be used for ambient energy harvesting. This relatively new research area has attracted much interest of scientific community, because of its huge potential for commercial applications. These could be, for instance, power supply devices used by monitoring sensors or to power autonomous structural operation.
Initial studies on piezoelectric materials exploited the linear constitutive relations formulated in early years of 20th century. However, most recent studies reveal nonlinear piezoceramic material behaviour, even when subjected to low electrical or mechanical loadings. In this research an energy harvester comprising a hub and clamped rotating beam with active material layer is studied. The research begins by proposing and conducting a laboratory experiment to confirm the nonlinear constitutive properties of the piezoceramic material. Specifically, the accelerance and mobility response characteristics are plotted, and the profiles of the backbone curves are examined. To identify and evaluate the electro-elastic coefficients of active material a nonlinear constitutive model representing the tested piezoceramics is developed. Next, its approximate solution is compared to the outcomes of laboratory tests. Finally, a system of nonlinear constitutive relations representing the properties of the tested piezoceramic material is written.
In the next stage, a mathematical model of structure comprising a hub and active composite beam is developed. In the analysis, the already identified nonlinear constitutive behaviour of the piezoceramic material is incorporated. The derived system of governing equations involves three integro-partial differential equations, mutually coupled. They represent the electromechanical behaviour of the beam (that is transverse displacement and transducer output voltage) and the hub rotation coordinate. Next, these state equations are solved numerically. Presented parametric studies capturing various design configurations and torque excitations scenarios provide deep insights into the system's performance and nonlinear dynamics.
Numerical simulation and optimisation study of vibration-based piezoelectric energy harvesting device
Zhan Zhelev
Energy harvesting, the process of capturing and converting energy from ambient sources, has gained significant attention as a sustainable solution for powering low-energy devices. The study examines analytically an electro-mechanical system designed to harvest energy - piezoelectric unimorph. The system comprises a composite cantilever beam with a piezoelectric layer bonded to a structural layer, and a tip mass attached to its free end.
The mechanical properties of the beam are modelled to capture the dynamic behavior under harmonic excitations. The vibrations of the structure induce strain in the piezoelectric material, leading to the generation of electrical voltage. Key parameters, such as the length of the piezoelectric layer, the thickness of the structural layer, the tip mass and the material properties of the structural layer, are systematically varied to evaluate their impact on the power output.
The numerical analysis, conducted using ANSYS APDL, reveals the optimal configurations that maximize energy conversion efficiency. The findings contribute to the development of more effective piezoelectric energy harvesters, with potential applications in powering wireless sensors, wearable devices, and other low-power electronics.
MECHANICS-BASED INTERPRETATION OF STRUCTURAL MONITORING DATA
Damage detection of elastic structures by vibration based methods and supervised learning
Stanislav Stoykov, Emil Manoach
Beams and plates are basic structural elements that find wide application in various engineering constructions. Early detection of damages and their accurate locations are critical for the operation and maintenance of these structures.
The current work presents and compares two types of numerical methods for damage detection. The first approach uses vibration-based methods and is based on a comparison of the time responses of intact and damaged structures. A selective criteria is imposed on the curvature of the structure and its time derivatives.
The second approach uses a supervised learning method. A neural network is designed and trained using the time responses of damaged structures. The training data is generated by solving the equation of motion of a damaged structure in the time domain for different external forces and various locations of the damage.
The numerical methods for damage localization are applied to beams and plates. Timoshenko’s theory is considered for beams, and Mindlin’s hypothesis is applied for the plate equation of motion. The finite element method is used for space discretization, and the Newmark time integration method is used to solve the systems of ordinary differential equations. Geometrical nonlinearity is assumed in both models. The damage to the structure is modeled by reducing the thickness of one or more finite elements.
Both methods are shown to localize damages precisely. The neural network has the advantage that it is capable of localizing damages even when the input data has additional noise, unlike the first approach, which uses derivatives in the selective criteria.
Nonparametric Phase Space Reconstruction: Methodology and Applications
Dayang Li, Maosen Cao, Emil Manoach
SIZE DEPENDENT CONTINUUM MECHANICS OF NANO-MICRO SCALE STRUCTURES
Dynamic Analysis of Axially Graded Nanobeams with Moving Support
Mustafa Arda, Tunahan Pamukcu
Axially graded nano structures have lots of potential application areas like nano mass sensors and fiber optics. Present study will be dealed with the vibration problem of axially graded carbon nanotubes with moving boundary condition. Carbon nanotube will be modeled as a hollow circular nano-beam with the help of nonlocal elasticity theory. Material properties variation will be considered in the length(axial) direction. Clamped-simply supported boundary conditions for the nanobeam will be considered. The simple support is considered a moving boundary which has a constant velocity. Effects of material variation, nonlocality, and support position to the vibration frequency of axially graded nanotube will be investigated. Present study results could be useful at design of nano sensor applications.
Coupled Axial-Torsional Dynamics of Carbon Nanotubes
Mustafa Arda, Metin Aydogdu
Carbon nanotubes are one of the promising materials for engineering applications. With their superior physical properties, nanoscale structures have lots of potential in aerospace, chemical, pharmaceutical, communication and CPU industries. To better understanding of the behavior of nanostructures in various physical environments, mechanical modeling has significant importance.
A carbon nanotube is a hollow rod structure and cylindrical shell theories can be used in modeling for this type of structures with three-dimensional stress-strain relations. Unimodal theories (like Rayleigh) can be used for considering only axial or torsional strain. Also, nonlocal elasticity theory should be used to include the size dependency effect in nanoscale.
Carbon nanotubes can be considered as a very thin structure according to its length. For the simplicity, a new model is proposed in the present study. A unimodal equation for the coupled axial-torsional deformation of hollow rod will be obtained with excluding the radial deformation. Nonlocal elasticity theory will be implemented to the proposed model for considering the length scale effect. Wave propagation results of the proposed model will be compared with cylindrical shell model. Free vibration case studies at various boundary conditions will be investigated with the novel rod model.
Present study results could be useful at design of fluid conveying carbon nanotube systems.
A generalized formulation of first strain gradient elasticity
Martin Lederer
Strain gradient elasticity is an enriched version of continuum mechanics, where in addition to the first gradient also the second gradient of displacements is considered in the expression for the elastic energy density. This leads to the advantage that size dependent material properties can be explained. However, the determination of the length scale parameters of the theory is a difficult task. Depending on the method used, one derives different values for the material dependent parameters. If one determines the parameters from fits to bending experiments performed with micro cantilever beams, then length scale parameters in the range of a few µm are derived. But if one instead uses the phonon dispersion relation for determination of the parameters, then length scales in the nm range are obtained. The present investigation attempts to resolve this contradiction by introducing additional terms, which are added to the energy density. By analogy to Mindlin’s theory, there is an energy contribution proportional to the square of strain gradients, whereby the correlated proportionality constants have the dimension of a force. However, in the new formulation also square roots of fourth order terms in strain gradients are allowed instead of using just quadratic forms of strain gradients. Consequently, one arrives at a theory with a larger number of material parameters, and it is therefore easier to distinguish different deformation modes from each other. In conclusion, one can describe the stiffness of all relevant deformation modes with one consistent set of material parameters. In order to evaluate this material behaviour, a Finite Element implementation based on the penalty method is elaborated. Finally, the simulation method is demonstrated for typical examples of small scaled structures.
Higher order Haar wavelet method for vibration analysis of nanostructures
Jüri Majak
Higher order Haar wavelet method (HOHWM), introduced by authors in 2018 in [1] as principal improvement of the widely used Haar wavelet method, is adapted for vibration analysis of uniform and functionally garded nanostructures. Study is focused on tehoretical and numerical convergence analysis, accuracy estimates and complexity analysis (numerical ans implementation complexities). The widely used Haar wavelet method introduced vy Chen and Hsiao in 1997 is utilised as reference solution. The HOHWM has been used with success for solving differential and integro-differential equations by several authors[2-5], but need still further study to cover more complex problems.
1. Majak, J.; Pohlak, M.; Karjust, K.; Eerme, M.; Kurnitski, J.; Shvartsman, B. S. New higher order Haar wavelet method: Application to FGM structures, Composite Structures,2018, 201, 72−78. DOI: 10.1016/j.compstruct.2018.06.013.
2. Arda, M. Majak, J. Mehrparvar, M. Longitudinal Wave Propagation in Axially Graded Raylegh–Bishop Nanorods. Mechanics of Composite Materials, 2024, 59 (6), 1109-1128, https://doi.org/10.1007/s11029-023-10160-4
3. Sorrenti, M., Di Sciuva, M., Majak, J. and Auriemma, F. Static Response and Buckling Loads of Multilayered Composite Beams Using the Refined Zigzag Theory and Higher-Order Haar Wavelet Method. Mechanics of Composite Materials, 2021, 57(1). https://doi.org/10.1007/s11029-021-09929-2
4. Majak, J., Shvartsman, B., Ratas, M., Bassir, D., Pohlak, M., Karjust, K., et al. Higher-order Haar wavelet method for vibration analysis of nanobeams. Mater Today Commun, 2020, 25. https://doi.org/10.1016/j.mtcomm.2020.101290
5. Mehrparvar, M., Majak, J., Karjust, K. and Arda, M. Free vibration analysis of tapered Timoshenko beam with higher order Haar wavelet method. Proceedings of the Estonian Academy of Sciences, 2022, 71(1). https://doi.org/10.3176/proc.2022.1.07
Multi-parameter optimization of polymer nanocomposites under combined loading
Tatyana Petrova, Elisaveta Kirilova, Boyan Boyadjiev, Rayka Vladova, Apostol Apostolov, Petia Dineva, Hongyu Tang
In the present study, the influence of the geometry at nano and micro level (layer thicknesses and length) and the magnitude of thermo-mechanical loading on the delamination in polymer nanocomposites, is theoretically investigated. The analytical solutions for the interface shear stress (ISS) in the middle layer of the structure are obtained, based on the application of 2D stress-function method and minimization of the strain energy [1]. Then, the theoretical criterion for delamination in the interface layer (non-linear equation in terms of debond length), based on the ISS model is formulated, including the structure geometry and loading as parameters. A multi-parameter optimization problem including this criterion is defined and solved. By simultaneously varying these parameters, the safety intervals of the parameters (without delamination) in the considered nanocomposite structures are obtained, for several case studies with different 2D nanomaterials. It was found, that the magnitude of the applied load mainly affects the magnitude of the ISS. Layers thicknesses mostly affect the type of ISS solution, especially the substrate thickness. The effect of layer length on ISS is weaker than that of layer thickness at a fixed load. The influence of temperature loading on the delamination appears to be important in cases where the magnitude of the mechanical load is close to the limit values for which delamination is observed. The obtained results can be used for fast prediction of delamination and appropriate design in similar nanostructured devices to assure their safety work.
Keywords: 2D nanomaterials, analytical modeling, multi-parameter optimization, nanocomposite geometry, safety load
Acknowledgement
The authors gratefully acknowledge the Bulgarian National Science Fund for its financial support of this work via the contract No. КП-06-Н57/3/15.11.2021 for project “Optimal safe loads and geometry for layered nanocomposites under thermo-mechanical loading”
References
[1] Petrova, T. St., Analytical modeling of stresses and strains in layered nanocomposite structures - opportunities and challenges, Bulgarian Chemical Communications. 55(3): 349-366, 2023.
MECHANICAL PROPERTIES OF BLOOD AND LYMPH VESSELS – THEORY, EXPERIMENT AND MEASUREMENT.
Deterioration of endothelial function caused by vascular collapse during blood pressure measurement: Phenomenon found in human and mechanisms examined in rabbits
Takeo Matsumoto
To investigate the impact of this phenomenon on endothelial function, we initially conducted the flow-mediated dilatation (FMD) test. In this test, a cuff wrapped around the forearm is pressurized, interrupting blood flow for 5 minutes, and the relative increase in diameter of the brachial artery is measured as the FMD value after releasing the pressure. FMD value serves as an indicator of endothelial function because the increase in wall shear stress resulting from the resumption of blood flow triggers nitric oxide production by endothelial cells, leading to the relaxation of smooth muscle cells and artery dilation. Fifteen adult male and female subjects aged 22 to 66 years underwent three cycles of vascular collapse, mimicking blood pressure measurement. Following this procedure, the FMD value decreased by 57% (P < 0.05).
To delve deeper into this phenomenon, we established an FMD reproduction system in the common carotid artery of Japanese White rabbits. Utilizing this system, we induced artery collapse to simulate blood pressure measurement and observed reductions in FMD values, vascular endothelial cell density, and sugar chain volume per cell by 55%, 11%, and 20%, respectively (P < 0.05) after collapse. These findings suggest that the observed decrease in endothelial function during human blood pressure measurement may be attributed to cell detachment and a reduction in the sugar chain layer caused by vascular collapse.
BIOMECHANICS AND TRANSPORT IN THE LYMPHATIC SYSTEM
James Moore
Lymphatic vessels serve to return interstitial fluid centrally to the blood system, a process that relies on their biomechanical properties and active pumping. Prior to its return to blood, lymph must be “filtered” for antigens as an important component of the immune system. Lymph Nodes serve as information collection/distribution hubs for innate and adaptive immunity, actively delivering antigen information to the appropriate cell types. The size of a human lymph node (order of cm) is such that diffusive and flow-mediated processes are required to distribute information effectively. Specialized blood vessels also run throughout lymph nodes, providing transmural transport of millions of lymphocytes. Small (<70kDa) antigens arriving in solution via afferent lymph are transported via conduits formed of collagen fibers, while larger antigens are distributed through the parenchyma. Antigen presenting cells follow chemokine gradients to find their way to the appropriate nodal compartments. Both antigens and chemokines can be transported by diffusion or advection. Using a combination of multiscale experimental and mathematical approaches, we have investigated the transport of solutes and cells from interstitial spaces into and through lymph nodes. Our results indicate that transport in conduits is nearly entirely diffusive, with collagen fibers providing anisotropic diffusivity. This delivers small amounts of solutes efficiently to targeted locations. Lymph flow is important for formation of chemokine gradients in the parenchyma, which behaves more as a porous medium. These vastly different transport regimes can be targeted for optimal immune response to vaccines, and are likely involved in progression of cancer metastases.
3D Fluid-Structure Interaction Model of Murine Contracting Lymphangions
Ghazal Adeli Koudehi, Charlotte Debbaut, Patrick Segers
The lymphatic system maintains tissue homeostasis by transporting the excess fluid from the interstitium and ultimately returning it to the venous circulation against an adverse pressure gradient and gravitational force. The spontaneous contractions of lymphangions, the building blocks of collecting vessels, and the secondary lymphatic valves play key roles in lymph propulsion. The aim of this study was to investigate lymph propulsion in a series of three contracting lymphangions in a 3D reconstructed model segmented from micro-CT scans of the collecting lymphatics in the hind limb of mice. We used Computational Fluid Dynamics and Fluid-Structure Interaction to study the behavior of flow within the collecting vessel, as well as the behavior and the deformations of the vessel wall and the poroelastic interstitium. The secondary valves were modelled as membranes with closed or open states depending on their permeability. We also performed a parametric study to evaluate their effect on the transport of lymph. The parameters having the most impact on the total volume of lymph propelled by active contraction of the lymphangions were the elastic modulus of the interstitium and the permeability of the secondary valves during the open states. Despite all the simplifications, our results and behavioral patterns were in harmony with literature, encouraging using these models as a stepping stone and gradually building up on their complexities, and providing a basis for a multi-scale model of the lymphatic network.
Non-Invasive Localisation of Coronary Artery Disease.
Steve Edward Greenwald, Simon Shaw
Stenotic coronary artery disease (i.e. partial blockage of the arteries feeding the heart muscle itself) produces disturbed flow downstream from the blockage, interacting with the artery wall, imparting energy to it and giving rise to low amplitude vibrations at audible frequencies. Some of this energy reaches the chest surface where it is detectable with suitable sensors as an acoustic signature, distinct from heart sounds and those from non-occluded vessels. We have carried out in-vitro experiments on a model chest filed with a soft-tissue mimicking gel, covered with a polyurethane “skin” and containing a latex “artery” containing 3-D printed stenoses of different geometry and fitted with a variety of sensors mounted on the skin or a short distance above it. With this set-up, we have proved the principle that signals associated with the presence of a stenosis can be detected at the skin surface. We report here the development of a device consisting of an array of sensors incorporated into a stick-on chest patch. The sensors transmit the signals wirelessly to a data capture unit from which, with post-processing, the characteristics associated with disturbed stenotic flow can be identified. Recent experiments have shown that not only can the presence of stenosis-associated disturbed blood flow be detected, but its position and severity can also be inferred. Currently an improved patch is under construction and a validation trial will be carried out, initially on healthy volunteers and subsequently on patients with chest pain undergoing simultaneous diagnostic CT scans.
Drug-mediated Mitigation of Maladaptive Inward Arterial Remodeling in Endovascular Therapy
Tarek Shazly
Drug-coated balloon (DCB) therapy is an emergent endovascular approach to treat obstructive arterial disease, with clinical studies showing superior performance compared to traditional interventional modalities. The recognized advantages of DCB therapy are generally attributed to the embodied leave-nothing-behind strategy, in which transient (~2-3 minutes) intravascular balloon inflation both compresses the lesion site and locally delivers an anti-proliferative (AP) drug to the arterial wall. Thus, with no permanent implant, DCBs offer a means to acutely restore (via lesion preparation/mechanical compression) and subsequently maintain (via AP drug activity) vessel patency. Although successful in the peripheral circulation, broader clinical use of DCBs is limited by an incomplete understanding of device- and patient-specific determinants of treatment efficacy, including late outcomes that are mediated by post-interventional maladaptive inward arterial remodeling. To address this knowledge gap, we propose a predictive mathematical model of pressure-mediated femoral artery remodeling following DCB deployment, with account of drug-based modulation of resident vascular cell phenotype and common patient co-morbidities, namely hypertension and endothelial cell dysfunction. Our theoretical findings suggest that in maladaptive post-DCB remodeling scenarios, restenosis is not prevented by traditional AP drug delivery, but that novel payloads which co-deliver AP and anti-contractile (AC) drugs may effectively prevent this maladaptive outcome. We then synthesized a series of experimental DCBs for co-delivery of representative AP and AC drugs, and quantified acute drug transfer to arterial tissue in an ex vivo model of DCB deployment. Taken together, our theoretical and experimental findings suggest that co-delivery of AP and AC drugs is feasible in the context of DCB therapy and could improve patient outcomes by mitigating post-interventional maladaptive inward arterial remodeling.
Establishing Robust and Reliable Computational Approaches to Integration Vascular Mechanics into the Clinical Setting
Lucas Timmins
Integrating Neural Networks and Mathematical Modeling for Enhanced Estimation of Stroke Volume
Nicos Stergiopulos
Integrating Neural Networks and Mathematical Modeling for Enhanced Estimation of Stroke Volume
Vicky Bikia, George Rovas, Nikos Stergiopulos
Laboratory of Hemodynamics and Cardiovascular Technology, EPFL, Switzerland
Background: Stroke volume (SV) is a pivotal indicator of heart function. The need for a non-invasive, precise, and dependable SV estimation method is unfulfilled. Meanwhile, breakthroughs in measurement systems and the emergence of wearable technologies have driven research toward the effective use of readily available data for improved hemodynamic monitoring.
Methods: We developed two methodologies to estimate SV utilizing a validated one-dimensional (1-D) cardiovascular model. Firstly, we applied an inverse-problem solving algorithm to estimate SV from age, weight, height, cuff BP and pulse wave velocity. This approach was based on the adjustment of the 1-D model and applied an optimization process for deriving a personalized profile of an individual’s arterial hemodynamics. Secondly, we created an artificial neural network (ANN)-enabled model which maps a subject’s radial BP waveform to the corresponding SV value. Of particular interest was to investigate whether the uncalibrated radial BP waveform contains sufficient information to derive SV. Specifically, a training/testing pipeline was adopted for the development of two ANN models using as input: the calibrated radial BP waveform (ANNcalradBP), or the uncalibrated radial BP waveform (ANNuncalradBP).
Results: Our first method was tested against measurements of SV derived from magnetic resonance imaging in healthy individuals covering a wide range of ages (n = 144; age range 18–85 years). The inverse method yielded satisfactory agreement between estimated and reference data (Figure 1), reporting a correlation of 0.83 and limits of agreement (95% confidence interval) equal to [-29.7, 32.7] mL. The ANN-inspired approach provided precise SV estimations across the extensive range of cardiovascular profiles (test size was 764), with accuracy being higher for the ANNcalradBP. The correlation coefficient and limits of agreement were found to be equal to (0.98 and [-5.5, 6.6] mL) and (0.95 and [-10.3, 8.8] mL) for ANNcalradBP and ANNuncalradBP, respectively (Figure 1).
Discussion: Our findings confirmed the inverse method's clinical utility and underlined the importance of physics-driven mathematical modeling in refining hemodynamic monitoringANN modelling can provide a computationally cost-efficient manner to translate measured data into critical biomarkers, such as SV. Importantly, the uncalibrated radial BP waveform contains sufficient information for accurately deriving SV. This methodology could pave the way for incorporating the model in wearable sensing systems, such as smartwatches or other consumer devices.
Mechanical properties of blood thrombi, with theory, experiment, and measurement
David Ku, Dongjune Kim, Yuhang Hu
Introduction: Blood thrombosis is the immediate cause of most heart attacks and strokes. While most prior authors assume that coagulation creates the thrombus, a contradiction arises as fibrin clots from coagulation may be too weak to stop arterial blood pressures (>150 mmHg).
Aims: Measure the strength of platelet-rich versus coagulation thrombi in comparison with coagulation thrombi.
Methods: Platelet-rich thrombi are made with human whole blood to create Shear-Induced Platelet Aggregation (SIPA) under stenotic arterial hemodynamics. Coagulation thrombi are made from stagnant human whole blood. We measure the material mechanical properties of elasticity and ultimate strength for thrombi using a mechanical testing machine. The measured elasticity and breakage strengths are used to make a computational model of these elastomers.
Results: The ultimate strength of SIPA clots averaged 4.6 ± 1.3 kPa, while whole blood coagulation clots had a strength of 0.63 ± 0.3 kPa (p < 0.05, Fig 1). The elastic modulus of SIPA clot was 3.8 ± 1.5 kPa, or 2.8 times higher than a whole blood coagulation clot (1.3 ± 1.2 kPa, p < 0.0001). The solid model shows lower stresses and reasonable deformation of SIPA thrombus under arterial pressures compared to the softer coagulation material that would break (Fig 2).
Conclusion: This study shows that the platelet-rich thrombi, formed quickly under high shear hemodynamics, are seven-fold stronger and three-fold stiffer compared to coagulation clots. A computational analysis of material strength using a force balance calculation shows a SIPA clot has sufficient strength to resist arterial pressure with a short length of less than 2 mm, consistent with coronary pathology. Coagulation clots are too weak.
BIOMECHANICS IN SPORT AND HEALTHCARE
Experimental ultrasound approach for studying knee intra-articular femur–tibia movements under different loads
Ivan Ivanov, Sergey Ranchev, Stoian Stoychev
The purpose of the present study was to develop an experimental model for the study of intra-articular knee movements depending on the function of the knee joint and involved muscle groups under isometric stretching conditions with different loads. The experimental procedure included an ultrasound examination of a knee joint after isometric stretching in healthy men (n = 32). The changes (in millimeters) in the distances between the femur and tibia were measured using an ultrasound sonographer at three stages. The first stage was performed on ten (n = 10) healthy men in five different sitting and upright positions. In the second and third experimental model stages, lower limbs loading was applied to 22 participants. Our hypothesis, which was confirmed, was that as a result of increased loads on the participant’s back, an intra-articular decrease in the femur–tibia cartilage surface distance would be observed. The accuracy of the created experimental model was improved over its three stages from 30% to 9%. Quantitative model data can help to create a mathematical model of the mechanical effects during the deformation of knee joint bone cartilage and it can also help outline some future tasks: increasing loading weights, enlarging participant groups, performing comparisons of men and women, and performing comparisons of healthy and pathological individuals.
This work has been supported by the Basic Research Project – КП-06-Н57/18 from 16.11.2021, funded by the Bulgarian National Science Fund.A set of anatomical databases for developing biomechanical models of the lower human limb
Rosica Raikova, Slavi Delchev, Ivan Ivanov, Silvia Angelova
Biomechanical models of the human lower limbs are developed and used in different areas – automobile design, rehabilitation, design of prosthetic and orthotic devices, achievement of high sports results, and even for criminal purposes. Many modelers have difficulties transferring anatomical data into mechanical concepts – i.e. modeling anatomical joints as one, two, or three degrees of freedom mechanical rotational joints with or without friction, to model the force which one muscle performs (its direction, application points, arm), etc.
The current work aims to systematize the specific anatomical and biomechanical data for the muscles of the lower limb and their action in the three main joints – hip, knee, and ankle – and to illustrate how a concrete model for concrete joints and motor tasks in norm and pathology can be developed. This will save a lot of time for non-specialists to orient in muscle-skeletal system and to develop simple or more complex biomechanical models and calculate muscle forces.
Using the data from different anatomical sources – textbooks and atlases, internet, etc. – two tables were composed. The first one consists of all muscles (34 in number) crossing the three main lower limb joints, used abbreviations, and their actions in hip, knee and ankle like flexor/extensor, abductor/adductor, internal/external rotator. In the second table, all these muscles are ordered concerning their importance in the motions of the joints. For each muscle, a model is developed showing its anatomical position, including attachments and functions. Simple biomechanical models in sagittal and frontal planes with the most important muscles are presented.
This work has been supported by the Basic Research Project – КП-06-Н57/18 from 16.11.2021, funded by the Bulgarian National Science Fund.Biomechanical jump characteristics of adolescent basketball female players after an isometric stretching program
Ivan Ivanov, Blagovest Glavev, Galia Rusimova, Sergey Ranchev
The effects of isometric stretching on the muscle-tendon-joint biomechanics in the literature data are contradictory. The aim of the study was to verify the effectiveness of the applied isometric stretching program for the improvement of jump performance of six adolescent basketball female players (17-19 years). The isometric stretching protocol with eleven exercises was used after the last daily regular training program, four times per week for 40 minutes. All exercises were repeated three times with detention duration 15 seconds for 11 weeks. Chronojump DIN-A1 contact platform (Spain), detecting a single jump with one phase of flight, was used to evaluate eight important biomechanical jump characteristics for the participants. The obtained Mann-Witney statistics of the measured parameters for our six participants shows contradictory results and individual stretching body adaptation. The obtained statistical significances in comparissons does not allow to make the important conclusions for the applied isometric stretching program effectiveness. The presented results underline that lower extremity control and performance are strongly related with the motion and synergy work of the upper limbs. In addition, the presented data draw attention to improving coordination between the lower and upper body movements, emphasizing the individual characteristics of each athlete for target increasing of lower limbs biomechanical performance.
This work has been supported by the Basic Research Project – КП-06-Н57/18 from 16.11.2021, funded by the Bulgarian National Science Fund.
Experimental study of pneumatic driven exoskeleton for rehabilitation and training
Pavel Venev, Dimitar Chakarov, Ivanka Veneva, Georgi Katsarov
This work includes the development of the upper limbs exoskeleton for training and rehabilitation. In order to meet the requirements of the rehabilitation exoskeletons for transparency and natural safety on the one hand and efficiency on the other, the work offers a pneumatic drive where positive pressure drive is combined with a vacuum pressure drive. The purpose of the work is to evaluate efficiency and transparency, where the exoskeleton is driven by rotary pneumatic actuators with vacuum pressure and to compare this approach with positive pressure actuation. In order to evaluate transparency, the force of interaction between the patient and the exoskeleton in passive mode is examined. A model of the force of interaction between the patient and the exoskeleton in passive mode has been created. The model is based on harmonious movements imposed in one joint on the exoskeleton. The simulations and experiments are carried out to evaluate the force of interaction between the patient and the exoskeleton due to the mechanical impedance of the device. The force of interaction is obtained from inertial, friction and gravitational forces, as well as from the elasticity of pneumatics. The experimentation of interaction between the patient and the exoskeleton is performed with a harmonious movement imposed by the patient in a joint on the exoskeleton. The force of interaction is evaluated in cases of pneumatic actuating, combining drives with pressure higher than atmospheric pressure and drives with vacuum pressure. Simulation and experimental results are shown graphically. A discussion is presented, as well as conclusions and instructions for future work.
Therapeutic effects of stretching
Elissaveta Zvetkova, Yordanka Gluhcheva, Ivan Ivanov, Eugeny Koytchev, Antonio Antonov, Sergey Ranchev
Characterized in biomedical terms, stretching exercises have been defined as movements applied by external and/or internal forces to increase muscle and joint flexibility, decrease muscle stiffness, elevate the joint range of motion (ROM), increase the length of the “muscle–tendon” morpho-functional unit, and improve joint, muscle, and tendon movements, contraction, and relaxation. The present review examines and summarizes the initial and recent literature data related to the physiological and therapeutic effects of static stretching (SS) on flexibility and other physiological characteristics of the main structure and the “joint–ligament–tendon–muscle” functional unit. The therapeutic effects of SS, combined with other rehabilitation techniques (massage, foam rolling with and without vibrations, hot/cold therapy, etc.), are discussed in relation to the creation of individual (patient-specific) or group programs for the treatment and prevention of joint injuries, as well as for the improvement of performance in sports. From a theoretical point of view, the role of SS in positively affecting the composition of the connective tissue matrix is pointed out: types I–III collagen syntheses, hyaluronic acid, and glycosaminoglycan (GAG) turnover under the influence of the transforming growth factor beta-1 (TGF-β-1). Different variables, such as collagen type, biochemistry, elongation, and elasticity, are used as molecular biomarkers. Recent studies have indicated that static progressive stretching therapy can prevent/reduce the development of arthrogenic contractures, joint capsule fibrosis, and muscle stiffness and requires new clinical applications. Combined stretching techniques have been proposed and applied in medicine and sports, depending on their long- and short-term effects on variables, such as the ROM, EMG activity, and muscle stiffness. The healing effects of SS on the main structural and functional unit—“joint–ligament–tendon–muscle”—need further investigation, which can clarify and evaluate the benefits of SS in prophylaxis and the treatment of joint injuries in healthy and ill individuals and in older adults, compared to young, active, and well-trained persons, as well as compared to professional athletes.
This work has been supported by the Basic Research Project – КП-06-Н57/18 from 16.11.2021, funded by the Bulgarian National Science Fund.Asymmetry in anthropometric characteristics of the limbs in youth tennis players
Albena Dimitrova
Tennis is characterized by high physical activity and frequently repeated motions, especially for the dominant upper limb. These motions may creates bilateral differences in morphological characteristics in this part of the body. The study aims to assess youth tennis players' anthropometric characteristics and determine the level and direction of the manifested asymmetry. Totally of 239 tennis players (152 boys and 87 girls) at the age 8-17 years are assessed. The anthropometric measurements are taken using the classical method of Martin-Saller (1957). Twenty-seven anthropometric features are bilaterally measured, because of the asymmetry analysis. A hand grip test (European Test of Physical Fitness - EUROFIT) is perform to defined static arm strength. The right hand grip strength (RHGS, kg) and left hand grip strength (LHGS, kg) are measured using a standard calibrated handgrip dynamometer at standing position with the shoulder adducted and neutrally rotated and elbow in full extension. The statistical analysis is made by software package SPSS 16.00 for Windows, and the following analyses are implemented: descriptive statistics, paired T-test (p≤0.05, p≤0.01, p≤0.001), One-Way ANOVA, as well as Post hoc procedures for multiple comparisons (Tukey, HSD-test), percentile method. The asymmetry coefficients of the assessed anthropometric features are calculated by Nacheva equation (1986), modified by us. The main results from the current study are: Tennis players from both sexes have higher values of forearm muscle mass compared to the muscle mass of the upper arm, which may be associated with the intensity of tennis training together with specific exercises in this part of the body. There are significant sexual differences, according to the hand grip strength, with priority for boys. Tennis training in youth athletes leads to anthropometric asymmetry in the dominant upper limbs. A moderate level of bilateral differences are reported in the forearm (forearm circumference, forearm skinfold) and muscle-fat ratios. The hand grip strength which have high level of asymmetry coefficient is indirect evidence of the large bilateral differences in the forearm, due to the higher loading of the muscles.
Keywords: tennis players; anthropometry; asymmetryThis work has been supported by the Basic Research Project – КП-06-Н57/18 from 16.11.2021, funded by the Bulgarian National Science Fund.
Influence of gender, skinfold thickness and body position on the biomechanical parameters of the trapezius muscle
Stela Ivanova, Hristina Tancheva
Background
The aim of the study is to investigate the influence of gender, skin and subcutaneous tissue thickness (skinfold thickness, SFT) and body positions on biomechanical parameters of the trapezius muscle in order the obtained results to be used to optimize specific trainings or for rehabilitation purposes.
Materials and methods
We used MyotonPRO device to objectively evaluate the biomechanical parameters of trapezius muscle: frequency (F [Hz]), decrement (D [log]), stiffness (S [N/m]), relaxation time (R [ms]) and creepability (C [De]). The value of SFT was assessed by caliper in the same place of measurement with MyotonPRO. Both measurements were taken in sitting, prone, supine and side-lying positions on 90 students (40 males and 50 females, age range 19-24 years), after completing an informed consent. Statistical data processing was carried out with SPSS software, using Student's t-test, ANOVA and Pearson correlation coefficient.
Results
The experimental results are processed and analyzed. On that base, the recommendations for sport and rehabilitation purposes are proposed.
Objectification of the impact of massage techniques in local areas on the tone, biomechanical and viscoelastic properties of the trapezius muscle in students
Stela Ivanova, Denka Marinova, Diana Dobreva, Gergana Taskova
Background
Massage is a widely used as alternative and complementary treatment in the prevention and recovery from various diseases and in sports. Most of the research methods, used in massage studies are subjective making them difficult for standartisation. The procedure subject of this study, based on the application of the MyotonPro® device, will provide an opportunity to evaluate the effect of the applied massage techniques on the biomechanical properties of the trapezius muscle.
The aim of the study is to evaluate the effect of basic massage techniques used in European massage on the mechanical properties of trapezius muscle at rest applying the developed procedure to students. The obtained results will be used to optimize massage therapies as part of physiotherapy treatment in the prevention and recovery from various diseases and in sports.
Methods
The developed procedure consists of measuring the following mechanical and viscoelastic muscle properties using MyotonPRO®: frequency (F [Hz]), decrement (D [log]), stiffness (S [N/m]), relaxation time (R [ms]) and creepability (C [De]) before and after application of effleurage, kneading, friction, petrissage and tapotement techniques. A randomized control trial was conducted on 83 students (age range 19-24 years) with the techniques of the Classical European massage. A mixed-design analysis of variance with gender as between subject comparison was used for assessing the differences between gender and each of the myometric parameters separately (F, D, S, R and C). Pearson correlation coefficient between initial and final values of F, D, S, R and C was conducted for males, females and males and females together to evaluate the effect of the different massage techniques.
Results
The experimental results are processed and analyzed. On that base, the recommendations for sport and rehabilitation purposes are proposed.
Keywords: massage, massage techniques, trapezius muscle, MyotonPro