2024
Středa 17. ledna 2024 v 10:00 (nová přednášková síň):
Power of Pulsating Liquid Jets
Dr. Josef Foldyna, oddělení kapalinových paprsků, Ústav geoniky AV ČR, Ostrava
The effects of high-speed water jets on disintegration of materials are well known - pure water jets are able to cut paper, wood, plastic, rubber, and thin metal sheets. Abrasive jets are capable of cutting, drilling, turning or milling not only metals, but also difficult to process materials such as composites, structural ceramics, high-strength alloys, glass, etc. Despite the undisputed technological advances made in recent years in the field of high-speed abrasive water jet applications, there is a constant pressure on the development of new technologies using only pure water jets. One potential approach is the utilization of the physical phenomenon created by the droplet's impact on a solid surface.
In the lecture, I will briefly explain what a high-speed liquid jet is and how is generated, what types of high-speed liquid jets we can encounter and what they are used for. The main part will be devoted to results of research on high-speed pulsating liquid jets at the Institute of Geonics. I will present possibilities of generating pulsating jets, their applications and I will also mention the problems we face.
2023
Pátek 24. listopadu 2023 ve 13:00 (nová přednášková síň) – dvě přednášky:
How to unlock quality consistency and repeatability to enable large-scale industrialization in Additive Manufacturing
Dr. Edson Costa Santos, Senior Application Development Manager ZEISS AM Technology, ZEISS Industrial Quality Solutions, Carl Zeiss Industrielle Messtechnik GmbH, Oberkochen, Germany
AI applications in industry and research
Martin Kovanda, doktorand FJFI ČVUT / Ústav termomechaniky AV ČR, Praha
Pdf pozvánka s medialonky obou přednášejících je k dispozici zde.
How to unlock quality consistency and repeatability to enable large-scale industrialization in Additive Manufacturing
Manufacturers rely on additive manufacturing when they want to boost production efficiency, customize parts, and achieve faster time to market - but how do you transform from rapid prototyping into end-use applications in the medical, aerospace and automotive industries? These benefits can only be achieved by ensuring consistent quality – from material and parameter development, ensuring printer equivalency, process qualification and stability. Digitized workflows based on artificial intelligence, enabling to improve quality, understand causes of failure, drive sustainable process improvements, and set standards for future series production in a holistic approach are the topics of this presentation.
AI applications in industry and research
In recent years, the potential of artificial intelligence has increased dramatically, as the hardware capabilities allowed training of ever deeper neural networks. From improving camera image quality to recommending songs, machine learning has become a part of our daily lives. The same revolution is now taking place in industry and research. For example, new deep learning methods allow to automate advanced material inspection, while other models may be used to detect anomalies in ultrasonic signal. This presentation covers the potential applications of AI in industry and research and should give an insight into the new opportunities that these new technologies represent.
Pondělí 2. října v 10:00 (nová přednášková síň)
How to build a „bridge“? Nature’s strategy for connecting hard and soft materials
Prof. Benny Bar-On, Department of Mechanical Engineering, Ben-Gurion University of the Negev, Izael
Load-bearing biological materials employ specialized bridging regions to connect material parts with substantially different mechanical properties (hard vs. soft). While such bridging regions have been extensively observed in diverse biomaterial systems that evolved through distinctive evolutionary paths—including arthropod parts, dental tissues, and marine threads—their mechanical origins and functional roles remain vague.
In my talk, I introduce a hypothesis that these bridging regions have primarily formed to minimize the near-interface stress effects between the connected material parts preventing their splitting failure, and obtain a simple theoretical law for the optimal mechanical properties of such bridging regions. I demonstrate this principle through Finite-Element simulations and physical experiments on a model synthetic-material system and verify its predictability for different biomaterial systems. The bridging principles of biological materials can be implemented into advanced material designs—paving the way to new forms of architected materials and composite structures with extreme load-bearing capabilities.
Reference: Uzan, A. Y., Milo, O., Politi, Y., & Bar-On, B. (2022). Principles of elastic bridging in biological materials. Acta Biomaterialia, 153, 320-330.
Čtvrtek 31. srpna v 10:30 (nová přednášková síň)
Theoretical and computational study on inelastic mechanics of cellular materials
Assistant Prof. Li-Wei Liu, Department of Civil Engineering, National Taiwan University, Taiwan
The cellular microstructure is observed in wood, cork, bone, and honeybees’ honeycomb, which possesses the characteristics of stiffness and light weight. The man-made material with cellular architecture called the cellular material is expected to possess similar features to natural materials. The investigation of elastic features of cellular materials has been conducted for several decades due to the attractive characteristics of their biological counterparts. However, biological/natural cellular materials usually demonstrate rate-dependent features as well as the existence of permanent deformation, while little attention has been paid to the inelastic properties of cellular materials. In this study, we use theoretical and computational tools to study the inelastic behavior of 2D cellular materials. To investigate the viscoelastic feature of the cellular materials, the unit-cell approach is adopted, and a viscoelastic model of 2D cellular materials is proposed. Then the analytical response of the material under various loadings is derived. After the validation of our model, the influence of microstructure on viscoelastic features of 2D cellular materials is observed and analyzed qualitatively and quantitatively. To investigate the plastic behavior of the cellular materials, a finite element analysis on the yield surface of 2D cellular materials is developed, and a representative block is selected to represent the effective feature of the cellular materials. After probing paths and preloading paths are designed, the initial and subsequent yield surfaces of the 2D cellular materials with different relative densities are detected, and the influence of relative density on the yield surface evolution is investigated. Further, phenomena of cellular materials, including the Bauschinger effect and the hardening behavior (isotropic, kinematic, rotational, and distortional) are observed from the yield surface evolution of the cellular materials. Based on the computational approach, we explore the mechanics of trabecular bone, which has non-periodic cellular microstructure and demonstrates asymmetric yield stresses in tension and compression. The computation shows that trabecular bone experiences the distortional yield surface and the appearance or disappearance of the Bauschinger effect during different loadings.
Úterý 11. července v 10:30 (nová přednášková síň)
Dual Boundary Element Method in Taiwan since 1986
Prof. Jeng-Tzong Chen, Distinguished Chair Professor, National Taiwan Ocean University, Keelung, Taiwan
Boundary element method (BEM) is an acceptable approach for simulating engineering problems. The theoretical bases of the dual boundary element method (DBEM) were presented by Hong and Chen in a general formulation which incorporates the displacement and the traction boundary integral equation. First, engineering applications for problems containing degenerate boundaries in Taiwan will be reviewed. It is well known that four degenerate problems, degenerate scale, spurious eigenvalue, fictitious frequency and degenerate boundary, may occur by using the BEM/BIEM. However, only the degenerate scale and degenerate boundary may appear at the same time. This is so-called double degeneracy. Two possibilities may occur. One is the double degeneracy of rigid line inclusion (degenerate boundary) and a critical length (degenerate scale) together. The other is the degenerate scale for the outer boundary and the inner degenerate boundary for doubly-connected problems. We introduce the objectivity of the degenerate kernel. The degenerate kernel is employed to analytically explain how the degenerate mechanism appears in the boundary integral formulation. It is found that a rigid line inclusion instead of a crack may have the possibility of double degeneracy. Even though the boundary density is polluted by the null space, the solution may be correct. Not only the analytical derivation is proposed but also the numerical experiment is also performed. Anti-plane shear and two-dimensional elasticity problems are both addressed. A linkage to the book cover of Gilbert Strang on linear algebra for the null space is also addressed.
Úterý 6. června v 10:30 (nová přednášková síň)
Phenomenological modelling of ductile fracture in metals using the element deletion technique
Doc. František Šebek, FSI VUT Brno
Ductile fracture is a phenomenon that occurs in metallic materials in many cases, from unwanted situations in the automotive industry to the desired output of manufacturing processes. Its modelling is approached by various techniques, ranging from the utilization of simple criteria based on the tensile tests to the complex models covering various stress states. The crack initiation and propagation, not in a sense of classical fracture mechanics, may itself be modelled by the node separation, phase field, extended finite element or meshless methods. The current talk will focus on a utilization of the element deletion technique, within an explicit finite element method, based on the phenomenological ductile fracture criteria coupled with a non-quadratic non-prismatic yield surface with the deviatoric associated flow rule. The calibrated material model, thus in the scope of continuum damage mechanics, will be verified towards the independent fracture tests to show its predictability. Finally, a broader industrial applicability will be presented along with the remarks and prospects for future studies.
Úterý 4. dubna v 10:00 (nová přednášková síň)
Nucleation Rates of Carbon Dioxide Gas-Hydrates
Dr. Bernd Rathke, Technische Thermodynamik, Universität Bremen, Německo
The formation of gas hydrates is one of the challenges in plant operation under conditions of elevated pressures, humidity and low temperatures. Furthermore, applications like the sequestration of carbon-dioxide, the basic understanding of processes in oil industry and natural gas processing are the most prominent applications, which urgently require a profound knowledge of the physico-chemical aspects of the underlying mechanisms.
Unfortunately, basic research on the kinetics of hydrate formation taking into account the different aspects of equilibrium thermodynamics and the kinetics of phase transitions is scarce. This contribution summarizes different experimental approaches to determine the formation of gas hydrates. Advantages and disadvantages of these techniques are explicated and discussed in the light of investigations on nucleation.
Against this background, we have been characterized onset-conditions of the formation of gas hydrates from carbon-dioxide saturated water and have been determined characteristic times and nucleation rates for different degrees of supersaturation. Such specific type of experiments should contribute to the understanding of the basics of hydrate formation.
For this purpose a set of experiments has been performed using a high pressure apparatus suitable up to pressures of p = 700 bar. The set-up consists of two independent parts, which allow for a preparation of binary mixtures under defined conditions and rapid kinetic studies of phase transitions induced by fast pressure changes, respectively. This concept allows for an independent control of temperature and pressure without a change of the composition of a sample.
Results indicate a strong variation of induction times or even nucleation rates of hydrate formation at different degrees of supersaturation and are discussed in terms of classical nucleation theories.
2022
Úterý 22. listopadu v 14:00 (nová přednášková síň)
Towards development of new equation of state for squalane
Ing. Aleš Blahut, Ph.D., Ústav termomechaniky AV ČR, v. v. i.
Currently, new fundamental equation of state for squalane is being developed at the Chair of Thermodynamics at the Ruhr University Bochum. One of the prerequisites for such development is a reliable database of thermodynamic properties. In case of liquid densities, majority of recent literature data was acquired with vibrating tube method, which however has certain limitations for measurements of viscous fluids such as squalane. Moreover, in order to provide reliable data, vibrating tube densimeters have to be properly calibrated with reference fluids, ideally in the entire temperature and pressure range of measurements.
The presentation introduces a background of equation-of-state development and focuses on new density measurements of squalane, which were carried out during a research stay at the Chair of Thermodynamics in Bochum using unique single-sinker densimeter with magnetic suspension. Experimental method, new results and several correlations representing experimental densities are presented. Because several squalane samples from the same batch were later investigated with vibrating tube densimeter at the Institute of Thermomechanics in Prague, reliability of vibrating tube method for density measurement of viscous fluids is discussed.
A. Blahut gratefully acknowledges funding within "Support for International Mobility of Researchers of the Institute of Thermomechanics, Czech Academy of Sciences, part II", no. CZ.02.2.69/0.0/0.0/18_053/0017555 of the Ministry of Education, Youth and Sports of the Czech Republic funded from the European Structure and Investment Funds (ESIF).
IMPORTANT GDPR NOTICE: By attending this event you consent that we may take photos from the talk (including the audience) and provide it to the Ministry of Education, Youth and Sports (MEYS). MEYS is the funding provider for the project "Support of international mobility of researchers of the Institute of Thermomechanics of the CAS, part II" and the processor of the provided data.
Úterý 8. listopadu v 10:00 (nová přednášková síň)
Modelling of Complex Signals in Nerves
Prof. Jüri Engelbrecht ve spolupráci s Dr. Kertem Tammem a Dr.Tanelem Peetsem,
Estonian Academy of Sciences a Department of Cybernetics, School of Science, Tallinn University of Technology
The propagation of signals in nerves is a fundamental physical process needed for understanding cognitive processes and mental phenomena. It involves not only electrical signals (action potential, ion currents) but also mechanical disturbances in nerve fibres and temperature changes. The modelling of dynamic processes in continua (leaving aside particle physics, astrophysics, etc. where relativity of motion is of importance) is based on the conservation of momentum which is usually known as Newton’s Second Law. The thermodynamic effects are modelled by Fourier's law (heat flux is related to temperature gradient) and Joule’s law (heat is related to electric current). Traditional models of nerve signals pay more attention to physiology which helps to explain biological phenomena. In order to explain all the phenomena in nerves, a broader view must be elaborated. According to general principles of complex systems, the first step of the bottom-up modelling needs to identify all the basic elements (basic physical processes) and their interactions with each other (couplings) so that many components are united to generate a whole: an ensemble of waves. A possible mathematical model following these ideas is derived. The governing equations for the components of the ensemble correspond either to the modified classical ones for describing the action potentials or are derived from the laws of physics resulting in a consistent system. The interaction of the components of the ensemble is realized by coupling forces. The numerical simulation has shown that the model can grasp the measured effects. The mathematical model generated by authors [1] is an attempt aiming to couple all the measurable effects of the signal propagation in nerves into a system and demonstrates the importance of basic sciences in developing plausible models. This is an interdisciplinary approach at the interface of physiology, physics, and mathematics but it can be said that physics shapes signals in nerves [2]. The ideas are also supported by philosophical analysis [3]. After establishing the sound backbone of the model, further modification of the modelling involving the influence of the internal structure of a fibre (myelin sheath, the cytoskeleton of the axoplasm, etc.) is possible. An example of the modelling of the myelin sheath demonstrates such a possibility [4].
References
[1] Engelbrecht J., Tamm K., Peets T. (2021) Modelling of Complex Signals in Nerves. Springer, Cham
[2] J.Engelbrecht, K.Tamm, T.Peets. (2022) Physics shapes signals in nerves. The European Physical Journal Plus, 137, 696
[3] J.Engelbrecht, K.Tamm, T. Peets. Signals in nerves from the philosophical viewpoint. Proc. Estonian Acad.Sci. (accepted, to appear in 2022)
[4] K.Tamm, T.Peets, J. Engelbrecht. Mechanical waves in myelinated axons. Biomechanics and Modeling in Mechanobiology - BMMB, 2022 (online available)
Středa 26. října ve 14:00, klub
Velká věda v Malém Tibetu
Mgr. Radka Kellnerová, Ph.D.
Ústav termomechaniky AV ČR, v. v. i.
Brontosauři v Himálajích pozvali 30 českých vědců do jedné z nejzapadlejších vesnic Malého Tibetu. Měli připravit prázdninovou školu pro děti do 15 let.
Jak to dopadlo? Zvládli vědci takovou výzvu v prostředí, kde stabilní elektřina, signál a pitná voda je pouhý luxus, technické zázemí na míle vzdálené potřebám pro experimenty a kulturní pozadí úplně odlišné od domoviny?
Přijďte si poslechnout povídání Radky Kellnerové o netradičních zážitcích našich výzkumníků v srdci budhismu 3500 metrů vysoko nad mořem.
Úterý 4. října ve 14:00, nová posluchárna
Robocasting: an additive manufacturing technique for fabricating micro-architectured ceramic scaffolds
Dr. Martin Koller
Ústav termomechaniky AV ČR, v. v. i.
Robocasting is a direct ink writing technique, where the filaments of pseudoplastic ceramic-based dispersions are extruded from a nozzle, following a route prescribed by a CAD model. The ceramic green-body scaffolds are printed layer-by-layer and then sintered to full density. This presentation shows the process of creating the printable inks, i. e. the aqueous dispersion of ceramic powders with highly shear-thinning behavior, based on SiC, Cr_{2}AlC, or Y_{2}O_{3}-stabilized ZrO_{2} ceramic powders, and the subsequent 3D printing of micro-architectured scaffolds at the Institute of Ceramics and Glass (ICV-CSIC) in Madrid, Spain. The periodic scaffold structure leads to phononic crystal behavior, which has been studied both numerically and experimentally at the Institute of Thermomechanics. The geometry of the scaffolds strongly affects their elastic and acoustic properties; the tetragonal scaffolds have strong elastic anisotropy, which leads to acoustic energy focusing along the direction of the ceramic rods, while the hexagonal scaffolds are in-plane isotropic in the low-frequency limit. At the higher frequencies of several MHz, frequency shear bands are observed, where the acoustic waves do not propagate and their energy is rather dissipated within the scaffold structure. Besides that, novel types of robocast scaffolds, e. g. multi-phase composite scaffolds, or electrically conductive scaffolds reinforced by graphene fillers are shown, highlighting the future prospects in the research of architectured ceramics.
M. Koller gratefully acknowledges funding within "Support for International Mobility of Researchers of the Institute of Thermomechanics, Czech Academy of Sciences, part II", no. CZ.02.2.69/0.0/0.0/18_053/0017555 of the Ministry of Education, Youth and Sports of the Czech Republic funded from the European Structure and Investment Funds (ESIF).
IMPORTANT GDPR NOTICE: By attending this event you consent that we may take photos from the talk (including the audience) and provide it to the Ministry of Education, Youth and Sports (MEYS). MEYS is the funding provider for the project "Support of international mobility of researchers of the Institute of Thermomechanics of the CAS" and the processor of the provided data.
Středa 27. července ve 14:00, nová posluchárna
Novel Design of a Device for Human Skin Viscoelastic Properties Measurement
Flavie Delouye and Perrine Bégon,
studenti z Institut National des Sciences Appliquées Centre-Val de Loire, Blois, Francie
Školitelé: Zdeněk Převorovský a Daniel Tokar
Human skin has a complex mechanical behavior which can be described as anisotropic and non- linearly viscoelastic. These properties are not well determined but they are of great interest e.g. in cosmetic industry and aesthetic medicine. This talk deals with a novel design of a device which measures the mechanical characteristics of the skin in-vivo, in particular the detailed mechanical design of the prototype device using 3D-printed components. The design of necessary mechanical components deals with a loading base in which are integrated ultrasonic transducers in order to transmit and receive ultrasonic signals propagating in the loaded skin. The loading base is built up with displacement sensor for the purpose of measurement of mechanical loading of the skin. The design of mechanical parts also includes a specific component integrating strain-gauges sensors in order to obtain the stress-strain of the skin tissue. Subsequent to the assembly of the whole device, verification of the mechanical components ensures the coherence of the entire design. The future work will be focused on electrical conception of the device for the motor control, strain-gauges and displacement sensor for stress-strain measurements, and finally the calibration and tests of the device in its complexity.
Středa 13. června v 10:30, nová posluchárna
Experimental mechanics and modeling to solve the challenges of manufacturing processes and materials performance
Associate Prof. Víctor Tuninetti, Department of Mechanical Engineering, Universidad de La Frontera, Temuco, Chile
Improving strength, toughness and reducing weight of conventional and new materials are one of the main challenges for today's engineers. We contribute to teaching and new knowledge transfer from experimental mechanics and finite element simulations of part design and manufacturing processes.
Among current developments the talk will include the following topics:
- Design of auxetic materials for ankle implant applications.
- Vibration analysis for industrial processing efficiency in the wood peeling process
- Characterization and modeling of Ti64 plasticity and damage for impact and manufacturing applications.
- Spatially varying filler microstructure in 3D printing fused deposition process based on topology optimization technique.
- Carbon nanotubes for the reinforcement of glass fiber composites.
- Strain rate sensitive behavior prediction of materials using artificial neural networks.
Středa 9. února v 10:30, nová posluchárna
From Point-to-Point Connections to Industrial CO2-Transport Networks – Contributions from Thermodynamics
Prof. Dr.-Ing. Roland Span, Chair of Thermodynamics, Faculty of Mechanical Engineering, Ruhr University Bochum, Německo
Concepts for Carbon Capture and Storage or Carbon Capture and Utilization (CCS/CCU) have always considered the transport of CO_{2} as part of the process chain. However, in many cases transport was considered established technology, or at least little technical problems were seen in the development of transport infrastructure. However, CCS and CCU concepts are no longer restricted to point-to-point connection between large CO_{2} sources (essentially power plants) and storage sites, but include CO_{2}-transport networks, in which multiple industrial emitters inject CO_{2}. The handling of fluctuating CO_{2} flows with different origin and separated using different capture technologies results in new challenges for CO_{2} transport. The talk will present a brief overview of these challenges, focusing on aspects relevant for research in the (wider) field of thermodynamics. With regard to the thermodynamic property basis required for the development of CO_{2}-transport networks, researchers both at Ruhr University and at IT CAS are part of an international network that experimentally and theoretically works on the development of accurate models for both scientific and industrial applications in this context for many years now. An overview of the results of this work will be presented.
References
- Jäger, V. Vinš, J. Gernert, R. Span, J. Hrubý: Phase equilibria with hydrate formation in H_{2}O + CO_{2} mixtures modeled with reference equations of state, Fluid Phase Equilib. 338 (2013) 100-113
- Gernert, R. Span: EOS–CG: A Helmholtz energy mixture model for humid gases and CCS mixtures, J. Chem. Thermodynamics 93 (2016) 274-293
- Jäger, V. Vinš, R. Span, J. Hrubý: Model for gas hydrates applied to CCS systems part III. Results and implementation in TREND 2.0, Fluid Phase Equilib. 429 (2016) 55-66
- Jäger, I.H. Bell, C. Breitkopf: A theroretically based departure function for multi-fluid mixture models, Fluid Phase Equilib. 469 (2018) 56-69
- T. Neumann, J. Poplsteinova Jakobsen, M. Thol, R. Span: A new model combining Helmholtz energy equations of state with excess Gibbs energy models to describe reactive mixtures, Chem. Eng. Sci. (2021) in press
Úterý 18. ledna ve 13:00, online
Low cycle fatigue behaviour of auxetic cellular structures using the inelastic energy approach
Dr. Branko Nečemer, Faculty of Mechanical Engineering, University of Maribor, Slovenia
The talk summarises the research work of low cycle fatigue behaviour of 2D auxetic cellular structures. In the presentation, the development and validation of the computational model based on the inelastic energy approach will be presented. In this study, research was focused on the mechanical characterisation of aluminium alloy 5083-H111, the development and validation of the appropriate computational model and, the numerical and experimental analysis of the given auxetic cellular structures characterised with a negative Poisson’s ratio. The experimental testing of the analysed aluminium alloy included the quasi-static and dynamic testing in the low-cycle fatigue regime. Dynamic tests were performed in a strain control at the strain ratios _ = −1 and _ = 0 at different amplitude strain levels. The experimental results were served as a basis for determining the material constants of the energy approach (c1, c2, c3 and c4) and the material parameters of the constitutive material model, which was then used in the subsequent computational analysis using the Simulia Abaqus software. In the computational model for fatigue life prediction, the algorithm of direct cyclic analysis integrated into the Simulia Abaqus software was used to accelerate the numerical simulation and determination of the fatigue life of the analysed samples. The proposed computational model was first validated based on the comparison of numerical and experimental results of flat and CT samples. Based on the good agreement between the computational and experimental results, the validated computational model was used as a basis for determining the fatigue life of the chiral and re-entrant auxetic structure.
2021
Středa 15. prosince ve 13:00, nová posluchárna
On spatio-temporal analysis of turbulent wake behind a circular cylinder
Prof. Václav Uruba, Ústav termomechaniky AV ČR, v.v.i.
a Fakultra strojní Západočeské univerzity
The method of spatio-temporal analysis of data is to be presented. The Oscillation Pattern Decomposition (OPD) method is intended for turbulent data analysis containing both random and pseudo-periodical parts. The method is based on approach defined by prof. Hasselmann for meteorological data Principal Oscillation Pattern (POP) employing Fokker-Planck evolution equation. An example of analysis of turbulent wake behind a circular cylinder will be presented. The three modes with corresponding frequencies characterized by Strouhal numbers 0.2, 0.4 and 0.6 respectively representing turbulence harmonic contents are to be shown.
References:
- Hasselmann, K., PIPs and POPs: The Reduction of Complex Dynamical Systems Using Principal Interaction and Oscillation Patterns, J of Geophysical Research, vol. 93, no. D9, pp 11,015-11,021, September 20, 1988.
- Uruba, V., Near wake dynamics around a vibrating airfoil by means of PIV and Oscillation Pattern Decomposition at Reynolds number of 65000, Journal of Fluids and Structures 55 (2015) pp 372–383.
Čtvrtek 11. listopadu ve 13:00, nová posluchárna
Technical challenges in the LISA project
and the contribution from the Czech Republic
Prof. Niels Lund, National Space Institute Astrophysics and Atmospheric Physics,
DTU, Kodaň, Dánsko, a Fyzikální ústav AV ČR, v. v. i., Praha
The Czech Republic will contribute to ESA’s LISA gravitational wave mission both in the scientific analysis efforts and by delivery of one of the delicate mechanisms on the ultraprecise Optical Benches which are at the heart of the LISA measurement scheme. The Czech instrument contribution is important – but we may hope it will never be used! This may sound strange, but it is like the Fire Brigade; we know it is important – but we hope it will never be user in our neighbourhood!
The LISA project will launch three satellites in a formation flying formation. The technical goal is to measure variations in the inter-satellite distances of 2.5 million km (2.5 10^{9} m) with a precision better than pico-meters (10^{-12} m), i.e. a relative error of 10^{-21}! Only if we can achieve this level of precision can we detect the small deformation of space caused by the gravitational waves!
In the lecture I shall first briefly describe the very complex measurement scheme of LISA and explain where the Czech contribution comes in.
Středa 20. října v 11:00, nová posluchárna
Slow dynamics effects in hysteretic elastic media: physical origin and potentiality for damage detection
Prof. Marco Scalerandi, DISAT, Department of Applied Science and Technology, Politecnico di Torino, Itálie
Slow dynamics in hysteretic elastic media consists in the variation over time of the ultrasonic wave velocity when a conditioning strain is applied to the material. The phenomenon consists in three phases: preconditioning, during which velocity is constant (linear velocity); conditioning (i.e. application of a large strain perturbation), during which velocity evolves slowly towards a new equilibrium value; relaxation (when the conditioning strain is set to zero), during which velocity relaxes back slowly to its linear equilibrium value.
This fully reversible effect was shown in materials with a very different microstructure: metal alloys, consolidated granular media (concrete and sandstones), cracked materials and unconsolidated granular media. The presence of contact interfaces between different grains and between crack surfaces seems to be the cause of slow dynamics, but understanding its physical origin (fluids redistribution, dislocations dynamics, sliding and friction …) is still an open issue, mainly because the same physical mechanisms are not taking place in all materials exhibiting elastic hysteresis.
Here, the main experimental observations related to the relaxation process are recalled and the dependence of the effects on some parameters discussed, in view of quantifying the behavior and highlight features, which are universal for all samples, and eventually features, which are not. Finally, some results are presented to discuss how slow dynamics could be used for materials characterization and damage detection. Slow dynamics is indeed a linear measurement (relaxation) of a nonlinear effect, thus it is expected to keep the sensitivity advantages intrinsic in nonlinear ultrasonic NDT while maintaining the simplicity of the experimental set-up typical of linear ultrasonic NDT.
Středa 29. září v 11:00, nová posluchárna
Heat conduction in microstructured solids
Dr. Dr. Arkadi Berezovski, Department of Cybernetics, School of Science, Tallinn University of Technology, Estonsko / Ústav termomechaniky AV ČR, Praha
The Fourier law is the cornerstone of heat transfer theory and practice. Being well applicable for homogeneous continua, the Fourier law is not sufficient for the description of heat conduction in inhomogeneous solids. Moreover, inner microstructure in a solid can be the source of a hyperbolic character of heat conduction. A variety of phenomenological hyperbolic heat conduction models has been proposed as discussed in [1, 2]. The common feature of the hyperbolic heat conduction models is the extension of the thermodynamic state space by heat flux and/or entropy flux. The most developed approach to the generalization of heat equation is provided by extended irreversible thermodynamics [3]. However, the hyperbolic heat conduction equation is obtained in this framework only under assumption of the independence of internal energy of heat flux. Such an assumption is inconsistent with the main constitutive postulate of the dependence of entropy (and, therefore, internal energy) on temperature and heat flux [3].
The thermodynamically consistent method of the extension of the state space is provided by the internal variable theory [4, 5]. Internal variables are used for accounting for the influence of inner microstructure on heat conduction. Two variants of the internal variable treatment are compared by means of the numerical simulation of two-dimensional heat conduction in a plate under a localised thermal pulse loading. Computations of the same problem by the different internal variable descriptions produce qualitatively dissimilar results. The single internal variable approach [5] leads to a diffusional type of the internal variable evolution. In contrast, the dual internal variable technique provides a wave-like evolution of the internal variables, and, as the consequence, the corresponding wave-like heat transfer. The results are obtained in the dimensionless form, and parameters of models are chosen to emphasize the features of each model.
[1] D. D. Joseph and L. Preziosi, Heat waves, Reviews of Modern Physics, vol. 61, pp. 41–73, 1989.
[2] B. Straughan, Heat Waves, Springer, New York, 2011.
[3] D. Jou, J. Casas-Vazquez, G. Lebon, ´ Extended Irreversible Thermodynamics, Springer, New York, 2010.
[4] B. D. Coleman, M. E. Gurtin, Thermodynamics with internal state variables, The Journal of Chemical Physics, vol. 47, pp. 597–613, 1967.
[5] G. A. Maugin, W. Muschik, Thermodynamics with internal variables. Part I. General concepts, Journal of Non Equilibrium Thermodynamics, vol. 19, pp. 217–249, 1994.
Středa 22. září v 10:00, nová posluchárna
Development of a Solver for Fully Coupled Particle-Laden Flows and Challenges for Model Order Reduction
Dr. Martin Isoz, Ústav termomechaniky AV ČR, v. v. i.
Particle-laden flows are commonly encountered in numerous aspects of day-to-day life ranging from technical applications such as fluidisation or filtration to medicinal problems, e.g. behavior of clots in blood vessels. However, computational fluid dynamics (CFD) simulations containing freely moving and irregularly shaped bodies are still a challenging topic. More so, if the bodies are densely distributed and large enough to affect the fluid flow. In this work, we present a newly developed finite volume solver for modeling flow-induced movement of arbitrarily-shaped solid particles. The modeling approach is based on a hybrid fictitious domain-immersed boundary method (HFDIB) for inclusion of the solids into the computational domain. The bodies movement and contacts are solved via the discrete element method (DEM). Unfortunately, the coupled HFDIB-DEM model structure causes significant limitations with respect to applications of standard projection-based methods of model order reduction (MOR). In the talk, we give an overview of the new solver implementation an capabilities and comment on the challenges the HFDIB-DEM approach poses for MOR.
Čtvrtek 9. září ve 13:00, nová posluchárna
Surface accretion of a pre-stretched half-plane: Biot’s problem revisited
Prof. Giuseppe Tomassetti, Roma Tre University
Motivated by experiments on dendritic actin networks exhibiting surface growth, we address the problem of its stability. We choose as a simple, reference geometry a biaxially stretched half plane growing at its boundary. Actin is modelled as a neo-Hookean material. A linear kinetic relation is assumed between growth velocity and a stress-dependent driving force for growth. The stability problem is formulated and results discussed for different loading and boundary conditions. Connections are drawn with Biot’s 1963 surface instability threshold.
Čtvrtek 24. června v 11:00, online
Fast Fourier Transformation and Finite Element Method
Prof. Miroslav Okrouhlík, Ústav termomechaniky AV ČR
Odkaz na přednášku je k dispozici zde.
Author intends to show the dispersion phenomenon in general from a historical perspective, also to inform about significant contributions of our forefathers, as Newton, Johan and Daniel Bernoulli’s, Jean Baptiste Joseph Fourier, and first of all to report about the dispersion topic and its role in the computational mechanics. The contemporary Fourier’s tools (as FFT), for the efficient treatment of engineering tasks in Finite Element Method, is reminded as well.
Pátek 18. června v 11:00, online
Thermomechanics of the Stefan’s solid-liquid phase transformation
Prof. Tomáš Roubíček, Ústav termomechaniky AV ČR
Záznam přednášky (heslo: 1*Bnl!V.)
The Stefan problem historically describes melting of ice or freezing (solidification) of water as a mere heat-transfer problem with a latent heat. This solid-liquid phase transition however naturally occurs in a mechanical context: melted liquid can flow while frozen solid exhibits some elasticity or some visco-elasticity and even may undergo some inelastic processes as fracture. This needs also to cope with the fluid-solid (so-called fluid-structure) interaction and calls for a model in Eulerian description. Of course, thermomechanical consistency is an ultimate attribute, too. The concepts of semi-compressible fluids, viscoelastic solids in Jeffreys' rheology, phase-field fracture, and nonsimple materials (known also as multipolar fluids) will be employed. Also superheating/supercooling effects will be involved, as well as a mathematical analysis briefly outlined. Some enhancements of this basic thermomechanical scenario will be mentioned, too.
Středa 26. května v 10:00, online
Laser Shock Peening (LSP) Laser Explosion and Shear Wave Propagation
Prof. ing. František Maršík DrSc., Ústav termomechaniky AV ČR
Odkaz na záznam přednášky je k dispozici zde.
Although the parameters of the laser pulse are known: the total light energy (5 J), the beam diameter (2.45 mm) and the pulse length (14 ns), the dynamics of the laser explosion itself is unknown. From the point of view of the studied application, the unknown quantities are: the magnitude of the generated pressure in the area of strongly superheated steam (or plasma), the rate of its expansion and its subsequent attenuation. The dynamics of the generated pressure pulse depends on the viscoelastic properties of the irradiated medium (304L austenitic steel) and the absorbing covering medium (water). Physical analysis and numerical simulation show that the magnitude and shape of the residual stress (reinforcement) depends on the choice of material model.
To describe the dynamics of an explosion, the starting point is the balance of the internal energy of the superheated gas (partially ionized water vapor) is needed. The amount of internal energy is given by the absorption of light energy. This energy is then transformed into the required expansion work and is reduced by radiation due to the high temperature.
The consequence of the high pressure magnitude (3-7 GPa) and the high expansion rates (10^{6}-10^{9} s^{-1}), shock waves are generated in both water and steel. Due to the existence of these waves, which propagate at a speed greater than the corresponding speed of sound, the pressure reaches extreme values and causes strong defor-mation of the material.
From the point of view of the subsequent strengthening of the material, the dynamics of the shock wave propagation in the steel is decisive. Modeling the consequences of a shock wave is, in addition to the standard elasticity, dependent on the plasticity model of the steel. Both the Ramberg-Osgood hardening model and the Bodner-Parton dislocation movement model are presented in the lecture.
The movement of dislocations can be characterized by the viscosity depending on the rate of deformation. In this way, the material strengthening is explained by overcoming atomic bonds, which coressponds to the hardening work. The movement of dislocations can be modeled by shear waves, which are strongly dispersive. In areas of high viscosity (before the shock wave) they precede the pressure shock wave. The concept of shear waves allows to describe with some accuracy the strengthening of the material due to extremely fast compression.
The presented analysis shows, that to achieve a higher residual stress at the same laser energy, it is more ad-vantageous to use a pulse of shorter length. For greater depth of reinforcement, it is necessary to use a longer pulse. Currently, an experiment is always needed to model LSP. The experimental residual stress data used were provided by the HiLASE Center Institute of Physics CAS. After calibration, the LSP process can also be used to determine the properties of the material under extremely fast loads.
Čtvrtek 29. dubna v 11:00, online
Slow Dynamics as a Multi-Relaxation Phenomenon
Dr. Jan Kober, oddělení Rázy a vlny v tělesech, Ústav termomechaniky AV ČR
Odkaz na staženou přednášku je k dispozici zde.
Slow dynamics is a phenomenon associated with elastic hysteresis. When a material is subjected to an external strain excitation, a gradual softening occurs (conditioning phase), once the excitation ends, the material slowly relaxes back to its original state (relaxation phase). This behavior was generally associated with consolidated granular materials such as rocks or concrete, but it was also found in damaged metals, where it manifests in a much more limited extent. The physical origins of slow dynamics are generally attributed to intergrain/interfacial mechanics and friction. As such, it is reasonable to expect, that the relaxation process incorporates some information about the material structure. It was shown, that the relaxation process can be interpreted as a superposition of exponential decays with varying time scales. This multi-relaxation model can be used as a stepping stone to a perhaps more physical model of continuous distribution of decay times. By analyzing relaxation curves of various materials, a link between the distribution peak location and grain size was found. Moreover, when a material damage is on a larger size scale than the microstructure, as is a case for e.g., cracks, bimodal relaxation times distributions were observed. The research of slow dynamics is challenging in various aspects ranging from the experimental management requiring fast and extremely precise velocity measurements, to data post-processing, where a careful parameter optimization is necessary.
Účastí na přednášce souhlasíte, že pořadatel smí pořídit snímek obrazovky účastníků a následně jej poskytnout Ministerstvu školství, mládeže a tělovýchovy (MŠMT). MŠMT je poskytovatelem financování projektu OP VVV, reg. č. CZ.02.2.69/0.0/0.0/18_053/0017555, „Podpora zahraničních stáží pracovníků Ústavu termomechaniky AV ČR“ a zpracovatelem poskytnutých dat.
Pondělí 8. března ve 13:30, online
Laser shock peening, principal, use and related phenomena
Dr. Jan Brajer, Centrum HiLASE, Fyzikální ústav AV ČR, Dolní Břežany
Odkaz pro připojení k přednášce je k dispozici zde.
Abstract: |
The laser shock peening (LSP) process using a Q-switched pulsed laser beam for surface modification. The development of the LSP technique and its numerous advantages over the conventional shot peening (SP) such as better surface finish, higher depths of residual stress and uniform distribution of intensity. The generation of shock waves, processing parameters, and characterization of LSP treated specimen is great topic for deeper understanding. Special attention will be given to the influence of LSP process parameters on residual stress profiles, material properties and structures. Based on the studies so far, more fundamental understanding is still needed when selecting optimized LSP processing parameters and substrate conditions. Furthermore, enhancements in the surface micro and nanohardness, elastic modulus, tensile yield strength and refinement of microstructure which translates to increased fatigue life, fretting fatigue life, stress corrosion cracking (SCC) and corrosion resistance will be discused with audience.
Středa 27. ledna ve 13:30, online
Numerical simulations of flexible multibody systems described by absolute nodal coordinate formulation
Ing. Radek Bulín, Ph.D., Západočeská univerzita, Fakulta aplikovaných věd
Odkaz pro připojení k přednášce je k dispozici zde.
Abstract: |
A large group of real mechanical problems can be modelled and analysed using the approaches of flexible multibody dynamics. The computational models in the form of differential-algebraic equations can be quite complex and therefore it is suitable to develop both efficient and accurate approaches for the dynamic analysis of such model. This talk will be dedicated to the description of various finite elements defined by the absolute nodal coordinate formulation (ANCF), which is suitable for modelling of flexible bodies that undergo large displacements, rotations and deformations. Eligible numerical technics for effective evaluation of the elastic forces as well as suitable integration schemes for multibody systems containing the ANCF elements will be discussed.
Středa 27. ledna ve 13:00, online
Dynamics of large rotating systems – methods and applications
doc. Ing. Michal Hajžman, Ph.D., Západočeská univerzita, Fakulta aplikovaných věd
Odkaz pro připojení k přednášce je k dispozici zde.
Abstract: |
Rotating mechanical systems are interesting systems from the viewpoint of inertia effects arising during the rotation and mutual interaction of subsystems. The presentation will be aimed at introducing the approaches for the modelling of such systems and a description of the software development suitable for the solution of real industrial problems. Main components of large rotating systems will be described, and their characteristics will be explained. The whole modelling methodology will be demonstrated in the typical industrial problems of large turbomachines in nuclear and conventional power plants. The modelling and dynamical analysis of oil journal bearings will be addressed in more detail.
2020
Čtvrtek 10. prosince ve 13:00, online
Experimentally Validated Enhanced Constitutive Model of NiTi-based Shape Memory Polycrystals
RNDr. Miroslav Frost, Ph.D., Ústav termomechaniky AV ČR
Odkaz pro připojení k přednášce je k dispozici zde.
Abstract: |
Shape memory alloys are metallic materials exhibiting unusual properties of being able to sustain and recover large strains and "remember" the initial configuration and return to it with temperature change. The peculiar mechanical response stems from rearrangements of the crystal lattice associated with a martensitic phase transformation induced by variation of temperature and/or variation of the applied mechanical load. The most common and practically utilized are polycrystals from nickel-titanium-based alloys, which exhibit many additional peculiarities of the mechanical response, e.g., the pronounced tension-compression asymmetry, the inclination towards strain localization, or appearance of an intermediate phase (R-phase). In the talk, the enhanced macroscopic constitutive model for NiTi SMA developed at the Institute of Thermomechanics will be introduced. It captures the mentioned phenomena via simple, albeit effective modifications of basic "building blocks" of a generic model formulated within the thermodynamics with internal variables. The enhancements of the model have been motivated by the state-of-the-art experimental research. The numerical implementation is based on the incremental energy approach. Simulations demonstrating the model's capabilities both at macro- and meso- scales will be presented and compared to experimental data.
Pondělí 2. listopadu ve 13:00, online
Design and analysis of membrane structures
Dr. Rostislav Lang, Ústav stavební mechaniky VUT Brno a FEM consulting, s.r.o.
Odkaz pro připojení k přednášce je k dispozici zde.
Abstract: |
The subject of the presentation are lightweight structures, composed mostly of membrane or cable parts. First, the special aspects during the design and analysis process will be introduced, which leads to the form finding and cutting patterns generation process, as well as to using the special wrinkling assumption in the analysis. Further, the methods for those processes will be presented and finally demonstrated by the calculation tool, developed during the doctoral research and implemented into the commercial software.
Středa 16. září v 10:00, posluchárna B
Corrosion study in subcritical and supercritical water: An electrochemical approach
Prof. Jan Macák, Ústav energetiky, Fakulta technologie ochrany prostředí VŠCHT Praha
Abstract: |
Interest in supercritical water (SCW) is motivated by its use for different purposes: supercritical water is used as a working fluid and coolant in fossil-fueled power plants, supercritical water oxidation systems are designed for destruction of dangerous waste and a supercritical water-cooled reactor (SCWR) was selected as one of the six Generation IV International Forum concepts selected for further investigation. The experimental corrosion data obtained for SCW supported the corrosion model assuming a superposition of two parallel corrosion processes: a "chemical oxidation" (CO) mechanism and an "electrochemical oxidation" (EO) mechanism. Validity of this model was confirmed by in-situ electrochemical impedance spectroscopy measurements.
Středa 13. května 2020, 13:00, přednáška v režimu online
High-order methods in simulations of fluid dynamics problems
Dr. Jan Pech, Ústav termomechaniky AV ČR
Odkaz pro připojení k přednášce je k dispozici zde.
Abstract: |
Solution accuracy is often a limiting factor for correct reproduction of physical phenomenons in numerical simulations. In the field of fluid dynamics, numerical inaccuracy may result in wrong flow instabilities or degeneration of flow structures. In previous decades, lower-order accurate methods were dominating for its robustness and ease of implementation, while resolution was compensated through sophisticated models.
Today, libraries implementing spectral/high-order (finite) elements are developed sufficiently (e.g. Nektar++) and bring an ambition to solve with high accuracy such systems as is the Navier-Stokes's. An additional benefit of the high-order approach is in insight to coefficient spectra over individual elements, since the decay of expansion coefficient spectra brings instant and computationally cheap information about quality of solution approximation in spatial coordinates.
The talk will introduce significant features of the spectral/hp element method and its application to problems currently investigated in Laboratory of CFD (D1), as is flow of heated/cooled fluid with variable material properties, fluid-structure interaction or flow separation in transition to turbulence.
Středa 6. května 2020, 13:00, přednáška v režimu online
Advanced Titanium Alloys for Medical Applications
Dr. Josef Stráský, Matematicko-fyzikální fakulta UK
Odkaz pro připojení k přednášce je k dispozici zde.
Abstract: |
Titanium alloys, despite originally developed for aerospace application, are considered as a 'material of choice' for various types of medical implants. In the last two decades, dedicated Ti alloys for the biomedical use have been developed, such as Ti-35Nb-6Ta-7Zr alloy. These alloys must meet several requirements including good biocompatibility, satisfactory strength and low modulus of elasticity to avoid so-called stress shielding effect. In designing Ti alloys we typically face trade-off between high strength and low modulus. Our important finding is that alloying by oxygen leads to a significant interstitial strengthening. However, simultaneously, oxygen addition leads to an adverse increase of elastic modulus. We now understand that the body centered cubic parent matrix (beta phase) exhibits the lowest elastic modulus when the stability of the beta phase is low due to the 'proximity' to martensitic beta to alpha'' transformation (anomalous softening). Oxygen causes relative beta phase stabilization resulting in increased stiffness. We therefore designed several alloys with reduced stability of beta phase by reducing the content of niobium and tantalum which are beta stabilizing elements. We successfully broke the strength-modulus trade-off and achieved biocompatible Ti-based material with unparalleled combination of yield stress exceeding 900 MPa and modulus of elasticity below 70 GPa.
Středa 29. dubna 2020, 13:00, přednáška v režimu online
Achievements, agreements and quarrels of forefathers of mechanics
Prof. Miloslav Okrouhlík, Ústav termomechaniky AV ČR
Odkaz pro připojení k přednášce je k dispozici zde.
Abstract:
The purpose of the lecture is to show that the original procedures, leading to discovery, or rather to invention, of mechanical laws and principles, that are taken for granted today, were lengthy, complicated and far from being straightforward. Following lifetime destinies of a few forefathers of mechanics we try to unveil the difficulties and complications that they witnessed in derivations of their laws and formulas and to show how their personal grievances and bitter quarrels, that sometimes lasted for decades, complicated not only their mutual relations but sometimes made obstacles in scientific communications between nations. We will concentrate on lives and works of Galileo Galilei, Johannes Kepler, Robert Hooke, Christiaan Huygens, Isaac Newton and Gottfried Wilhelm Leibniz trying to bring their narratives into proper relations. |
Úterý 21. dubna 2020, 13:00, přednáška v režimu online
Application of boundary element type methods in computational aerodynamics
Dr. Chandra Shekhar Prasad, Ústav termomechaniky AV ČR
Abstract:
Boundary elements type methods (BEM) are well known method in many engineering application of numerical simulation. BEM for the aerodynamic or hydrodynamic flow modeling was first introduced in 1940s in NASA by Falkner as vortex lattice method (VLM) and after 60s by Hess & Smith and it is termed as Panel method (PM), since then it became very popular in aircraft industry and NASA has setup special research department for PM research. In BEM or PM unlike field based CFD methods, only solid boundary in the flow field is needed to be discretized which greatly reduce the computational cost. These methods are good compromise of speed and accuracy and being used by many aircraft manufactures for aerodynamics and aero-elasticity calculation. During the talk mathematical back ground, numerical implementation and some example will be discussed. |
Středa 15. dubna 2020, 13:00, online
Models of „semi-compressible“ fluids as a compromise between incompressible fluids and compressible gasses
Prof. Tomáš Roubíček, Ústav termomechaniky AV ČR
Abstract:
Starting from a so-called quasi-incompressible fluid models developed originally as an approximation of incompressible models, various models will be developed which are slightly compressible but, anyhow, not fully compressible as full gas-dynamical models. Being called "semi-compressible", particular models will enjoy the attributes: |
Středa 11. března 2020, 10:00, posluchárna B
Thermomechanics in optical fibre drawing, splicing, and everyday use
Prof. Pavel Honzátko, Ústav fotoniky a elektroniky AV ČR
Abstract:
Overview of the optical fibre drawing technology in the Institute of Photonics and Electronics of the Czech Academy of Sciences (ÚFE) will be presented. Some future trends in the fibre technology will be discussed together with related thermomechanical problems which include preform shaping by laser ablation, capillary drawing, hollow core and air-clad fibre drawing, polymer and metal coating, thermomechanical polymer stripping, fibre fusion splicing and tapering, splice testing, fibre cooling, and mitigation of acousto-optical effects. |
Čtvrtek 27. února 2020, 14:00, posluchárna B
Structural Design and Analysis at OHB System AG
Dr. Markus Geiß, Structural and Thermal Development Engineer, OHB System AG, Weßling, Německo
Abstract:
This lecture gives an introduction to the field of structural design and analysis of optical space instruments at OHB System AG. OHB System AG is one of the three leading space companies in Europe which specializes in design and manufacturing of high-tech solutions for space, science and industrial applications. To start the lecture, special considerations of spacecraft structures are discussed and the typical tasks of a structural development engineer are presented. Next, design highlights of optical instrument structures and their analysis methodologies are reviewed. After a brief discussion of topology optimization and additive manufacturing used for design and manufacturing of optical space instruments, thermos-elastic aspects of spacecraft structures are addressed. Finally, the process of assembly, integration and testing (AIT) is discussed, which typically concludes the hardware development-cycle of space structures. |
Pondělí 17. února 2020, 10:00, posluchárna B
Experimental and Numerical Procedures for Calibration of Advanced Phenomenological Models of Metal Plasticity
Ing. Slavomír Parma, Ph.D, Ústav termomechaniky AV ČR, v. v. i.
Abstract:
The talk summarizes the research done within author’s internship at the Northern Arizona University, AZ, USA. Elastic domain of metals is bounded by the yield surface. When the material is loaded over the elastic limit, changes in size, position, and shape of the yield surface are observed. The new experimental procedure that employs hollow tubular specimens was developed to detect these phenomena. In the experiment, specimens are axially preloaded over the initial elastic limit to develop a distorted shape of the yield surface and consequently loaded by a sequence of combined axial load and torque to carefully probe the boundary of the elastic domain. This experimental methodology requires (i) a real-time evaluation of the effective plastic strain, and (ii) a real-time conditional control of the experiment. The numerical part of this research is focused on the calibration of current models of phenomenological plasticity on the experimental data of multiaxial ratcheting. These kinds of models usually feature about 10–20 material parameters and need to be calibrated numerically. The proposed calibration procedure employs numerical integration of models combined with the optimization method based on the gradient descent. The algorithm is coded in FORTRAN language. Tato přednáška se koná v návaznosti na projekt OP VVV EF16_027/0008500 - Podpora zahraničních stáží pracovníků Ústavu termomechaniky AV ČR (2018-2020, MSM/EF) |
Středa 8. ledna 2020, 10:00, posluchárna B
First-principles calculations of elastic constants for complex systems
Ing. Martin Zelený, Ph.D., fakulta strojní VUT v Brně
Abstract:
First principles or ab initio means to perform the calculations of the properties of a system from fundamental quantum mechanics with no parametrization or knowledge of experimental data. The Density Functional Theory (DFT) forms the basis for most of current first-principles methods and it is able to provide the exact information about the electronic structure of the system under study and subsequently about its internal energy. Nowadays, employing DFT for calculations of elastic properties is routine task for system with ideal crystal lattices. On the other hand, estimation of elastic properties is still challenging for systems with chemical disorder or with defect in crystal lattice despite large computational resources available to the scientific community. The recently introduced stress-strain method together with the DFT package VASP is able to provide the full matrix of elastic constants with reasonable computational cost also for large supercells describing lattices with defects, e.g. grain boundaries or twin boundaries. Large supercells are also required for description of alloys with chemical disorder by the supercell-based quasirandom structures (SQS). Further directional optimization of SQS (do-SQS) supercell is necessary to obtain proper matrix of elastic constants that will reflect the symmetry of the simple ordered lattice. Using of the stress-strain method and do-SQS approach will be demonstrated for Ni3Si and Ni3Al intermetallics, CoCrNi high entropy alloy and Ni2MnGa magnetic shape memory alloy. |