2017 Programme

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17. 8. 2017, 11:00

Modelling extreme deformation and dynamic behaviour of materials using mesh-less methods

Dr. Anton Tkachuk, Institute for Structural Mechanics, University of Stuttgart, Stuttgart, Germany

in collaboration with Anne Schäuble, Prof. Manfred Bischoff

Standard explicit dynamic simulation relies on diagonal or lumped mass matrices. Lumped mass enables a trivial computation of the nodal accelerations from the total force vector. Moreover, critical time step estimators and contact-impact algorithms for such mass types are well understood and developed. A disadvantage of the exlicit time integration with the lumped mass is huge number of the time steps even for short time dynamics. Recently, several approaches for reciprocal mass matrix that allows higher time steps and reduction of the total computational cost were proposed. A reciprocal mass is a sparse inverse of mass matrix that usually has a mask/structure of consistent mass or stiffness matrix. It can be constructed directly and cheaply either with variational or with algebraic methods. Achievable speed-up with respect to lumped mass is from 20% to 50%.

In this talk, an overview of existing approaches of construction reciprocal mass matrices is given and recent advances in reciprocal mass matrices for impact algorithms, time step estimation and assessment of the error in heterogeneous materials are presented.
 

17. 8. 2017, 10:00

Multi-Scale Structural Gradients Optimize  the Bio-Mechanical Functionality of the Spider Fang

Dr. Benny Bar-On, Laboratory for the Mechanics of Complex Materials, Department of Mechanical Engineering, Ben-Gurion University of the Negev


The spider fang is a natural injection needle, built as a multi-scale composite material with outstanding mechanical properties. In this study we introduce a hierarchical modeling for the spider fang, based on computer tomography and SAXS measurement, and analyze the correlation between the fang architectural motifs and its macroscopic elastic behavior. Analytical methods and Finite-Element simulations are used for the mechanical analysis and the effects of small- and large-scale structural gradients on the macroscopic mechanical properties are investigated.

It is found that the multi-scale structural gradients of the spider fang optimize its performances in term of load-bearing stiffness and strength, and that the naturally evolved fang architecture provides optimal mechanical properties compared to other alternative structural configurations.

 

26. 6. 2017, 10:00

Modelling extreme deformation and dynamic behaviour of materials using mesh-less methods

Dr. Raj Das, Sir Lawrence Wackett Aerospace Research Centre, School of Engineering, RMIT University, Australia

The seminar will present overview of computational mechanics research at the Centre for Multifunctional and Composite Materials of RMIT University, Australia. Our research covers both fundamental and applied aspects of material behaviour and failure processes. This presentation will encompass computational modelling of material deformation, damage and fracture using multi-scale techniques in conjunction with mesh-less methods, novel composite materials development and damage tolerance structural optimisation.

Multi-scale modelling of damage and fracture progression linking nano to macro scales and associated development of coupled computational modelling tools will be highlighted. The strengths of mesh-less methods will be illustrated with reference to both low to high-speed impact induced fractures and small to large scale problems. These include several dynamic fracture and fragmentation processes, such as hypervelocity impact fracture, nano-scale machining, large scale geo-mechanical failures (magma intrusion, caving, slope stability, etc).

One of our core areas to be presented is novel impact and blast resistant, light weight composite material developments for aerospace components subjected to high-speed loading and extreme deformations, as occurs in the cases of debris impact on spacecrafts, bird strike on aircraft engines, blast induced failures, etc. Lastly novel shape and topology optimisation methodologies for damage tolerance optimisation, i.e. maximising the residual strength and fatigue life, of aero-structures will be highlighted. Case studies from projects with Royal Australian Air Force and Defence Science and Technology Organisation will be presented to demonstrate the practical implementation and utilities of the developed design and analysis methodologies.

 

 

19. 6. 2017, 10:00

Additive Manufacturing of metals: Past, today and tomorrow

Dr. Edson Costa Santos, SENAI Innovation institute in Laser Processing, Joinville, Santa Catarina, Brazil

 
The lecture will be addressed the following topics in Additive Manufacturing (AM) of metals:

· Draw some observations from various attitudes to AM world-wide.
· Review shortly various additive manufacturing technologies, their virtues and drawbacks.
· Address cases, in which additive can/cannot replace conventional manufacturing - problems with distortions, variability in micro-structure and consequences, etc.
· Comparison of additive and conventional micro-structures and their impact on macro-mechanical properties: strength, fragility, fatigue, impact resistance, etc.
· Use of additive manufacturing for meta-materials (auxetic and other) for the purposes of "energy absorption or distribution" and "mechanical strength with low weight".
· Additive manufacturing process certification and/or serial production of components - competitiveness in terms of both function and price.
· Design of components for Additive Manufacturing - material only there "where needed" and the related development of software (e.g. topology optimization).
· Future of AM - visions and expectations.
· Describe and introduce FIESC - SENAI focus in AM.

Dr. Edson Costa Santos spent more than a decade in various laboratories related to Additive Manufacturing in Europe, South America and Japan. In the presentation, it will be drawn from his experience, and present a view of current AM layout – technologies, directions and main leaders.
 

15. 6. 2017, 10:00

Quasibrittle Failure Probability and Scaling

Prof. Zdeněk P. Bažant, Northwestern University, Evanston, Illinois, USA

 
The size effect on structural strength and its probability distribution function (pdf) is a complex problem for quasibrittle materials because their failure behavior transits from quasi-plastic at small sizes to brittle at large sizes. These are heterogeneous materials with brittle constituents in which the size of inhomogeneity, or representative volume element (RVE), is not negligible compared to the structure size. Aside from concrete, the archetypical example, they include fiber composites, coarse-grained ceramics, rocks, sea ice, snow slabs, wood, bone, foam, stiff soil, dry snow,ccarton, etc., and on the micro- or nano-scale, all brittle materials become quasibritle. Since the break probability is known exactly only for interatomic bonds (being equal to frequency), Kramer’s rule of transition rate theory is applied to nano-crack jumps. Based on proving the rules of multiscale transition of tail probabilities of break to material scale, the probability distribution function (pdf) of strength of one macro-scale representative volume element (RVE) is shown to have a Weibullian tail, calibrated to reach to probability circa 0.001, the rest being Gaussian. On the structure scale, only Type 1 failure is considered, i.e., the structure fails as soon as the first RVE fails. Hence the weakest-link model applies on the structure scale. But, crucially, the number of links is finite, because of non-negligible RVE. For increasing structure size, the Weibullian portion gradually spreads into the Gaussian core. Only in the infinite size limit the distribution becomes purely Weibull, but, importantly, with a zero threshold. Based on an atomistic derivation of the power law for subcritical macro-crack growth, a similar Gauss-Weibull transition is shown to apply to structure lifetime. The theory is then extended to the size dependence of Paris law and Basquin law for fatigue fracture, to statistics of fatigue lifetime, and to residual strength after a period of preload. The theory is shown to match the existing experimental results on the monotonic strength, residual strength after preload, static and fatigue crack growth rates, and static and fatigue lifetimes, including their distributions and size effects on the distributions. There are three essential consequences: 1) The safety factors must depend on structure size and shape; 2) To predict the pdf of strength, the size effect tests of mean strength suffice; 3) To predict the static and fatigue lifetimes, it suffices to add tests of initial subcritical crack growth rate. An interesting mathematical analogy predicting the lifetime of nano-scale high-k dielectrics is also pointed out. Finally, a new “fishnet” statistics for strength of biomimetic nacre-like lamellar structures, modelled as a square fishnet pulled diagonally, is presented. This simple model differs from the weakest-link model as well as the fiber bundle model. The pdf is found again to transit from Gaussian to Weibullian, but in a different way.
 

30. 5. 2017, 15:00

ISG-Israel Smart Grid consortium and Large-Scale Power System Dynamics

Prof. Yuval Beck, Head of Power Engineering, Faculty of Engineering, Holon Institute of Technology, Israel
Prof. Yoash Levron, Professor of Electrical Engineering, Faculty of Electrical Engineering, Technion – Israel Institute of Technology

A Smart Grid demonstrator was implemented within the framework of the "Israel Smart Grid
– ISG" Magnet project. The goal of the project was to implement and develop technologies for
optimizing and controlling Smart Grids. The main achievement of the project is its operation
system which is hierarchical in nature. Namely, the control and commands are not centralized
but rather distributed from top levels downwards. Every such control level can also be selfcontained.
The project consists of a demonstration of rout of electric power that is delivered to
a modern "Procumer" (Producer and Consumer), precisely upon its request, with minimum
power failures, energy optimization and minimal electricity costs. The demonstrator is
constructed in various sites and controlled by a virtual network. The virtual network consists
of controllable loads and some generators. Some of the actual controllable loads are motors,
air conditioning chiller, air treatment units and others. The loads are controlled by an
Intelligent Home Gateway Unit (IHGU) which operates in accordance to the contract between
the grid and the prosumer or consumer. The system also controls, via web services, two
remote sites of the Israel Electric Company – IEC, consisting of a virtual neighborhood.

Large-scale dynamics models of power systems are mostly based on time-varying phasors.
However, with increasing integration of distributed and renewable sources into existing power
grids, the assumption of time-varying phasors (or quasi-static models) becomes less accurate,
and may even lead to misleading conclusions regarding the system dynamics and stability.
During the lecture I will briefly review and compare several types of dynamic models,
describe several paradoxes that result from misuse of these models, and describe our group's
approach to this problem.

 

3. 5. 2017, 10:00

Non-standard damped oscillators

Prof. Dalibor Pražák, Department of Mathematical Analysis, Faculty of Mathematics and Physics, Charles University in Prague

Damped oscillators of the form x'' + a(x)x' + b(x) = f(t) are classical models in mechanics and for regular enough a(.), b(.), say C1 or Lipschitz, the mathematical theory is very well understood. Non-standard analysis (NSA), on the other hand, is a rather strong and abstract logical framework. Using NSA, various mathematical theories can be embedded into larger universes with non-standard ("ideal") elements. The simplest and most famous examples are infinitely large and small numbers (which are thought by some advocates of NSA to be fatally missing from Calculus for nearly 200 years by now.)

Curiously enough, some nonstandard choices of the functions a(.) and b(.), taking infinitely large values, or with infinitely steep growth, are natural models of some "non-standard" mechanical elements: damper with Coulomb's friction, inextensible string, or more generally, collision of a moving mass with a wall.
In our talk, we will see how these situations can be modelled within the framework of NSA. We show that interesting dynamics can occur and even more, new interesting questions can be asked.
 

5. 4. 2017, 10:00

Implosive magnetocumulative generator for effective energy conversion

Dr. Jiří Šonský, Institute of Thermomechanics of the CAS, v. v. i.

History of magnetohydrodynamic generators goes back to 1832 when Michael Faraday tried first experiments. Magnetocumulative generators were developed by Andrei Sakharov at the start of the 1950s, but up to these days such devices are not used for public energetics and remain in experimental, often military development. Therefore we have developed new thermal plasma source for magnetohydrodynamic or magnetocumulative generator suitable for general energetics. The thermal plasma is created from combustible mixture by implosion - spherical compression driven by convergent detonation wave. The detonation wave is initiated by a weak spark and by means of deflagration to detonation transition in detonation tube. Convergent polyhedral shape of the detonation wave is formed by large number of vents opened to hemispherical combustion chamber. Propagation of the detonation wave and its multiple branches is tracked by an array of ionization probes. Resulting high velocity plasma is ejected from a nozzle near the geometrical center of the device. The plasma is observed by capturing emitted light by hi-speed camera to determine plasma velocity. Construction of the implosion plasma source and possible variants of extraction of electrical energy from kinetic energy of the plasma by interaction of high velocity plasma with seed magnetic field will be also discussed in this presentation.
 

1. 3. 2017, 10:00

Atmospheric Boundary Layer: main characteristics and methods of the research in context of continuum mechanics

Prof. Zbyněk Jaňour, Institute of Thermomechanics of the CAS, v. v. i.

 
The bulk of the fluid on the Earth's surface can be found in the atmosphere and the oceans. Geophysical Fluid mechanics investigate it. The flow inside the area near the Earth's surface is called Atmospheric Boundary Layer in a certain analogy with the classical theory of fluid mechanics. Its properties, methods of research, taking into account their shortcomings; will be discussed in the following paragraphs.
1) Introduction: the introduction of the concept of and the reasons for its monitoring;
2) The basic characteristics (Equations of motion in continuum mechanics approximation, Flow in a rotating coordinate system, The temperature stratification, Turbulence and determinism)
3) Modelling (experimental methods, numerical methods)
4) Tasks solved in the Laboratory of Environmental Aerodynamics;
5) New problems to solve:
Verification and validation of mathematical models,
Many scales problem;
6) Conclusion: applications
 

14. 2. 2017, 10:00

Modelling of complex processes in nanopowder fabrication using thermal plasma flows

prof. Masaya SHIGETA, Joining and Welding Research Institute, Osaka University, Japan

 

Thermal plasmas have been expected as a promising tool for mass-production of nanopowders [1] because thermal plasmas offer a distinctive thermal-fluid field involving high temperature, high chemical reactivity and variable properties. Furthermore, thermal plasmas have steep temperature gradients at their fringes where many small nanoparticles are produced rapidly from the material vapour as a result of the highly supersaturated state. However, it is still difficult to investigate the formation mechanism of nanoparticles generated in/around a thermal plasma because the process involves remarkably intricate mass transfer of phase conversions in micro-second scales. Moreover, the plasma fringe is fluid-dynamically unstable and consequently it forms a turbulent mixing field composed of multiscale eddies [2]. The growing nanoparticles are transported by the complicated convection as well as diffusion and thermophoresis. In this lecture, several modelling works to simulate those complex processes are explained.

 

1. 2. 2017, 10:00

Highlights of plasma spraying in the life of one researcher

Dr.  Tomáš Chráska, Institute of Plasma Physics of the CAS, v. v. i.

Thermal spraying techniques are coating processes in which melted or heated materials are sprayed onto a surface. There is a great variety of feedstock materials that can be thermally sprayed including solid powders and suspension liquids. There is also a great variety of thermally sprayed coatings used for many different applications including for example the thermal barrier coatings in jet engines. Plasma spraying process is a member of the thermal spraying family of techniques. It uses plasma gun to generate a plasma jet that melts feedstock materials. This talk is not going to present a complete overview of plasma sprayed coatings and their applications. It will rather present a set of interesting and sometimes intriguing examples of what can achieved by plasma spraying. The examples will include nanopowders, epitaxial growth of crystals in plasma sprayed coatings, amorphous and nanocomposite coatings, spraying of suspension and more.
 

4.1. 2017, 10:00

Usage of time reversal signal processing in nondestructive diagnostics of materials and structures

Dr. Zdeněk Převorovský, Institute of Thermomechanics of the CAS, v. v. i.

 

Time reversal processing of acoustic and ultrasonic signals (TRA) is very effective tool for complicated problems solution in many fields like nondestructive testing and evaluation (NDT/NDE) of materials and structures. TRA enables waves focusing in time and space and therefore precise localization and reconstruction of wave sources in strongly inhomogeneous, anisotropic, and dispersive media. Properties of TRA may be used to signal processing in acoustic emission (AE), nonlinear elastic wave spectroscopy (NEWS), and also e.g. in seismology, medicine, telecommunications, etc.

TRA principles will be mentioned in the talk, and its potentials in AE source location and identification will be discussed. Outlined will be also some questions of a new approach to that inverse problems solution by using the ultrasonic signal transfer from a real body onto its laboratory and/or numerical model where they can be analyzed more easily.
 


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