Institute of Thermomechanics Seminar Programme

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.

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.‏

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.

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.

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.

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.

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.

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)

(The lecture will be held in Czech)

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.

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, Cr₂AlC, or Y₂O₃-stabilized ZrO₂ 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.

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.

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.

Concepts for Carbon Capture and Storage or Carbon Capture and Utilization (CCS/CCU) have always considered the transport of CO₂ 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 CO₂ sources (essentially power plants) and storage sites, but include CO₂-transport networks, in which multiple industrial emitters inject CO₂. The handling of fluctuating CO₂ flows with different origin and separated using different capture technologies results in new challenges for CO₂ 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 CO₂-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

1. Jäger, V. Vinš, J. Gernert, R. Span, J. Hrubý: Phase equilibria with hydrate formation in H₂O + CO₂ mixtures modeled with reference equations of state, Fluid Phase Equilib. 338 (2013) 100-113
2. Gernert, R. Span: EOS–CG: A Helmholtz energy mixture model for humid gases and CCS mixtures, J. Chem. Thermodynamics 93 (2016) 274-293
3. 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
4. Jäger, I.H. Bell, C. Breitkopf: A theroretically based departure function for multi-fluid mixture models, Fluid Phase Equilib. 469 (2018) 56-69
5. 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

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.

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.

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 (2.5 10⁹ 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.

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.

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.

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.

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.

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.

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.

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 (10⁶-10⁹ 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.

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.

By attending this online event you consent that we may take a screenshot of the participants and provide it to the Ministry of Education, Youth and Sports (MEYS). MEYS is the funding provider for project CZ.02.2.69/0.0/0.0/18_053/0017555, "Support of international mobility of researchers of the Institute of Thermomechanics of the CAS" and the processor of the provided data.

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.

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.