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Microstructure modifications of Al-Si-coated press-hardened steel 22MnB5 by laser welding
Šebestová, Hana; Horník, Petr; Mika, Filip; Mikmeková, Šárka; Ambrož, Ondřej; Mrňa, Libor
2024 - anglický
Weld microstructure depends on the characteristics of welded materials and parameters of welding technology, especially on the heat input that determines the peak temperature and the cooling rate. When the coated sheets are welded, the effect of the chemical composition of the coating must be also considered even though its thickness is only a few tens of microns. During 22MnB5+AlSi laser welding experiments, the ferrite-stabilizing elements of coating modified the weld metal microstructure. Ferrite appeared in a quenched weld metal. The rapid cooling rate accompanying welding with a focused beam limited the homogenization of the weld metal which resulted in the formation of ferritic bands in the regions rich in Si and especially in Al. On the other hand, a high level of homogenization was reached when welding with the defocused beam. The ferritic islands uniformly distributed in the weld metal were formed at 0.4 wt% and 1.6 wt% of Si and Al, respectively. The doubled heat input reduced the Al content to 0.7 wt% insufficient for the ferrite formation at still relatively high cooling rates. Predicting the distribution of ferrite in the weld metal is challenging due to its dependence on various factors, such as cooling rate and the volume of dissolved coating, which may vary with any modifications made to the welding parameters. Klíčová slova: laser welding; high-strength steel; microstructure; heat input; ferrite stabilization Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Microstructure modifications of Al-Si-coated press-hardened steel 22MnB5 by laser welding

Weld microstructure depends on the characteristics of welded materials and parameters of welding technology, especially on the heat input that determines the peak temperature and the cooling rate. ...

Šebestová, Hana; Horník, Petr; Mika, Filip; Mikmeková, Šárka; Ambrož, Ondřej; Mrňa, Libor
Ústav přístrojové techniky, 2024

Influence of ball material on the resulting fatigue life of thermal sprayed HVOF coatings in dynamic impact testing
Duliškovič, J.; Daniel, Josef; Houdková, Š.
2024 - anglický
Dynamic impact wear, i.e. contact between two components in the presence of high cyclic local loads, is a challenging failure mode that occurs in many mechanical applications. Many previous studies have confirmed that dynamic impact testing is suitable for evaluating the contact fatigue of thermal sprayed coatings. However, the effect of the test parameters on the resulting lifetime is unclear. The aim of this study describes the effect of the ball material used in the dynamic impact test on the resulting fatigue life of the HVOF thermal sprayed coating. Three test balls made of WC/Co alloy, Si3N4 silicon nitride and 440 C steel were chosen for this study. Dynamic impaction testing was carried out on the Cr3C2-NiCr coating, which was sprayed by HVOF on a 1.2376 high-speed steel substrate. The impact lifetime was described by the number of critical impacts, i.e. the number of impacts before coating fatigue occurs. Furthermore, the depth and volume of impact craters were measured. Using scanning electron microscopy (SEM), the surface of the impacts as well as the microstructure of the coating on the cross-section in the region of the impacts were observed. Furthermore, the mechanism of crack propagation in the coating and the microstructure of the indentor were investigated. Klíčová slova: dynamic impact test; HVOF; Cr3C2-NiCr; fatigue Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Influence of ball material on the resulting fatigue life of thermal sprayed HVOF coatings in dynamic impact testing

Dynamic impact wear, i.e. contact between two components in the presence of high cyclic local loads, is a challenging failure mode that occurs in many mechanical applications. Many previous studies ...

Duliškovič, J.; Daniel, Josef; Houdková, Š.
Ústav přístrojové techniky, 2024

Functional Tungsten-based thin films and their characterization
Košelová, Zuzana; Horáková, L.; Sobola, Dinara; Burda, Daniel; Knápek, Alexandr; Fohlerová, Z.
2024 - anglický
Anodizing is a technique by which thin oxide layers can be formed on a surface. Thin oxide layers have been found to be useful in a variety of applications, including emitters of electrons. Tungsten is still a common choice for cold field emitters in commercial microscopy applications. Its suitable quality can be further improved by thin film deposition. Not only the emission characteristic can be improved, but also the emitter operating time can be extended. Tungsten oxide is known for its excellent resistance to corrosion and chemical attack due to its stable crystal structure and strong chemical bonds between tungsten and oxygen atoms. Many techniques with different advantages and disadvantages have been used for this purpose. Anodization was chosen for this work because of the controllable uniform coverage of the material and its easy availability without the need for expensive complex equipment. The anodizing process involves applying an electrical potential to tungsten while it is immersed in an electrolyte solution. This creates a thin layer of tungsten oxide on the surface of the metal. The thickness and properties of the resulting oxide layer can be controlled by adjusting the anodization conditions, such as the electrolyte solution, voltage, and the duration of the process. In this work, H3PO4 was used as the electrolyte to test whether these tungsten oxide layers would be useful for electron emitters, for use in electron guns and other devices that require high-quality electron emitters. The properties were evaluated using appropriate techniques. In general, anodization of tungsten to form thin layers of tungsten oxide layers is a promising technique for producing high quality electron emitters. Klíčová slova: cold-field emission; thin layer deposition; tungsten oxide; resonance enhanced tunneling; anodization Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Functional Tungsten-based thin films and their characterization

Anodizing is a technique by which thin oxide layers can be formed on a surface. Thin oxide layers have been found to be useful in a variety of applications, including emitters of electrons. Tungsten ...

Košelová, Zuzana; Horáková, L.; Sobola, Dinara; Burda, Daniel; Knápek, Alexandr; Fohlerová, Z.
Ústav přístrojové techniky, 2024

Vrt DO 599. Paleomagnetický a magnetomineralogický výzkum. Závěrečná zpráva
Kdýr, Šimon; Pruner, Petr; Uçar, Hakan; Elbra, Tiiu
2024 - český
Klíčová slova: palaeomagnetism; rock-magnetism; greigite Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Vrt DO 599. Paleomagnetický a magnetomineralogický výzkum. Závěrečná zpráva

Kdýr, Šimon; Pruner, Petr; Uçar, Hakan; Elbra, Tiiu
Geologický ústav, 2024

Využití termografie v průzkumech památek
Valach, Jaroslav; Eisler, Marek
2024 - český
Termografie rozšiřuje portfolia metod, jimiž lze studovat vlastnosti objektů okolního světa. Ještě před deseti lety byla tato technologie kvůli potenciálním vojenským aplikacím úzkostlivě kontrolována, což omezovalo konkurenci v nabídce výrobků a výrobců a vedlo k vysokým cenám zařízení založených na těchto principech. Teprve díky uvolnění této kontroly došlo k rozmachu výroby a zpřístupnění zjednodušených výrobků masovému využití, takže si dnes může zákazník například pořídit chytrý telefon s doplňkovým termografickým modulem jen s malým navýšením ceny. Spolu s dostupností zařízení pozorujeme nárůst uplatnění termografie v mnoha oblastech, včetně průzkumů staveb a památek. Thermography expands the portfolio of methods that can be used to study the properties of objects in the surrounding world. As recently as a decade ago, the technology was frowned upon for potential military applications controlled, which limited competition in the offer of products and manufacturers and led to high prices of devices based on these principles. It was only thanks to the release of this control that it happened to expand production and make simplified products available to mass use, so get it today for example, a customer can purchase a smartphone with an additional thermographic module for only with a small price increase. Together with the availability of equipment, we observe an increase in the application of thermography in many areas, including surveys of buildings and monuments. Klíčová slova: thermography; survey of historical structures Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Využití termografie v průzkumech památek

Termografie rozšiřuje portfolia metod, jimiž lze studovat vlastnosti objektů okolního světa. Ještě před deseti lety byla tato technologie kvůli potenciálním vojenským aplikacím úzkostlivě ...

Valach, Jaroslav; Eisler, Marek
Ústav teoretické a aplikované mechaniky, 2024

Flow simulations approach for flocculation tanks
Idžakovičová, Kristýna; Bílek, Vojtěch; Haidl, Jan; Isoz, M.; Pivokonský, Martin
2024 - anglický
Flocculation in water treatment facilities plays a key role in the separation of colloidal inorganic and organic substances. Its optimization leads to a significant increase in its efficiency and savings of operational costs. However, it is currently based on trial-and-error experimental approaches. In this contribution, we focus on flow modeling in stirred flocculation tanks that would, after coupling with a calibrated model of particle aggregation, enable simulationbased flocculation optimization. Despite the abundance of literature on stirred tank modeling, there is no universal agreement on the methodology used to describe turbulence nor on the approach to the computational mesh creation. Consequently, there is no unified methodology for simulations and their validation. To address this, we present a best-practice methodology for economical, yet reliable flow simulations in the said device. This methodology includes the choice of the turbulence model, the approach to the design of a high quality mesh suitable for arbitrary geometries, and results evaluation. It is developed based on an extensive literature review, a multitude of flow simulations using several meshes of progressively higher quality and resolution, and various strategies to converge to steady-state flow conditions. The simulation quality indicators used here involve comparison with the experimental data on fluid velocity, stirrer power output, and flow rate through the impeller zone. Additionally, the resulting flow simulation models are compared using tracer transport simulations, hinting at their potential for coupling with particle aggregation models. Klíčová slova: flocculation tank; stirring; MRF; CFD; OpenFOAM Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Flow simulations approach for flocculation tanks

Flocculation in water treatment facilities plays a key role in the separation of colloidal inorganic and organic substances. Its optimization leads to a significant increase in its efficiency and ...

Idžakovičová, Kristýna; Bílek, Vojtěch; Haidl, Jan; Isoz, M.; Pivokonský, Martin
Ústav pro hydrodynamiku, 2024

Galloping of insulated bundled overhead line - nonlinear numerical analysis in time domain
Macháček, Michael; Hračov, Stanislav
2024 - anglický
Our contribution focuses on a 3D numerical nonlinear analysis of galloping in a specific bundled overhead line with ice accretion. We studied the susceptibility to this self-excited oscillation, critical onset wind speeds, and global dynamic response of a very low-tensioned line with simulated icing observed on similar real conductors. Due to the highly nonlinear mechanical behavior of such a flexible cable, we employed the Newmark integration method combined with the iterative Newton-Raphson method. We analyzed two numerical models of the overhead line loaded by the wind: one assuming nonlinearity only in the wind load, while retaining the linearity of the mechanical system itself, and the other representing a fully nonlinear system including geometrical nonlinearity. Our analysis revealed that the determined critical wind speeds for the onset of galloping are in relatively close ranges for both models. However, numerical simulations with the fully nonlinear system indicated significantly lower amplitudes of limit cycle oscillations, especially at higher wind speeds, compared to the linear model of the line. This underscores the necessity of using fully nonlinear models during the design stage of such low-tensioned aerial conductors. Klíčová slova: aerial bundled conductors; wind effects; galloping; limit cycle oscillation Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Galloping of insulated bundled overhead line - nonlinear numerical analysis in time domain

Our contribution focuses on a 3D numerical nonlinear analysis of galloping in a specific bundled overhead line with ice accretion. We studied the susceptibility to this self-excited oscillation, ...

Macháček, Michael; Hračov, Stanislav
Ústav teoretické a aplikované mechaniky, 2024

Numerical study of the steady airflow in the human respiratory system during inhaling and exhaling
Lancmanová, Anna; Bodnár, Tomáš
2024 - anglický
This paper presents some of the initial results of the numerical simulations of a steady turbulent flow in human upper airways during inhalation and exhalation. The mathematical model is based on the system of Reynolds-Averaged incompressible Navier-Stokes equations complemented by the SST k − ω turbulence model. The simulations were performed using finite-volume open source solver OpenFOAM on a realistic three-dimensional geometry. The main aim of this particular study is to verify the computational setup with special focus on appropriate choice and implementation of boundary conditions. The prescribed boundary conditions are chosen to mimic the physiological conditions during normal breathing cycle. This study aims to gain an insight into the airflow behavior during the inhalation and exhalation process by comparing the results of two distinct simulations corresponding to two different (opposite) flow rates . The obtained local flow rates and flow fields for both cases are presented and mutually compared. This initial work should serve as a foundation for future more complex simulations that will include the time-dependent and compressible effects. Klíčová slova: human airways; incompressible Navier-Stokes; OpenFOAM Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Numerical study of the steady airflow in the human respiratory system during inhaling and exhaling

This paper presents some of the initial results of the numerical simulations of a steady turbulent flow in human upper airways during inhalation and exhalation. The mathematical model is based on the ...

Lancmanová, Anna; Bodnár, Tomáš
Matematický ústav, 2024

Verification of the dynamic properties of a new model turbine wheel with free blades
Voronova, Evgeniya; Procházka, Pavel; Maturkanič, Dušan; Mekhalfia, Mohammed Lamine; Hodboď, Robert
2024 - anglický
The paper describes a new experimental model of turbine wheel with free blades. Verification tests were carried out using a Doppler laser vibrometer, with a focus on the equipment parameters under various conditions. The measurements were performed in the Laboratory of Rotational Laser Vibrometry originated at the Institute of Thermomechanics AS CR, v.v.i. Klíčová slova: natural frequencies; vibration; blade; laser; turbine Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Verification of the dynamic properties of a new model turbine wheel with free blades

The paper describes a new experimental model of turbine wheel with free blades. Verification tests were carried out using a Doppler laser vibrometer, with a focus on the equipment parameters under ...

Voronova, Evgeniya; Procházka, Pavel; Maturkanič, Dušan; Mekhalfia, Mohammed Lamine; Hodboď, Robert
Ústav termomechaniky, 2024

Numerical evaluation of mass-diffusive compressible fluids flows models
Bodnár, Tomáš; Fraunié, P.
2024 - anglický
This contribution presents first numerical tests of some recently published alternative models for solution of viscous compressible and nearly incompressible models. All models are solved by high resolution compact finite difference scheme with strong stability preserving RungeKutta time stepping. The two simple but challenging computational test cases are presented, based on the double-periodic shear layer and the Kelvin-Helmholtz instability. The obtained time-dependent flow fields are showing pronounced shear and vorticity layers being resolved by the standard as well as by the new mass-diffusive modified models. The preliminary results show that the new models are viable alternative to the well established classical models. Klíčová slova: compressible Navier-Stokes; nearly incompressible flow; mass diffusion; compact finite-difference Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Numerical evaluation of mass-diffusive compressible fluids flows models

This contribution presents first numerical tests of some recently published alternative models for solution of viscous compressible and nearly incompressible models. All models are solved by high ...

Bodnár, Tomáš; Fraunié, P.
Matematický ústav, 2024

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