Number of found documents: 1014
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First-principles study of interface energies in Fe-Al-based superalloy nanocomposites
Miháliková, Ivana; Slávik, Anton; Friák, Martin; Všianská, Monika; Koutná, N.; Holec, David; Šob, Mojmír
2018 - English
Fe-Al-based nanocomposites with a superalloy-type of microstructure constitute a very promising class of materials. They possess a great potential as an alternative to the currently used steel grades in high temperature applications. Intermetallics-containing nanocomposites, such as those with the Fe3Al compound being one of the phases, may open a way towards future automotive and energy-conversion technologies with lower fuel consumption and reduced environmental impact. We employ quantum-mechanical calculations to analyze relations between ordering tendencies of Al atoms in the disordered Fe-18.75at.%Al phase on one hand and thermodynamic, structural and magnetic properties of Fe-Al-based nanocomposites on the other. When comparing supercells modeling disordered Fe-Al phase with different atomic distribution of atoms we find out that the supercell without 1st and 2nd nearest neighbor Al-Al pairs has a lower energy than that mimicking a perfect disorder (a special quasi-random structure, SQS). Further, coherent interfaces with (001), (110) and (1-10) crystallographic orientations between Fe3Al compound and SQS Fe-Al phase have higher energies than those exhibiting atomic distribution without 1st and 2nd nearest neighbor Al-Al pairs. Keywords: Ab initio calculations; Fe-Al based superalloys; Interface energies; Nanocomposites Available at various institutes of the ASCR
First-principles study of interface energies in Fe-Al-based superalloy nanocomposites

Fe-Al-based nanocomposites with a superalloy-type of microstructure constitute a very promising class of materials. They possess a great potential as an alternative to the currently used steel grades ...

Miháliková, Ivana; Slávik, Anton; Friák, Martin; Všianská, Monika; Koutná, N.; Holec, David; Šob, Mojmír
Ústav fyziky materiálů, 2018

Estimation of equilibrium hydrogen pressure - A new method
Čermák, Jiří; Král, Lubomír
2018 - English
A new method is proposed to estimation of hydrogen pressure in equilibrium with hydride phase in a hydrogen\nstorage material. It is applicable both for hydrogen absorption and desorption in cases where the hydride phase\nis formed by nucleation and growth mechanism. The proposed method saves considerably the experimental\ntime replacing the conventional time consuming measurement of pressure-composition isotherms, the so\ncalled PCT curves. The proposed evaluation procedure is illustrated using hydrogen chemi-sorption at\ntemperatures 623 K, 573 K and 523 K in chosen hydrogen storage alloys Mg-Si-C, Mg-Li-C and Mg-Na-C. Keywords: hydrogen storage; magnesium; new method Available at various institutes of the ASCR
Estimation of equilibrium hydrogen pressure - A new method

A new method is proposed to estimation of hydrogen pressure in equilibrium with hydride phase in a hydrogen\nstorage material. It is applicable both for hydrogen absorption and desorption in cases ...

Čermák, Jiří; Král, Lubomír
Ústav fyziky materiálů, 2018

Měření velmi pomalých creepových deformací materiálů pro pokrytí jaderného paliva technikou helikoidních vzorků.
Kloc, Luboš
2018 - Czech
Metodika umožňuje přímé experimentální stanovení creepových vlastností povlakových trubek jaderného paliva za podmínek, při kterých probíhá creepová deformace velmi pomalu, takže není standardními metodami creepových zkoušek měřitelná. Takto získané údaje jsou potřebné ve výpočtových kódech pro modelování chování paliva v reaktoru i mimo\nněj při skladování paliva a dosud chybí, popřípadě jsou získávany nespolehlivou extrapolací. Presented technique enables experimental determination of the creep properties of the nuclear fuel cladding tubes under conditions leading to very low strain rates, which are not measurable by conventional creep tests. The data are necessary for the numerical modeling of the nuclear fuel behavior inside the reactor or during the fuel storage and are not available or used to be replaced by the data obtained by unreliable extrapolation. Keywords: measurement of slow creep deformation; helicoid specimen test; creep rate measurement Available at various institutes of the ASCR
Měření velmi pomalých creepových deformací materiálů pro pokrytí jaderného paliva technikou helikoidních vzorků.

Metodika umožňuje přímé experimentální stanovení creepových vlastností povlakových trubek jaderného paliva za podmínek, při kterých probíhá creepová deformace velmi pomalu, takže není standardními ...

Kloc, Luboš
Ústav fyziky materiálů, 2018

HYDROGEN STORAGE PROPERTIES OF GRAPHENE OXIDE MATERIALS PREPARED BY DIFFERENT WAYS
Král, Lubomír; Čermák, Jiří; Bytesnikova, Z.
2018 - English
Graphene-based materials show unique properties. These single layered materials consist of 2D structure of carbon atoms, belong to the strongest known materials, that are very mechanically flexible, optically transparent and that are excellent electrical and thermal conductors. Recently, several studies on these types of materials have highlighted the potential of this material for hydrogen storage (HS) and raised new hopes for the development of an effective solid-state HS media. In the present paper, the structure and HS properties of graphene oxide (GO) and chemically reduced graphene oxide (rGO) produced by different procedures were studied. Hydrogen sorption characteristics of GO and rGO were measured using the Sieverts-type gas sorption analyzer PCT-Pro Setaram Instrumentation. The study of HS was carried out at temperature range from 198 K to 423 K under hydrogen pressure from 1x10(-4) to 4 MPa. \n\nFor the HS point of view, the advantage of GO or rGO compared to graphene, is the presence of multiple chemical groups that can be used for introducing modifiers and their superior spreading on the materials surface. The suitably functionalized GO or rGO materials could potentially exhibit outstanding HS properties. Keywords: Graphene oxide; hydrogen storage; adsorption; adsorption Available at various institutes of the ASCR
HYDROGEN STORAGE PROPERTIES OF GRAPHENE OXIDE MATERIALS PREPARED BY DIFFERENT WAYS

Graphene-based materials show unique properties. These single layered materials consist of 2D structure of carbon atoms, belong to the strongest known materials, that are very mechanically flexible, ...

Král, Lubomír; Čermák, Jiří; Bytesnikova, Z.
Ústav fyziky materiálů, 2018

Estimation of mechanical parameters of thin films using finite element analysis
Tinoco Navaro, Hector Andres; Holzer, Jakub; Pikálek, Tomáš; Buchta, Zdeněk; Lazar, Josef; Chlupová, Alice; Kruml, Tomáš; Hutař, Pavel
2018 - English
This study shows a methodology to estimate mechanical parameters of thin films by means of a bulge\ntest and a numerical approach. The methodology is based on the combination of finite element analysis with a\nclassical analytical method. Finite element modelling was conducted for monolayer (Si3N4) membranes of 2x2mm\nwith the aim to approximate both the load-deflection curves experimentally measured and the classical loaddeflection\nanalytical model. Error functions were constructed and minimized to delimit a coupled solution space\nbetween Young’s modulus and Poison’s ratio. In a traditional bulge test analysis only one of the elastic properties\ncan be determined due to that there is not unique solution in the estimations of these parameters. However, both\nelastic parameters were determined through the proposed numerical procedure which compares the deformed\nsurfaces for a specific set of optimal elastic parameters computed. Results shows that the estimated elastic\nproperties agree with corresponding values determined by other methods in the literature Keywords: bulge test; thin film; finite element analysis; elastic properties Available at various institutes of the ASCR
Estimation of mechanical parameters of thin films using finite element analysis

This study shows a methodology to estimate mechanical parameters of thin films by means of a bulge\ntest and a numerical approach. The methodology is based on the combination of finite element ...

Tinoco Navaro, Hector Andres; Holzer, Jakub; Pikálek, Tomáš; Buchta, Zdeněk; Lazar, Josef; Chlupová, Alice; Kruml, Tomáš; Hutař, Pavel
Ústav fyziky materiálů, 2018

CALIBRATION FUNCTIONS FOR EDGE CRACKS UNDER SELECTED BENDING LOADS.\n\n
Seitl, Stanislav; Miarka, Petr
2018 - English
Fatigue cracks are found during the regular structural inspections. To precisely\ndescribe/suggest of fatigue cracks propagation throughout structure and for designed\nservice life, the knowledge of calibration functions is important. The cracks usually\npropagate from the edge or the surface of the structural element. The theoretical model\nof fatigue crack propagation is based on linear fracture mechanics (Paris law). Steel\nstructural elements are subjected to various bending load (three- and four-point bending,\npure bending etc.). The calibration functions for the edge cracks are calculated for\nvarious load and appropriate polynomial function independent on the distance are\nproposed for 3PB and 4PB load. Keywords: Fracture mechanics; calibration curve; stress intensity factor; Paris-Erdogan Law; edge Available at various institutes of the ASCR
CALIBRATION FUNCTIONS FOR EDGE CRACKS UNDER SELECTED BENDING LOADS.\n\n

Fatigue cracks are found during the regular structural inspections. To precisely\ndescribe/suggest of fatigue cracks propagation throughout structure and for designed\nservice life, the knowledge of ...

Seitl, Stanislav; Miarka, Petr
Ústav fyziky materiálů, 2018

Evaluation of thin discontinuities in planar conducting materials using the diffraction of electromagnetic field
Savin, A.; Nový, F.; Fintová, Stanislava; Steigmann, R.
2017 - English
The current stage of nondestructive evaluation techniques imposes the development of new electromagnetic (EM) methods that are based on high spatial resolution and increased sensitivity. In order to achieve high performance, the work frequencies must be either radifrequencies or microwaves. At these frequencies, at the dielectric/conductor interface, plasmon polaritons can appear, propagating between conductive regions as evanescent waves. In order to use the evanescent wave that can appear even if the slits width is much smaller that the wavwelength of incident EM wave, a sensor with metamaterial (MM) is used. The study of the EM field diffraction against the edge of long thin discontinuity placed under the inspected surface of a conductive plate has been performed using the geometrical optics principles. This type of sensor having the reception coils shielded by a conductive screen with a circular aperture placed in the front of reception coil of emission reception sensor has been developed and 'transported' information for obtaining of magnified image of the conductive structures inspected. This work presents a sensor, using MM conical Swiss roll type that allows the propagation of evanescent waves and the electromagnetic images are magnified. The test method can be successfully applied in a variety of applications of maxim importance such as defect/damage detection in materials used in automotive and aviation technologies. Applying this testing method, spatial resolution can be improved. Keywords: Electromagnetic fields; Electromagnetic wave reflection; Electromagnetic waves; Geometrical optics; Image resolution; Nondestructive examination; Testing Available at various institutes of the ASCR
Evaluation of thin discontinuities in planar conducting materials using the diffraction of electromagnetic field

The current stage of nondestructive evaluation techniques imposes the development of new electromagnetic (EM) methods that are based on high spatial resolution and increased sensitivity. In order to ...

Savin, A.; Nový, F.; Fintová, Stanislava; Steigmann, R.
Ústav fyziky materiálů, 2017

Effect of the load eccentricity on fracture behaviour of cementitious materials subjected to the modified compact tension test
Seitl, Stanislav; Ríos, J. D.; Cifuentes, H.; Veselý, V.
2017 - English
Fracture properties of quasi-brittle cementitious composites are typically determined from the load–displacement response recorded during a fracture test by using the work-of-fracture method or possibly other relevant fracture models. Our contribution is focused on a set of experimental tests which are used to study the fracture behaviour on notched dog-bone-shaped specimens made of cementitious materials. These specimens are subjected to modified compact tension (ModCT) test under a specific range of eccentricity of the tensile load. This type of test generates a stress state in the specimen ligament which combines a direct tension with a defined level of bending due to eccentricity of the tensile load. Several values of relative notch length are also considered. While the crack propagates, a variety of stress states, resulting in variations in the crack-tip stress and deformation constraint, appears in the ligament zone because of the changes in the eccentricity of the applied load, which influences the fracture behaviour of the investigated specimens. The K-calibration, T-stress, CMOD and COD curves for ModCT specimens are introduced and variations of these curves with varying load eccentricity are discussed. Keywords: constraint; fracture; load accentricity Available at various institutes of the ASCR
Effect of the load eccentricity on fracture behaviour of cementitious materials subjected to the modified compact tension test

Fracture properties of quasi-brittle cementitious composites are typically determined from the load–displacement response recorded during a fracture test by using the work-of-fracture method or ...

Seitl, Stanislav; Ríos, J. D.; Cifuentes, H.; Veselý, V.
Ústav fyziky materiálů, 2017

Crack initiation in austenitic stainless steel sanicro 25 subjected to thermomechanical fatigue
Petráš, Roman; Škorík, Viktor; Polák, Jaroslav
2017 - English
Thermomechanical fatigue experiments were performed with austenitic stainless Sanicro 25 steel. Several amplitudes of mechanical strain in a wide temperature interval (250-700 °C) were applied to the specimens. Mechanical response was recorded and fatigue lives were obtained. Scanning electron microscopy combined with FIB technique was used to study the mechanism of crack initiation in in-phase and in out-of-phase thermomechanical cycling. Different mechanisms of the crack initiation were found in these two types of loading. During in-phase loading fatigue cracks start in grain boundaries by cracking of the oxide. Cracks grew preferentially along grain boundaries which resulted in rapid crack initiation and low fatigue life. In out-of-phase loading multiple cracks perpendicular to the stress axis developed only after sufficiently thick oxide layer was formed and cracked in low temperature loading half-cycle. The cracks in oxide allowed localized repeated oxidation and finally also cracking. The cracks grow transgranularly and result in longer fatigue life. Keywords: Damage mechanism; FIB cutting; Localized oxidation-cracking; Sanicro 25 steel; Thermomechanical fatigue Available at various institutes of the ASCR
Crack initiation in austenitic stainless steel sanicro 25 subjected to thermomechanical fatigue

Thermomechanical fatigue experiments were performed with austenitic stainless Sanicro 25 steel. Several amplitudes of mechanical strain in a wide temperature interval (250-700 °C) were applied to the ...

Petráš, Roman; Škorík, Viktor; Polák, Jaroslav
Ústav fyziky materiálů, 2017

Lifetime Assessment of Particulate Ceramic Composite with Residual Stresses
Náhlík, Luboš; Majer, Zdeněk; Štegnerová, Kateřina; Hutař, Pavel
2017 - English
A micro-crack propagation in particulate ceramic based composite was studied using finite element method (FEM). Subcritical crack growth (SCG) was numerically simulated under complex load conditions (mechanical loading and loading by internal residual stresses). The effect of residual stresses on the crack propagation was studied. Two-dimensional computational model of particulate ceramic composite with material properties corresponding to low temperature co-fired ceramics (LTCC) was developed. The results indicate that the presence of residual stresses significantly reduces values of stress intensity factor in the vicinity of composite surface and the direction of residual stresses around the particles contributes to the micro-crack deflection from the particles. The time to failure of the composite under mechanical loading was determined. Results obtained contribute to a better understanding of the role of residual stresses during micro-crack propagation in ceramic particulate composites. Keywords: ceramic particulate composite; sub-critical crack growth; residual stresses Available at various institutes of the ASCR
Lifetime Assessment of Particulate Ceramic Composite with Residual Stresses

A micro-crack propagation in particulate ceramic based composite was studied using finite element method (FEM). Subcritical crack growth (SCG) was numerically simulated under complex load conditions ...

Náhlík, Luboš; Majer, Zdeněk; Štegnerová, Kateřina; Hutař, Pavel
Ústav fyziky materiálů, 2017

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