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Three-dimensional characterization of polymer foams using X-ray dark-field imaging
Senck, S.; Plank, B.; Gusenbauer, C.; Salaberger, D.; Vavřík, Daniel; Santer, W.; Kastner, J.
2017 - anglický
Due to the low cost, the ease of processing, and excellent material properties, polymer foams are used in various applications, e.g. packaging, building and construction, furnitures and bedding, and the automotive and aerospace sector. The mechanical response of polymer foams is primarily influenced by density and morphology. While foam density can be determined with high precision, cell morphology is more difficult to determine since the size distribution of foam cells differs in three dimensions. However, using conventional methods, e.g. optical light microscopy or scanning electron microscopy, it is very difficult to obtain three-dimensional information and to differentiate between the strut system and cell walls. An alternative for the three-dimensional characterization of foam morphology is micro-computed tomography (XCT). But even non-destructive techniques like XCT are not able to characterize anisotropic foams if the thickness of single struts and cell walls is below the physcial resolution of the respective XCT system. In this contribution we therefore investigate different polymeric foam samples using a Talbot-Lau grating interferometer XCT (TLGI-XCT) system. We show that the obtained darkfield contrast images show a high contrast and a strong signal at struts and cell walls, facilitating the segmentation of foam cells in various examples.
Klíčová slova:
foam; micro-computed tomography; Talbot-Lau grating interferometer XCT; dark-field imaging
Dokument je dostupný na externích webových stránkách.
Three-dimensional characterization of polymer foams using X-ray dark-field imaging
Due to the low cost, the ease of processing, and excellent material properties, polymer foams are used in various applications, e.g. packaging, building and construction, furnitures and bedding, and ...
Wind-tunnel simulation of thermally unstable atmospheric flow in complex terrain
Pospíšil, Stanislav; Kuznetsov, Sergeii; Kozmar, H.; Michalcová, V.
2017 - anglický
The paper presents the characteristics of thermal effects in wind-tunnel simulations of the atmospheric boundary layer (ABL) flow developing above complex terrain. The differences and instable stratification effects between the windward and leeward sides of a hill model were investigated for different wind characteristics. Particularly, the buoyancy effects on the structure of the ABL simulation in a wide range of stability conditions expressed by Richardson number are studied.
Klíčová slova:
instable stratification; wind tunnel; Richardson number
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Wind-tunnel simulation of thermally unstable atmospheric flow in complex terrain
The paper presents the characteristics of thermal effects in wind-tunnel simulations of the atmospheric boundary layer (ABL) flow developing above complex terrain. The differences and instable ...
Analytical approach of slender structure vibration due to random component of the wind velocity
Náprstek, Jiří; Hračov, Stanislav
2017 - anglický
Along wind random vibration of slender structures represents one of the most important aeroelastic effects resulting from wind - structure interaction. The theoretical model being based on one-dimensional elements with continuously distributed mass and stiffness has been introduced in this paper. The system has been considered to be linear self-adjoint with strongly non-proportional linear damping due to both material of the structure and presence of vibration dampers. The additive random excitation continuously distributed in time and space is Gaussian, therefore the response is Gaussian as well. Consequently, mathematical mean value and correlation function are satisfactory for the full description of the generalized solution of the respective PDE in the stochastic meaning. The general results have been obtained mostly in the form of analytical formulae for important cases of input spectral densities. A numerical example dealing with real structure is presented.
Klíčová slova:
random vibrations; slender structures; wind load; non-proportional damping; distributed-parameter systems
Dokument je dostupný na externích webových stránkách.
Analytical approach of slender structure vibration due to random component of the wind velocity
Along wind random vibration of slender structures represents one of the most important aeroelastic effects resulting from wind - structure interaction. The theoretical model being based on ...
Remarks on inverse of matrix polynomials
Fischer, Cyril; Náprstek, Jiří
2017 - anglický
Analysis of a non-classically damped engineering structure, which is subjected to an external excitation, leads to the solution of a system of second order ordinary differential equations. Although there exists a large variety of powerful numerical methods to accomplish this task, in some cases it is convenient to formulate the explicit inversion of the respective quadratic fundamental system. The presented contribution uses and extends concepts in matrix polynomial theory and proposes an implementation of the inversion problem.
Klíčová slova:
matrix polynomial; polynomial eigenvalues; structural vibration
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Remarks on inverse of matrix polynomials
Analysis of a non-classically damped engineering structure, which is subjected to an external excitation, leads to the solution of a system of second order ordinary differential equations. Although ...
Formace polymorfů uhličitanu vápenatého během mísení koncentrovaných roztoků solí
Ševčík, Radek; Mácová, Petra; Viani, Alberto
2017 - český
Příspěvek se zaobírá předběžnými výsledky studia gelové fáze vznikající během mísení koncentrovaných roztoků solí během syntézy polymorfů uhličitanu vápenatého. Během reakce bylo odebráno několik vzorků, které byly následně zkoumány instrumentálními technikami jako např. skenovací elektronový mikroskop. This contribution is demonstrated the preliminary results of the study of the gel phase forming during the synthesis of calcium carbonate's polymorphs. Several samples were collected during the reaction and they were subsequently investigated with instrumental techniques e.g. scanning electron microscopy.
Klíčová slova:
CaCO3; polymorphs; synthesis; vaterite; ACC
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Formace polymorfů uhličitanu vápenatého během mísení koncentrovaných roztoků solí
Příspěvek se zaobírá předběžnými výsledky studia gelové fáze vznikající během mísení koncentrovaných roztoků solí během syntézy polymorfů uhličitanu vápenatého. Během reakce bylo odebráno několik ...
Creep behavior of oak pegs under tension in dry and wet conditions
Kunecký, Jiří; Kloiber, Michal; Hasníková, Hana; Hrivnák, Jaroslav; Sebera, V.; Tippner, J.; Milch, J.
2017 - anglický
Carpentry joints equipped with oak pegs are well suited for applications where increased moisture is not a significant factor. However, for the use in outside conditions, e.g. in the repairs of historical timber bridges, the pegs connecting the two halves of the joint may deform and change their original shape. This happens especially during long-term loading and can be augmented significantly by environmental changes. Creep behavior can play a crucial role for some types of carpentry joints, especially when precise contacts of faces in the joint are required. The paper deals with experimental testing of creep behavior of 16 mm oak pegs under dry and wet conditions loaded by the combination of shear and bending between two planks made of spruce in the direction parallel to grain. The results show that the decrease of theoretical stiffness values over time plays a significant role in the force redistribution in the joint especially due to moisture effects (drop 50-70% of the original stiffness).
Klíčová slova:
creep; oak; pegs; dowels; tension
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Creep behavior of oak pegs under tension in dry and wet conditions
Carpentry joints equipped with oak pegs are well suited for applications where increased moisture is not a significant factor. However, for the use in outside conditions, e.g. in the repairs of ...
Stochastic stability of the generalized van der Pol system under random additive excitation
Náprstek, Jiří; Fischer, Cyril
2017 - anglický
The paper is motivated by a series of wind tunnel experiments investigating aeroelastic SDOF and TDOF section models of various shape and aeroelastic properties. It reveals that most of them can be theoretically modeled by the van der Pol – Duffing type equations or their combination adjusting degree of individual nonlinear terms or their coefficients. It should be emphasized that this character of the system response is very stable and is obvious in linear as well as in weakly nonlinear domain when the post-critical effect emerges. Moreover, many special effects identified by an experimental way evoke properties recognized in the pure theory of differential equations.
Klíčová slova:
wind tunnel experiment; random excitation; stochastic stability
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Stochastic stability of the generalized van der Pol system under random additive excitation
The paper is motivated by a series of wind tunnel experiments investigating aeroelastic SDOF and TDOF section models of various shape and aeroelastic properties. It reveals that most of them can be ...
Using homogenization and nanoindentation for microfibril angle determination of spruce
Kucíková, L.; Hrbek, Vladimír; Vorel, J.; Šejnoha, M.
2017 - anglický
This paper is concerned with the evaluation of microfibril angle of spruce. The microfibril angle is defined as an inclination of microfibrils from the longitudinal axis, the direction of lumens. It is well known and further supported by the present study that the microfibril angle has a great influence on the final mechanical properties of wood. This angle could be measured either directly using, e.g. polarisation microscopy, X-ray diffraction, infrared spectroscopy, or indirectly, as used in this study, by employing the nanoindentation measurements. Therein, the measured indentation modulus is compared with that obtained numerically using the anisotropic theory of indentation. The latter one depends on the entire stiffness matrix derived through homogenization and the searched microfibril angle. In view of the cell wall microstructure, the effective cell properties were found using the two-step micromechanical homogenization adopting both the Self-consistent and Mori-Tanaka methods.
Klíčová slova:
spruce; nanoindentation; microfibril angle; homogenization
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Using homogenization and nanoindentation for microfibril angle determination of spruce
This paper is concerned with the evaluation of microfibril angle of spruce. The microfibril angle is defined as an inclination of microfibrils from the longitudinal axis, the direction of lumens. It ...
Optimization of acquisition parameters in radiography and tomography
Vopálenský, Michal; Vavřík, Daniel; Kumpová, Ivana
2017 - anglický
Contrast in radiographic images is one of the most important parameters both in radiography and tomography. This paper investigates the possibility of adjusting the radiographic parameters, in particular the accelerating voltage of the X-ray tube, in order to maximize the difference in the detected intensity in response to the change in the thickness of the investigated object. Simplified model is used that allows finding the optimal operating point for the given material and thickness without an apriory knowledge about the X-ray spectra, detector response and the material composition. It is shown that in case of small changes of thickness, the optimum is met when the detected intensity behind the object of certain thickness is at approx. 37 % of the full range (i.e., of the value that would be measured under the same conditions in the absence of the object). In spite of simplifications made, the theoretical model shows a very good correspondence with the experimental data. A brief consideration is made on the case of a general difference in thicknesses.
Klíčová slova:
X-ray computed tomography; X-ray radiography; X-ray optimization; image processing; contrast
Dokument je dostupný na externích webových stránkách.
Optimization of acquisition parameters in radiography and tomography
Contrast in radiographic images is one of the most important parameters both in radiography and tomography. This paper investigates the possibility of adjusting the radiographic parameters, in ...
Early stage microstructure development of cement paste modified by crystalline admixture
Hrbek, V.; Petráňová, Veronika; Němeček, J.
2017 - anglický
This paper deals with the effect of crystalline admixture (CA) on the microstructure of the cementitious composite in first four weeks after the production. Previous investigation was performed on reference specimens consisting of pure cement. Samples of pure CA and cement modified by 1% of CA with the same w/c ratio were examined and compared in this paper. By investigation of these materials on the micro-scale, it is possible to identify the development of the microstructure of each and determine the impact of the modification. For this purpose, backscattered electrons microscopy (BSE) with energy-dispersive X-ray spectroscopy (EDX) was used for phase analysis as well as instrumental nano-indentation to obtain micro-mechanical properties. Presented results show the evolution of mechanical properties and microstructure in time and the impact of the crystalline admixture on modified cement.
Klíčová slova:
cement; crystalline admixture; microstructure; nanoindentation; phase; SEM
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Early stage microstructure development of cement paste modified by crystalline admixture
This paper deals with the effect of crystalline admixture (CA) on the microstructure of the cementitious composite in first four weeks after the production. Previous investigation was performed on ...
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