Number of found documents: 573
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Stability of a bar influenced by small and large imperfections
Náprstek, Jiří; Fischer, Cyril
2020 - English
The geometrical and physical imperfections of systems can drastically reduce their critical loading. These imperfections are usually of stochastic character and, therefore, they act as random parametric perturbations of coefficients of corresponding differential equations. In this paper, the imperfections are introduced as multidimensional statistics on the set of a large number of realizations of the same system. As far as the amount of information is small or the imperfections themselves cannot be considered small, the convex analysis is preferable. The paper compares results obtained by both stochastic and convex analyses for hyperprism and demonstrates when each of them is more convenient to be used. Besides of the hyper-prism, the possibilities and properties of other modifications of convex method are considered, especially those based on the definition of imperfection zone marked as a centric hyper-ellipsoid or as an eccentric hyper-ellipsoid. The analytical background was brought up to the level when only a few configurations of imperfections are sufficient to be evaluated numerically. These configurations are obtained by means of the convex analysis as points of extreme critical loading using the Lagrange method of constrained extremes. Keywords: convex domain method; system stability; hyper-prism and hyper-ellipsoid domains Available at various institutes of the ASCR
Stability of a bar influenced by small and large imperfections

The geometrical and physical imperfections of systems can drastically reduce their critical loading. These imperfections are usually of stochastic character and, therefore, they act as random ...

Náprstek, Jiří; Fischer, Cyril
Ústav teoretické a aplikované mechaniky, 2020

Random response of a dynamic system under polynomial of a white noise
Náprstek, Jiří; Fischer, Cyril
2020 - English
Many types of external additive random excitation of dynamic systems admit to be modelled as a combination of powers of a Gaussian noise. Such a type of excitation produces a non-Gaussian response even if the dynamic system is linear and the excitation is additive only. Although the excitation as a whole is non-Gaussian, the problem can be transformed into the form of a linear system with an additive and multiplicative white noise excitation which _nally produces a non-Gaussian response. The general method of transformation, the respective FPK equation, basic stochastic moments of the response, and a demonstrative example are discussed. Keywords: non-Gaussian excitation; nonlinear filtering; Kronecker algebra Available at various institutes of the ASCR
Random response of a dynamic system under polynomial of a white noise

Many types of external additive random excitation of dynamic systems admit to be modelled as a combination of powers of a Gaussian noise. Such a type of excitation produces a non-Gaussian response ...

Náprstek, Jiří; Fischer, Cyril
Ústav teoretické a aplikované mechaniky, 2020

Damage detection using a cogwheel load - numerical case study
Bayer, Jan
2020 - English
A moving impulse load generated by a heavy cogwheel (CW) can be used as a testing excitation for bridges. This proposed type of dynamic load acts along the entire driving path, its intensity is adjustable, and it can be very efficient in the case of resonance. However, higher harmonic components are an inevitable effect of this type of loading, complicating the vibration analysis. The present study investigates suitable procedures to detect and locate damage in structures using a cogwheel load and only one or a few transducers mounted on the bridge. This arrangement seems to be sensitive to early damage indication. Damage localization is also possible, but further research will be required to increase its credibility. Keywords: bridge testing; damage detection; impulse loading; vibration analysis; bridge health monitoring Available at various institutes of the ASCR
Damage detection using a cogwheel load - numerical case study

A moving impulse load generated by a heavy cogwheel (CW) can be used as a testing excitation for bridges. This proposed type of dynamic load acts along the entire driving path, its intensity is ...

Bayer, Jan
Ústav teoretické a aplikované mechaniky, 2020

Region of interest tomography
Vopálenský, Michal; Kumpová, Ivana
2020 - English
Během standardního tomografického procesu jsou tomografická data (projekce) nabírána s takovým geometrickým nastavením, že stín celého objektu se vejde do šířky detektoru pro všechny pořízované úhly. Vzhledem k limitované šířce detektoru však tento přístup znamená též omezení maximálního dosažitelného rozlišení pro daný průměr vzorku. Je možné zvýšit zvětšení, ale potom se projektovaný objekt na aktivní plochu detektoru nevejde, což způsobuje jisté problémy při rekonstrukci, avšak umožňuje dosažení vyššího rozlišení v oblasti, která je na všech projekcích viditelná (region of interest). Výzkum byl zaměřen na vyhodnocení možností ROI tomografie na tomgrafu TORATOM. During standard tomographic process, the acquisition of tomographic data (projections) is performed with geometric adjustment of the tomographic assembly ensuring that the projection of the entire width of the examined object fits onto the detector during the full object rotation. Because of the limited width of the detector, this approach has a limited maximum achievable resolution for the given specimen diameter. It is possible to make higher magnification, but then the projections do not fit onto the detector, which causes certain problems during the reconstruction, but allows achieving of higher resolution in the always-visible region of the object (the region of interest). The research was focused on evaluation of ROI tomography approach on the TORATOM tomograph. Keywords: X-ray computed tomography; resolution; Region-of-Interest tomography Available at various institutes of the ASCR
Region of interest tomography

Během standardního tomografického procesu jsou tomografická data (projekce) nabírána s takovým geometrickým nastavením, že stín celého objektu se vejde do šířky detektoru pro všechny pořízované úhly. ...

Vopálenský, Michal; Kumpová, Ivana
Ústav teoretické a aplikované mechaniky, 2020

Multifold stationary solutions of an auto-parametric non-linear 2DOF system
Fischer, Cyril; Náprstek, Jiří
2020 - English
A non-linear 2DOF model of a bridge girder with a bluff cross-section under wind loading is used to describe the heave and pitch self-excited motion. Existence conditions of stationary auto-parametric response for both the self-excited case and an assumption of a harmonic load form a non-linear algebraic system of equations. Number of distinct solutions to this algebraic system depends on the frequencies of two principal aero-elastic modes and other system parameters. Thus, the system may possess none, one, or several stationary solutions, whose stability has to be checked using the Routh-Hurwitz conditions. If all quantities entering the system are continuous functions, individual solutions may exhibit (piecewise) continuous dependence on selected system parameters. Thus, multiple identified solutions to the system for a given set of parameters may actually belong to a single solution branch and their values can be determined from the knowledge of the solution branch. Such a situation may significantly simplify assessment of stability of the particular solutions and/or provides an applicable overall description of the system response. Keywords: multifold solution; algebraic system; aero-elastic system; stationary vibration Available at various institutes of the ASCR
Multifold stationary solutions of an auto-parametric non-linear 2DOF system

A non-linear 2DOF model of a bridge girder with a bluff cross-section under wind loading is used to describe the heave and pitch self-excited motion. Existence conditions of stationary auto-parametric ...

Fischer, Cyril; Náprstek, Jiří
Ústav teoretické a aplikované mechaniky, 2020

Distribution of hydration products in the microstructure of cement pastes
Hlobil, Michal
2020 - English
This case study focuses on the quantification of the amorphous hydrate distribution in the microstructure of hardened cement paste. Microtomographic scans of the hardenend cement paste were thresholded based on histogram image analysis combined with microstructural composition obtained from CEMHYD3D hydration model, to separate unhydrated cement grains, crystalline and amorphous hydrates, and capillary pores. The observed spatial distribution of the amorphous hydrate exhibited a strong spatial gradient as the amorphous gel tended to concentrate around dissolving cement grains rather than precipitate uniformly in the available space. A comparative numerical study was carried out to highlight the effect of the spatially (non)uniform hydrate distribution on the compressive strength of the hardened cement paste. Keywords: cement paste; hydration products; microstructure; modelling Available at various institutes of the ASCR
Distribution of hydration products in the microstructure of cement pastes

This case study focuses on the quantification of the amorphous hydrate distribution in the microstructure of hardened cement paste. Microtomographic scans of the hardenend cement paste were ...

Hlobil, Michal
Ústav teoretické a aplikované mechaniky, 2020

Distribution of the external wind pressure coefficients on the surface of the façade of the high‐rise building "KINEUM" determined from the wind tunnel measurements
Hračov, Stanislav; Macháček, Michael
2019 - English
The report describes the procedure and results of the measurements of aerodynamic model of the planned high-rise building in Gothenburg in Sweden. The determined wind pressure distribution on individual surfaces of the building for several wind directions are presented in the form of external pressure coefficients. The obtained pressure coefficients are subsequently used by the client for the design of façade elements loaded by the wind. Keywords: external pressure coefficient; wind load; high-rise building Available at various institutes of the ASCR
Distribution of the external wind pressure coefficients on the surface of the façade of the high‐rise building "KINEUM" determined from the wind tunnel measurements

The report describes the procedure and results of the measurements of aerodynamic model of the planned high-rise building in Gothenburg in Sweden. The determined wind pressure distribution on ...

Hračov, Stanislav; Macháček, Michael
Ústav teoretické a aplikované mechaniky, 2019

Moving dynamic test loads for road bridges - a case study
Bayer, Jan; Urushadze, Shota
2019 - English
Two types of dynamic testing load for bridges - driving sprung mass and driving impulse load - areexamined from the drive-by identification point of view. Numerical studies in ANSYS using contact elements and in MATLAB using a modal solution together with coupling force were performed. Equations for the estimation of the impulse load caused by a rolling cogwheel were suggested. The experiments confirmed that a driving impulse load is more efficient in exciting bridge vibrations. This, together with the fact that impulse loading is not sensitive to surface roughness, leads to the conclusion that, as a method for bridge health monitoring, impulse loading is more promising than using a driving sprung mass Keywords: drive-by identification; bridge testing; moving sprung mass; impulse loading; vibration measurements; laboratory experiments Available at various institutes of the ASCR
Moving dynamic test loads for road bridges - a case study

Two types of dynamic testing load for bridges - driving sprung mass and driving impulse load - areexamined from the drive-by identification point of view. Numerical studies in ANSYS using contact ...

Bayer, Jan; Urushadze, Shota
Ústav teoretické a aplikované mechaniky, 2019

Probability density determination by means of Gibbs entropy probability density
Náprstek, Jiří; Fischer, Cyril
2019 - English
A method of random response investigation of a nonlinear dynam-ical system is discussed. In particular, the solution of the probability density of a single/multi-degree of freedom (SDOF/MDOF) system response is investigated. Multiple stable equilibrium states with possible jumps of the snap-through type among them are considered. The system is Hamiltonian with weak damping excited by a set of non-stationary Gaussian white noises. The solution, which is based on the Gibbs principle of the maximum entropy of probability, can be employed in various branches of engineering. The search for the extreme of the Gibbs entropy functional is formulated as a constrained optimization problem. The secondary constraints follow from the Fokker-Planck equation (FPE) for the system considered or from the system of ordinary di_erential equations for the stochastic moments of the response derived from the relevant FPE Keywords: Fokker-Planck equation; Gibbs entropy functional; maximum entropy; probability density principle Available at various institutes of the ASCR
Probability density determination by means of Gibbs entropy probability density

A method of random response investigation of a nonlinear dynam-ical system is discussed. In particular, the solution of the probability density of a single/multi-degree of freedom (SDOF/MDOF) system ...

Náprstek, Jiří; Fischer, Cyril
Ústav teoretické a aplikované mechaniky, 2019

System response with random imperfections in coefficients on the space of realizations
Náprstek, Jiří; Fischer, Cyril
2019 - English
The contribution is concerned with the analysis of the simultaneous effect of a random perturbation and a white noise in the coefficient of the system. The excitation of the system of the 1-st order is described by the sum of a deterministic signal and an additive white noise which is partly correlated with the parametric noise. The random perturbation in the parameter is considered as a statistics in a set of realizations. It reveals that the density of probability of perturbations must be limited in the phase space, otherwise the system would lose the stochastic stability in probability. The width of the permissible zone depends heavily on the intensity of the parametric noise, the extent of correlation with the additive excitation noise and the type of probability density. The general explanation is demonstrated on cases of normal, uniform and truncated normal densities of probability. Keywords: noisy signal; filtration; random imperfections Available at various institutes of the ASCR
System response with random imperfections in coefficients on the space of realizations

The contribution is concerned with the analysis of the simultaneous effect of a random perturbation and a white noise in the coefficient of the system. The excitation of the system of the 1-st order ...

Náprstek, Jiří; Fischer, Cyril
Ústav teoretické a aplikované mechaniky, 2019

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