Volume 1, number 1 (2007)

Permanent URI for this collection

http://hdl.handle.net/11025/30238

Browse

Showing 1 - 20 out of 45 results

An improved method for estimating fatigue life under combined stress
(University of West Bohemia, 2007) Balda, Miroslav; Svoboda, Jaroslav; Fröhlich, Vladislav
The paper deals with a complicated problem of estimating fatigue lives of machine parts exposed to various kinds of dynamic loading. An origin of the presented method can be found 25 years ago when a simple algorithm of strain energy density cumulation was applied for evaluation of uniaxial harmonic and random fatigue tests. Recently, the method has been adopted for multiaxial loading. Unfortunately, results of its application have shown rather big variance. This paper presents the improved method, which yields much lower scattering of estimated fatigue lives compared with earlier approaches.
The problems at investigation of state of stress of thick orthotropic plate
(University of West Bohemia, 2007) Červená, Olga; Hora, Petr
The determination of state of stress of thick orthotropic plate is outlined here. The analytical formulations of dispersion curves for arbitrary direction of wave propagation in orthotropic plate, which we defined earlier, are used to obtain results. The false roots appearing in the numerical computation of thick plate dispersion curves from orthotropic materials are mentioned. The displacements and the stresses of the thick orthotropic plate are presented too.
The influence of the notches on the natural frequencies
(University of West Bohemia, 2007) Dekýš, Vladimír; Kopas, Peter; Mazúr, Ján; Malcho, Milan
The simplification of the FEM model (the missing of a notch or a crack) generally means the modification of the natural frequencies. An analysis of these changes is matter of this paper. The direct and semicircle notches were made on the testing specimens (by cut depth in the specimens) and the naturals frequencies were measured and computed. The results are compared. In the paper is discussed the choice of the FEM element for FEM model and the comparison with experiment is made too. The influence of the crack on the natural frequencies is discussed in the case of specimen for high cycle fatigue test. The changes of the first three frequencies are compared for specimens with and without crack.
A stochastic response of vibrating systems containing random parameters
(University of West Bohemia, 2007) Dupal, Jan; Králíček, Jan
The paper deals with an approach to the response limit estimation of the vibrating systems in the transient state. A gradient and perturbation methods for derivatives of the response with respect to the random parameters are shown. This sensitivity analysis can be performed for linear systems in the analytical way or in numerical way for both linear and nonlinear systems. In case of nonlinear systems it is supposed small deviations of random parameters from hyperbolic point of the system and that no bifurcations and changes of phase portrait will occur.
Numerical analysis of hydrogen and methane propagation during testing of combustion engines
(University of West Bohemia, 2007) Dvořák, Václav
The research of gas-fuelled combustion engines using hydrogen or methane require accordingly equipped test benches which take respect to the higher dangerous of self ignition accidents. This article deals with numerical calculations of flow in laboratory during simulated leakage of gas-fuel from fuel system of tested engine. The influences of local suction and influences of roof exhausters on the flow in the laboratory and on the gas propagation are discussed. Results obtained for hydrogen and for methane are compared. Conclusions for design and performance of suction devices and test benches are deduced from these results.
Contribution to finite element modelling of airfoil aeroelastic instabilities
(University of West Bohemia, 2007) Horáček, Jaromír; Sváček, Petr; Růžička, Martin; Feistauer, Miloslav
Nonlinear equations of motion for a flexibly supported rigid airfoil with additional degree of freedom for controlling of the profile motion by a trailing edge flap are derived for large vibration amplitudes. Preliminary results for numerical simulation of flow-induced airfoil vibrations in a laminar incompressible flow are presented for the NACA profile 0012 with three-degrees of freedom (vertical translation, rotation around the elastic axis and rotation of the flap). The developed numerical solution of the Navier – Stokes equations and the Arbitrary Eulerian-Lagrangian approach enable to consider the moving grid for the finite element modelling of the fluid flow around the oscillating airfoil. A sequence of numerical simulation examples is presented for Reynolds numbers up to about Re 105, when the system loses the aeroelastic stability, and when the large displacements of the profile and a post-critical behaviour of the system take place.
Bifurcation phenomena in internal dynamics of gear systems
(University of West Bohemia, 2007) Hortel, Milan; Škuderová, Alena
The impact effects in gear mesh represent specific phenomena in the dynamic investigation of highspeed light transmission systems with kinematic couplings. They are caused of greater dynamic than static-elastic deformations in meshing gear profiles. In term of internal dynamics they are influenced among others by time heteronomous stiffness functions in gear mesh and resonance tuning of stiffness level. The damping in gear mesh and in gear system is concerned significantly in the amplitude progress, greatness and phase shift of relative motion towards stiffness function alternatively towards its modify form in gear mesh. In consequence of these and another actions rise above resonance characteristics certain singular locations with jump amplitude course.
On scaling of human body models
(University of West Bohemia, 2007) Hynčík, Luděk; Nováček, Vít; Bláha, Pavel; Chvojka, Ondřej; Krejčí, Petr
Abstract Human body is not an unique being, everyone is another from the point of view of anthropometry and mechanical characteristics which means that division of the human body population to categories like 5%-tile, 50%-tile and 95%-tile from the application point of view is not enough. On the other hand, the development of a particular human body model for all of us is not possible. That is why scaling and morphing algorithms has started to be developed. The current work describes the development of a tool for scaling of the human models. The idea is to have one (or couple of) standard model(s) as a base and to create other models based on these basic models. One has to choose adequate anthropometrical and biomechanical parameters that describe given group of humans to be scaled and morphed among.
Blade dynamic stress analysis of rotating bladed disks
(University of West Bohemia, 2007) Kellner, Josef; Zeman, Vladimír
The paper deals with mathematical modelling of steady forced bladed disk vibrations and with dynamic stress calculation of the blades. The blades are considered as 1D kontinuum elastic coupled with three-dimensional elastic disk centrally clamped into rotor rotating with constant angular speed. The steady forced vibrations are generated by the aerodynamic forces acting along the blade length. By using modal synthesis method the mathematical model of the rotating bladed disk is condensed to calculate steady vibrations. Dynamic stress analysis of the blades is based on calculation of the time dependent reduced stress in blade cross-sections by using Hubert-Misses-Hencky stress hypothesis. The presented method is applied to real turbomachinery rotor with blades connected on the top with shroud.
The behaviour of a tube bundle near the stability limit
(University of West Bohemia, 2007) Klášterka, Hynek
Tube heat exchangers are inseparable components of a great number of energetic machinery, where one fluid flows through tubes and the other fluid flows around the tubes. Heat transfer occurs between these two fluids. Apart from the problem of heat transfer, the problem of fluid-structure interaction is very important too. Mainly the fluid flowing around the tubes may be very dangerous, because it causes vibrations of these tubes. Intensity of vibration depends on the velocity of the flow. Under the certain, so-called critical velocity, vibration amplitudes can have random pattern. The objective of this article is to determine the probability of up crossing of some fixed level. It is necessary to avoid such regimes of operations, in which the damage of a heat exchanger as a consequence of flow-induced vibration could be caused.
Heart valve viscoelastic properties: a pilot study
(University of West Bohemia, 2007) Kochová, Petra; Klepáček, Jan; Hlubocký, Jaroslav; Mokráček, Aleš; Špaténka, Jaroslav; Nováček, Vít
The effects of cryopreservation on the biological tissue mechanics are still largely unknown. Generalized Maxwell model was applied to characterize quantitatively the viscoelastic behavior of sheep mitral heart valve tissue. Three different groups of specimens are supposed to be tested: fresh tissue specimens (control group), cryopreserved allografts from tissue bank and allografts already used as tissue replacements taken from the animals approximately one year after the surgery. Specific aim of this study is to determine whether or not the treatment used for storage in tissue bank influences significantly the mechanical properties and behavior of the tissue. At the moment, only the first group of specimens was examined. The methodology presented in this paper proved suitable to complete the study.
Optimization of lumping schemes for plane square quadratic finite element in elastodynamics
(University of West Bohemia, 2007) Kolman, Radek; Plešek, Jiří; Gabriel, Dušan; Okrouhlík, Miloslav
The effectiveness of explicit direct time-integration methods is conditioned by using diagonal mass matrix which entails significant computational savings and storage advantages. In recent years many procedures that produced diagonally lumped mass matrices were developed. For example, the row sum method and diagonal scaling method (HRZ procedure) can be mentioned. In this paper, the dispersive properties of different lumping matrices with variable mass distribution for the plane square 8-node serendipity elements are investigated. The dispersion diagrams for such lumping matrices are derived for various Courant numbers, wavelengths and the directions of wave propagation.
Comparative analysis of influence selected geometrical parameters on stress concentration in the surrounding of inclusion
(University of West Bohemia, 2007) Kopas, Peter; Handrik, Marián; Sága, Milan; Melicher, Richard
In this paper we are focused on influence of selected geometrical characteristics as are: inscribed circle diameter, circumscribed circle diameter, eccentricity, ovality and radius of curvature of inclusion on stress concentration around these defects modelling using by FEM. This task was solved as plane stress. From this point of view there are monitored and evaluated there factors: maximum value of stress along the loading, across the loading, shear stress and equivalent stresses. There will be also presented algorithms for automatic generation of models that make possible to practice statistical data processing.
Chaos in drive systems
(University of West Bohemia, 2007) Kratochvíl, Ctirad; Houfek, Martin; Koláčný, Josef; Kříž, Roman; Houfek, Lubomír; Krejsa, Jiří
The purpose of this article is to provide an elementary introduction to the subject of chaos in the electromechanical drive systems. In this article, we explore chaotic solutions of maps and continuous time systems. These solutions are also bounded like equilibrium, periodic and quasiperiodic solutions.
Stress wave propagation in thin long-fiber carbon/epoxy composite panel: numerical and experimental solutions
(University of West Bohemia, 2007) Kroupa, Tomáš; Červ, Jan; Valeš, František
The article deals with experimental and numerical analysis of stress wave propagation in a thin long fiber carbon/epoxy composite material. Experiments were performed on in-plane loaded square composite panels with dimensions 501mm £ 501mm £ 2:2 mm. The panels have several fiber orientations (0±, 30±, 60± and 90± measured from the loaded edge). They were loaded by in-plane impact of steel sphere. The impact area was on the edge, exactly 150mm from top left corners corner of the panels. The loading force was approximated by a time dependent function. Its shape was obtained from three dimensional contact analysis, which was performed on smaller area of panel. The function was used in further plane stress analysis of the whole panels. The comparison of the numerical and experimental results was executed. An attempt at determination of velocity of propagation of Rayleigh waves on the loaded edge was performed and the results are discussed in the paper. Further directions of the research are proposed.
Modelling of acoustic transmission through perforated layer
(University of West Bohemia, 2007) Lukeš, Vladimír; Rohan, Eduard
The paper deals with modeling the acoustic transmission through a perforated interface plane separating two halfspaces occupied by the acoustic medium. We considered the two-scale homogenization limit of the standard acoustic problem imposed in the layer with the perforated periodic structure embedded inside. The homogenized transmission conditions govern the interface discontinuity of the acoustic pressure associated with the two halfspaces and the magnitude of the fictitious transversal acoustic velocity. By numerical examples we illustrate this novel approach of modeling the acoustic impedance of perforated interfaces.
Influence of particles-matrix interphase on stress distribution in particulate composite with polymer matrix
(University of West Bohemia, 2007) Majer, Zdeněk; Hutař, Pavel; Náhlík, Luboš; Knésl, Zdeněk
Abstract In this paper fracture behaviour of particulate composite (CaCO3 – PP) is studied. Attention is focused mainly to the influence of interphase between particles and matrix on stress distribution and on micro-crack propagation in composite matrix. The composite was modeled as three-phase continuum and numerically simulated on a microscopic scale using the finite element program ANSYS. Simplified two-dimensional model is used for estimation of hypothetical micro-cracks path prediction. The influence of interphase properties on fracture toughness for particle reinforced polymer composite is discussed.
Hammer crusher: influences of design and execution of vibroprotection and machine properties on vibration intensity
(University of West Bohemia, 2007) Makovička, Daniel; Šmejkal, Jiří
On an example of a hammer crusher the author analyzes the factors participating in the proper operation of any machine. Their number includes the estimate of load magnitude, the safety reserves in its determination, the influence of material characteristics of the machine, vibration isolation and subgrade, the influence of the execution of vibroprotection arising from installation inaccuracies and, last but not least, the influence of the machine design. The individual factors are assessed quantitatively and qualitatively on the example of a comparison of response produced by a theoretical model and experimental results.
Thick-walled anisotropic elliptic tube analyzed via curvilinear tensor calculus
(University of West Bohemia, 2007) Mareš, Tomáš
After a brief introduction into the tensor calculus, the thick-walled anisotropic elliptic tube is analyzed. A procedure of the analysis is described in a stepwise manner. A choice of the appropriate coordinate systems is the first step. The second step consists of the determination of corresponding metric tensors. Then the elasticity tensor of a local orthotropy is transformed into a global computational coordinate system. Next the appropriate Christoffel symbols of the second kind are determined and the total potential energy of the system is expressed. At the end the solution is approximated by a Fourier series and for given geometrical values and loading the numerical results are obtained and graphically represented. It must be said that throughout the calculation the free software only was used and for the numerical operations an old laptop is sufficient. The author regards both the former and the latter as a great advantage of the demonstrated method.
Control rod drive WWER 1000: tuning of input parameters
(University of West Bohemia, 2007) Markov, Petr; Valtr, Oldřich
The article picks up on the contributions presented at the conferences Computational Mechanics 2005 and 2006, in which a calculational model of an upgraded control rod linear stepping drive for the reactors WWER 1000 (LKP-M/3) was described and results of analysis of dynamical response of its individual parts when moving up- and downwards were included. The contribution deals with the tuning of input parameters of the 3rd generation drive with the objective of reaching its running as smooth as possible so as to get a minimum wear of its parts as a result and hence to achieve maximum life-time.

Browse

Recent Submissions