Volume 3, number 2 (2009)
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Item Mathematical modelling of biogenous filter cake and identification of oilseed material parameters(University of West Bohemia, 2009) Očenášek, Jan; Voldřich, JosefMathematical modeling of the filtration and extrusion process inside a linear compression chamber has gained a lot of attention during several past decades. This subject was originally related to mechanical and hydraulic properties of soils (in particular work of Terzaghi) and later was this approach adopted for the modeling of various technological processes in the chemical industry (work of Shirato). Developed mathematical models of continuum mechanics of porousmaterials with interstitial fluid were then applied also to the problem of an oilseed expression. In this case, various simplifications and partial linearizations are introduced in models for the reason of an analytical or numerical solubility; or it is not possible to generalize the model formulation into the fully 3D problem of an oil expression extrusion with a complex geometry such as it has a screw press extruder. We proposed a modified model for the oil seeds expression process in a linear compression chamber. The model accounts for the rheological properties of the deformable solid matrix of compressed seed, where the permeability of the porous solid is described by the Darcy’s law. A methodology of the experimental work necessary for a material parameters identification is presented together with numerical simulation examples.Item Dynamic wheelset drive load of the railway vehicle caused by short-circuit motor moment(University of West Bohemia, 2009) Zeman, Vladimír; Hlaváč, ZdeněkThe paper deals with mathematical modelling of dynamic response of the railway vehicle wheelset drives caused by short-circuit traction motor torque. The individual wheelset drive with hollow graduated shaft is one of subsystems of the two-axled vehicle bogie with two wheelset drives. The model respects the viscoelastic suspension of the both engine stators with gear housings mounted on the bogie frame and all the other couplings among bogie drive components. The dynamic response is investigated in dependence on longitudinal creepage and forward velocity of the vehicle at the moment of the sudden short-circuit in one asynchronous traction motor. The method is applied to bogie of the electric locomotive developed for speed about 200 km/h by the company ŠKODA TRANSPORTATION, s. r. o. The wheelset drive vibration is confronted with stability conditions of the whole bogie.Item A computational investigation of vibration attenuation of a rigid rotor turning at a variable speed by means of short magnetorheological dampers(University of West Bohemia, 2009) Zapomněl, Jaroslav; Ferfecki, PetrRotors of all rotating machines are always slightly imbalanced. When they rotate, the imbalance induces their lateral vibration and forces that are transmitted via the bearings into the foundations. These phenomena are significant if the rotor accelerates or decelerates and especially if it passes over the critical speeds. The vibration can be reduced if the rotor supports are equipped with damping elements. To achieve optimum performance of the damper, the damping effect must be controllable. At present time, semiactive magnetorheological squeeze film dampers are a subject of intensive research. They work on a principle of squeezing a thin film of magnetorheological liquid. If magnetic field is applied, the magnetorheological liquid starts to flow only if the shear stress between two neighbourhood layers exceeds a limit value which depends on intensity of the magnetic field. Its change enables to control the damping force. In the mathematical models, the magnetorheological liquid is usually considered as Bingham one. Application of the computer modelling method for analysis of rotors supported by rolling element bearings and magnetorheological squeeze film dampers and turning at variable angular speed requires to set up the equations of motion of the rotor and to develop a procedure for calculation of the damping force. Derivation of the equations of motion starts from the first and second impulse theorems. The pressure distribution in the thin lubricating film can be described by a Reynolds equation modified for the case of Bingham liquid. In cavitated areas, it is assumed that pressure of the medium remains constant. The hydraulic force acting on the rotor journal is then obtained by integration of the pressure distribution around the circumference and along the length of the damper. Applicability of the developed procedures was tested by means of computer simulations and influence of the control of the damping force on vibration of the rotor was analyzed.Item An evaluation of the stress intensity factor in functionally graded materials(University of West Bohemia, 2009) Ševčík, Martin; Hutař, Pavel; Náhlík, Luboš; Knésl, ZdeněkFunctionally graded materials (FGM) are characterised by variations in their material properties in terms of their geometry. They are often used as a coating for interfacial zones to protect the basic material against thermally or mechanically induced stresses. FGM can be also produced by technological process for example butt-welding of polymer pipes. This work is focused on a numerical estimation of the stress intensity factor for cracks propagating through FGM structure. The main difficulty of the FE model creation is the accurate description of continual changes in mechanical properties. An analysis of the FGM layer bonded from both sides with different homogenous materials was performed to study the influence of material property distribution. The thickness effect of the FGM layer is also discussed. All analyses are simulated as a 2D problem of an edge cracked specimen. In this paper, the above effects are quantified and conclusions concerning the applicability of the proposed model are discussed.Item Utilization of random process spectral properties for the calculation of fatigue life under combined loading(University of West Bohemia, 2009) Svoboda, Jaroslav; Balda, Miroslav; Fröhlich, VladislavThe contribution includes the results of experimental works aiming to find a new methodology for the calculation of fatigue life of structures subjected to operational loading from a combination of forces and moments of random character. Considering the fracture mechanics theory, then the damaging of material is both in the micro- and macro-plastic area connected with the rise of plastic deformation and hence with the plastic transformation rate which depends on the amount of supplied energy. The power spectral density (PSD) indicating the power at individual frequencies in the monitored frequency band yields information about the supplied amount of energy. Therefore, it can be assumed that there is a dependence between the PSD shape and the size of damage and that the supplied power which is proportional to the value of dispersion s2 under the PSD curve could be a new criterion for the calculation of fatigue life under combined loading. The searching for links between the spectral properties of the loading process and the fatigue life of structure under load is dealt with by new Grant GA No. 101/09/0904 of the Czech Technical University in Prague and the Institute of Thermomechanics of the Academy of Sciences of the Czech Republic, v.v.i.Item Glass fibre reinforced cement based composite: fatigue and fracture parameters(University of West Bohemia, 2009) Seitl, Stanislav; Keršner, Zbyněk; Bílek, Vlastimil; Knésl, ZdeněkThis paper introduces the basic fracture mechanics parameters of advanced building material – glass fibres reinforced cement based composite and its fracture and fatigue behaviour is investigated. To this aim three-point bend (3PB) specimens with starting notch were prepared and tested under static (l–d diagram) and cyclic loading (Paris law and W¨ohler curve). To evaluate the results, the finite element method was used for estimation of the corresponding values of stress intensity factor for the 3PB specimen used. The results obtained are compared with literature data.Item A refined description of the crack tip stress field in wedge-splitting specimens – a two-parameter fracture mechanics approach(University of West Bohemia, 2009) Seitl, Stanislav; Knésl, Zdeněk; Veselý, Václav; Řoutil, LadislavThe paper is focused on a detailed numerical analysis of the stress field in specimens used for the wedge splitting test (WST) which is an alternative to the classical fracture tests (bending, tensile) within the fracture mechanics of quasi-brittle building materials, particularly cementitious composites. The near-crack-tip stress field in the WST specimen is described by means of constraint-based two-parameter fracturemechanics in the paper. Different levels of constraint in the vicinity of the crack tip during fracture process through the specimen ligament are characterized by means of the T -stress. Two basic shapes ofWST specimen – the cube-shaped and the cylinder-shaped one – are investigated and the determined near-crack-tip stress field parameters are compared to those of compact tension (CT) specimens according to the ASTM standard for classical and round geometry. Particular attention is paid to the effect of the compressive component of the loading force (complementing the splitting force) acting on the loaded side of the specimen and its reaction from the opposite part of the specimen on the stress field in the cracked body. Several variants of boundary conditions on the bottom side of the specimen used for this kind of testing procedure are also considered. The problem is solved numerically by means of the finite element method and results are compared with data taken from the literature.Item Analytical and numerical investigation of trolleybus vertical dynamics on an artificial test track(University of West Bohemia, 2009) Polach, Pavel; Hajžman, Michal; Soukup, Josef; Volek, JanTwo virtual models of the ŠKODA 21 Tr low-floor trolleybus intended for the investigation of vertical dynamic properties during the simulation of driving on an uneven road surface are presented in the article. In order to solve analytically vertical vibrations, the trolleybus model formed by the system of four rigid bodies with seven degrees of freedom coupled by spring-damper elements is used. The influence of the asymmetry of a sprung mass, a linear viscous damping and a general kinematic excitation of wheels are incorporated in the model. The analytical approach to solving the ˇSKODA 21 Tr low-floor trolleybus model vibrations is a suitable complement of the model based on a numerical solution. Vertical vibrations are numerically solved on the trolleybus multibody model created in the alaska simulation tool. Both virtual trolleybus models are used for the simulations of driving on the track composed of vertical obstacles. Conclusion concerning the effects of the usage of the linear and the nonlinear spring-damper elements characteristics are also given.Item Mathematical modeling and dynamic simulation of a class of drive systems with permanent magnet synchronous motors(University of West Bohemia, 2009) Mikhov, Mikho RachevThe performance of a two-coordinate drive system with permanent magnet synchronous motors is analyzed and discussed in this paper. Both motors have been controlled in brushless DC motor mode in accordance with the rotor positions. Detailed study has been carried out by means of mathematical modeling and computer simulation for the respective transient and steady-state regimes at various load and work conditions. The research carried out as well as the results obtained can be used in the design, optimization and tuning of such types of drive systems. They could be also applied in the teaching process.Item Numerical simulation of plastic deformation of aluminium workpiece induced by ECAP technology(University of West Bohemia, 2009) Melicher, RichardThe objective of this paper was to show some numerical simulations which can be very helpful for optimal settings in extrusion process by equal channel angular pressing (ECAP) technology. Using the finite element (FEM) software ADINA all basic types of used shape of the inner and outer corner of die were modeled. The influence of the tool geometry for plastic strain by simple shear into the aluminum workpiece during extrusion process of the ECAP was analyzed. It was also examined the influence of the pressing force for all usually used variants of die corners.Item Various methods of numerical estimation of generalized stress intensity factors of bi-material notches(University of West Bohemia, 2009) Klusák, Jan; Profant, Tomáš; Kotoul, MichalThe study of bi-material notches becomes a topical problem as they can model efficiently geometrical or material discontinuities. When assessing crack initiation conditions in the bi-material notches, the generalized stress intensity factors H have to be calculated. Contrary to the determination of the K-factor for a crack in an isotropic homogeneous medium, for the ascertainment of the H-factor there is no procedure incorporated in the calculation systems. The calculation of these fracture parameters requires experience. Direct methods of estimation of Hfactors need choosing usually length parameter entering into calculation. On the other hand the method combining the application of the reciprocal theorem (Ψ-integral) and FEM does not require entering any length parameter and is capable to extract the near-tip information directly from the far-field deformation.Item Analysis of composite car bumper reinforcement(University of West Bohemia, 2009) Kleisner, Václav; Zemčík, RobertThe presented work summarizes the present state of car passive safety testing according to European methodologies. The main objective is to analysis a bumper reinforcement made of composite materials. The bumper is tested according to RCAR (Research Council for Automobile Repairs) methodology using numerical simulation. Individual proposed variants are compared with the existing steel construction which does not comply with manufacturers specifications. The PAM-Crash software is used for the simulation. The numerical model is using shell elements and the Ladev`eze model for the description of behaviour of composite materials. The methodology for the set-up of the numerical model in PAM-crash is firstly validated by comparison of experiment and analytical results.Item A model of engineering materials inspired by biological tissues(University of West Bohemia, 2009) Holeček, Miroslav; Moravec, Fanny; Vychytil, JanThe perfect ability of living tissues to control and adapt their mechanical properties to varying external conditions may be an inspiration for designing engineering materials. An interesting example is the smooth muscle tissue since this “material” is able to change its global mechanical properties considerably by a subtle mechanism within individual muscle cells. Multi-scale continuum models may be useful in designing essentially simpler engineering materials having similar properties. As an illustration we present the model of an incompressible material whose microscopic structure is formed by flexible, soft but incompressible balls connected mutually by linear springs. This simple model, however, shows a nontrivial nonlinear behavior caused by the incompressibility of balls and is very sensitive on some microscopic parameters. It may elucidate the way by which “small” changes in biopolymer networks within individualmuscular cells may control the stiffness of the biological tissue, which outlines a way of designing similar engineering materials. The ‘balls and springs’ material presents also prestress-induced stiffening and allows elucidating a contribution of extracellular fluids into the tissue’s viscous properties.Item The multi-axial material fatigue under the combined loading with mean stress in three dimensions(University of West Bohemia, 2009) Fojtík, František; Fuxa, JanThis contribution describes the application of Fuxa’s conjugated strength criterion on the experimental results under combined loading of specimens made from common construction steel 11523.0, melt T31052. The specimens were stepwise loaded by the torque amplitude, combination of torque amplitude and tension pre-stress, further by the amplitude of the torque in combination with inner overpressure and axial tension force. The last set of specimens was loaded by the torque amplitude in combination with inner and external overpressure and with axial tension force. To obtain the data required as the input values for the conjugated criterion the stress/strain analysis of the specimens by the finite element method in software ANSYS was performed. The experiments were performed on modified testing machine equipped by overpressure chamber.Item Implementation of skeletal muscle model with advanced activation control(University of West Bohemia, 2009) Kocková, Hana; Cimrman, Robertis suitable for a 3D muscle representation. It respects the microstructure of the muscle which is represented by three basic components: active fibers, passive fibers and a matrix. For purposes of presented work the existing material models suitable for the matrix and passive fibers are used and a new active fiber model is proposed. The active fiber model is based on the sliding cross-bridge theory of contraction. This theory is often used in modeling of skeletal and cardiac muscle contractions. In this work, a certain simplification of the cross-bridge distribution function is proposed, so that the 3D computer implementation becomes feasible. The new active fiber model is implemented into our research finite element code. A simple 3D muscle bundle-like model is created and the implemented composite model (involving the matrix, passive and active fibers) is used to perform the isometric, concentric and excentric muscle contraction simulations.Item Identification of low cycle fatigue parameters(University of West Bohemia, 2009) Balda, MiroslavThe article describes a new approach to the processing of experimental data coming from low-cycle fatigue (LCF) tests. The data may be either tables from the standard tests, or a time series of loading processes and corresponding numbers of cycles to damage. A new method and a program for the evaluation of material parameters governing the material behavior under a low cycle loading have been developed. They exploit a minimization procedure for an appropriate criterion function based on differences of measured and evaluated damages.