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New method of complex modulus estimation of prepressed rubber by the FE model parameter tuning – rubber-damped wheel application
Šulc, Petr ; Pešek, Luděk ; Bula, Vítězslav ; Cibulka, Jan ; Boháč, T. ; Tašek, H.
The main goal of the study was to develop a method for estimation of the frequency dependence of material constants of the pre-pressed hard synthetic rubbers. It was motivated the need to identify material constants, i.e. Young modulus and loss factor, of rubber segments pressed between the disk and the rim of a rubber-damped railway wheel. The rubber segments are pre-pressed about 20% strain level during the production of the wheel. Hard synthetic rubber materials exhibit complex thermalfrequency behavior with nonlinear dependence on static preload. The standard experimental procedures that evaluate the frequency dependence of the material are based on the vibrations of a cantilever beam that consists of a metal and a rubber layer. The new estimation method of the complex modulus of elasticity of rubber is based on the tuning of rubber constants of an FE wheel model according to the results of natural frequencies and mode shapes of the wheel ascertained from the experiment. Numerical FE model of the wheel consisted of the cyclic model of 1/24 sector of the wheel with an angle of 15° and containing one rubber segment and was created in ANSYS 14.5. Damping model of rubber is described by a special case of proportional damping. For calculating eigenvalues of the problem, the Lanczos method was used for the wheel as an undamped system and QR damped method for the damped system. The experimental modal analysis of the rubber-damped wheel pressed on the shaft took place at room temperature in the dynamic laboratory. Modal tests were performed in configuration with an exciter acting first in the axial direction and consequently in the radial direction of the wheel. Responses to the exactions were measured in three directions at 144 points. The identification of eigenvalues and mode shapes of the wheels was made separately for excitations both in radial and in axial direction.
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Stress analysis under torsion loading hard rubber for a large deformation and hyperelastic behaviour
Šulc, Petr ; Pešek, Luděk ; Bula, Vítězslav ; Cibulka, Jan ; Košina, Jan
The paper deals with a stress analysis of hard rubber under large torsion deformations. This study was motivated by effort to enhance the theoretical background for experimental evaluation of material behaviour of hard rubbers on our test rig. First the Mooney Rivlin model (MRM) for shear case of loading was developed and then MRM constants were attained by fitting of the MRM to the experimental torsion-deformation curve. Then the tuned MRM cylindrical model was tested under torsion loading for evaluation of stress state. Besides the radial distribution of shear stress and strain the attention was paid for evaluation of axial stresses. It could help to assess the influence of the tension stresses on the tangential deformations of the test sample during large torsion.
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Application of pulse excitation for forced vibration of selected eigenmode of blade wheel under rotation
Pešek, Luděk ; Bula, Vítězslav ; Cibulka, Jan
Application of pulse excitation for forced vibration of selected eigenmode of blade wheel under rotation. The paper deals with a solution of multipoint electromagnetic pulse excitation of turbine blade wheel under rotation for analysis of its dynamic behaviour. Since a width of electromagnetic pulses should decrease with revolution speed, magnetomotoric forces acting to the blades decrease, too. So, the application of more “high-speed” electromagnets distributed along a circumference of the wheel that multiply the excitation potential seems to be inevitable for such a stiff structure. It, however, requires accurate synchronization of the electromagnets with wheel revolutions and with a dynamic response of the selected blade. Furthermore impulse excitation causes, in general, transient oscillations of many superposed eigenmodes of the wheel. Therefore an attention, herein, was paid to develop an appropriate control algorithm for the pulse excitation of electromagnets and to find a proper strategy of electromagnet interconnections to gain a multiplepoint excitation for excitation of selected eigenmodes.
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The test of the time-difference method for the torsion vibration of the rottating shaft
Pešek, Luděk ; Bula, Vítězslav ; Cibulka, Jan
The contribution deals with testing of the time difference method by using the encoder for evaluation of the torsion vibration of the rotating shaft under dynamic torque moment excitation. The testing of accuracy of this method was first performed on the numerical simulations of the pulse signals and then on the real experimental data gained from the encoder. Then the method was implemented into dSPACE control system for on-line processing. The achieved experimental results are in accordance with the numerical assumptions.
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Air-pressure characteristics and visualization of bubbling effect in water resistance therapy
Radolf, Vojtěch ; Horáček, Jaromír ; Bula, Vítězslav ; Laukkanen, A. M.
This study investigates the influence of a widely used method in voice training and therapy, phonation into a resonance tube with the outer end submerged in water (‘water resistance therapy’ with bubbling effect). Acoustic and electroglottographic (EGG) signals and air pressures in the mouth cavity were registered and the formation of bubbles was studied using high speed camera. Bubbling frequency dominates in the spectra of the pressure signal being about 15 dB higher than the amplitude of the first harmonic, which reflects the fundamental frequency of the vocal folds’ vibration. Separation of the bubbles 10 cm under water surface starts when the buoyancy force acting on the bubble is approximately equal to the aerodynamic force in the tube.
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