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How to test the reliability of instruments used in mirotremor horizontal-to-vertical spectral ratio measurements
Izidor Tasič, Franc Runovc, 2010, izvirni znanstveni članek

Opis: The reliability of a horizontal-to-vertical spectral ratio (HVSR) curve depends on the results obtained by a verified seismological system. Seismic microzonation provides the basis for a site-specific risk analysis and it can be evaluated using the microtremor HVSR method, where the data are recorded using modern seismological systems. Changes in the transfer function of seismological systems affect the HVSR curve and, consequently, also its interpretation, if these changes are not detected and taken into consideration while performing the microtremor spectral calculations. The reliability of the seismic microzonation performed by such a procedure becomes questionable. An algorithm is developed with a two references system, where the influence of the transfer function on the HVSR curve by the tested system can be evaluated without any a-priori knowledge regarding the transfer functions of any of the systems. This approach is applied to a Lennartz Le-3D/5s seismometer and to a TROMINO seismological system, where two Streckeisen STS2 seismometers are used as the reference systems.
Ključne besede: seismic microzonation, horizontal-to-vertical spectral ratio method, ambient vibrations, microtremor, seismic system transfer function, reliability and calibration of seismic systems
Objavljeno v DKUM: 11.06.2018; Ogledov: 1010; Prenosov: 55
.pdf Celotno besedilo (201,91 KB)
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A microtremor HVSR study of the seismic site effects in the area of the town of Brežice (SE Slovenia)
Andrej Gosar, 2009, izvirni znanstveni članek

Opis: The wider region of Brežice is one of the most seismically active areas of Slovenia. The most damaging earthquake occurred in 1917, when Brežice was hit by an ML=5.7 earthquake that had an VIII MSK maximum intensity. Seismic site effects due to Quaternary sediments can be expected in the whole area of the town. Since there is a lack of boreholes and geophysical data to assess the site effects, the microtremor (ambient vibrations) horizontal-to-vertical spectral ratio (HVSR) method was applied at 113 free-field measuring points. In general, clear spectral peaks were obtained, but there were also examples of flat spectral ratios indicating a low impedance contrast between the sediments and the bedrock. A map of the sediment resonance frequency was drawn, which shows a frequency range of 1–15 Hz. In the southern part of the town higher frequencies correspond to the thin sediments (the Brežice terrace and the middle Sava terrace) and the high HVSR peak amplitudes indicate a great impedance contrast with the Sarmatian and Badenian bedrock. The northern part is characterized by very low frequencies, indicating thick sediments (Plio-Quaternary gravel) and a small impedance contrast with Pontian marl. The transition between the high and low frequencies is relatively steep. In addition, measurements were performed in 12 characteristic houses to assess the longitudinal and transverse building frequencies. The established building frequency range of 5–10 Hz indicates some areas in the southern part of the town where there is a potential danger of soil-structure resonance. Two buildings show direct evidence of a potential soil-structure resonance. According to this study, more pronounced seismic site effects are expected in the southern part of Brežice.
Ključne besede: ambient vibrations, microtremors, horizontal-to-vertical spectral ratio (HVSR), seismic site effects, soil-structure resonance
Objavljeno v DKUM: 06.06.2018; Ogledov: 1271; Prenosov: 236
.pdf Celotno besedilo (403,95 KB)
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Influence of vibrations on residual stresses distribution in welded joints
Ivan Samardžić, Tomaž Vuherer, Dejan Marić, Pejo Konjatić, 2015, izvirni znanstveni članek

Opis: This paper presents research experience related to implementation of vibrations on residual stress relief with the equipment for treatment by vibration method. The paper provides explanation of effects achieved by vibration method when implemented during welding, and elaborates advantages and disadvantages of that method applied after welding process. In order to present effects of vibration on solid structures, there was experimental analysis performed and compared with heat treatment.
Ključne besede: welding, vibrations, residual stress, heat treatment
Objavljeno v DKUM: 03.07.2017; Ogledov: 1103; Prenosov: 125
.pdf Celotno besedilo (213,77 KB)
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Methods for detecting fatigue cracks in gears
Aleš Belšak, Jože Flašker, 2009, objavljeni znanstveni prispevek na konferenci

Opis: A crack in the tooth root is the least desirable damage caused to gear units and it often causes failure of gear unit operation. For fault analyses presented in this article, gear units with real damages or faults produced on the basis of numerical simulations of real operating conditions are used; tests were carried out in a laboratory test plant. Damages in gear units can be identified by monitoring vibrations. The influences of a crack in a single-stage gear unit on produced vibrations are presented. Significant changes in tooth stiffness are caused by a fatigue crack in the tooth root whereas, in relation to other faults, changes of other dynamic parameters are more expressed. Non-stationary signals are analysed, using the family of Time Frequency Analysis tools, which include Wavelets and Joint Time Frequency Analyses.
Ključne besede: gear unit, vibrations, tooth root, time frequency analysis tool
Objavljeno v DKUM: 30.06.2017; Ogledov: 1134; Prenosov: 389
.pdf Celotno besedilo (705,13 KB)
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Predictions of experimentally observed stochastic ground vibrations induced by blasting
Srđan Kostić, Matjaž Perc, Nebojša Vasović, Slobodan Trajković, 2013, izvirni znanstveni članek

Opis: In the present paper, we investigate the blast induced ground motion recorded at the limestone quarry “Suva Vrela” near Kosjerić, which is located in the western part of Serbia. We examine the recorded signals by means of surrogate data methods and a determinism test, in order to determine whether the recorded ground velocity is stochastic or deterministic in nature. Longitudinal, transversal and the vertical ground motion component are analyzed at three monitoring points that are located at different distances from the blasting source. The analysis reveals that the recordings belong to a class of stationary linear stochastic processes with Gaussian inputs, which could be distorted by a monotonic, instantaneous, time-independent nonlinear function. Low determinism factors obtained with the determinism test further confirm the stochastic nature of the recordings. Guided by the outcome of time series analysis, we propose an improved prediction model for the peak particle velocity based on a neural network. We show that, while conventional predictors fail to provide acceptable prediction accuracy, the neural network model with four main blast parameters as input, namely total charge, maximum charge per delay, distance from the blasting source to the measuring point, and hole depth, delivers significantly more accurate predictions that may be applicable on site. We also perform a sensitivity analysis, which reveals that the distance from the blasting source has the strongest influence on the final value of the peak particle velocity. This is in full agreement with previous observations and theory, thus additionally validating our methodology and main conclusions.
Ključne besede: blasting, vibrations, surrogate data, deterministic chaos, stochasticity
Objavljeno v DKUM: 19.06.2017; Ogledov: 941; Prenosov: 330
.pdf Celotno besedilo (1,45 MB)
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Torsional vibrations at guide-vane shaft of pump-turbine model
Andrej Predin, 1997, izvirni znanstveni članek

Opis: This article focuses on the problem of guide-vane vibrations of reversible pump-turbines, especially, in the pump mode. These vibrations are transmitted to the guide-vane shaft torque. The guide-vane vibrations are caused by the impeller exit flow, which has a turbulent and partly nondeterministic property. Experimentally determined flow velocities at the impeller exit are given. The mathematical models for theoretical torsional vibration prediction formulated using linear and nonlinear differential equations are presented. The results of theoretical calculations are compared with measurements results. The possibility of transferring the parameters from the model to the prototype is discussed.
Ključne besede: pump-turbines, oscillations, guide-vane shaft, torsional vibrations
Objavljeno v DKUM: 15.06.2017; Ogledov: 1255; Prenosov: 393
.pdf Celotno besedilo (3,80 MB)
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Emergency gate vibration of the pipe-turbine model
Andrej Predin, Roman Klasinc, 2000, izvirni znanstveni članek

Opis: The vibration behaviour of an emergency gate situated on a horizontal-shaft Kaplan turbine is studied. The analysis and transfer of the dynamic movements of the gate are quite complex. In particular the behaviour is examined of the emergency gate for the case when the power unit is disconnected from the system or there is a breakdown of the guide vane system at the moment when the maximal head and capacity are achieved. Experimental-numerical methods both in the time domain and in the frequency are employed. Natural vibrations characterize a first zone, corresponding to relatively small gate openings. As the gate opening increases, the vibration behaviour of the gate becomes increasingly dependent on the swirl pulsations in the draft tube of the turbine. Finally, the data transfer from the model to the prototype by use of the dinamic similitude law is discussed.
Ključne besede: water turbines, emergency gates, vibrations, vibration measurement
Objavljeno v DKUM: 15.06.2017; Ogledov: 1253; Prenosov: 346
.pdf Celotno besedilo (2,45 MB)
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Sand as a medium for transmission of vibratory signals of prey in antlions Euroleon nostras (Neuroptera: Myrmeleontidae)
Dušan Devetak, Bojana Mencinger Vračko, Miha Devetak, Marko Marhl, Andreja Špernjak, 2007, izvirni znanstveni članek

Opis: European pit-building antlions (Euroleon nostras/ Geoffroy in Fourcroy/) detect their prey by sensing the vibrations that prey generate during locomotory activity. The behavioural reactions and some of the physical properties of substrate vibrations in sand are measured to observe signal transmission through the substrate. The frequency range of the signals of four arthropod species (Tenebrio molitor, Pyrrhocoris apterus, Formica sp. and Trachelipus rathkei) is 0.1-4.5 kHz and acceleration values are in the range ▫$400 {mu}m s^{-2} to 1.5 mm s^{-2}$▫. Substrate particle size and the frequency of prey signals both influence the propagation properties of vibratory signals. The damping coefficient at a frequency 300 Hz varies from 0.26 to 2.61 dB ▫$cm^{-1}$▫ and is inversely proportional to the size of the sand particle. The damping coefficient is positively correlated with the frequency of the pulses. Vibrations in finer sand are attenuated more strongly than in coarser sand and, consequently, an antlion detects its prey only at a short distance. The reaction distance is defined as the distance of the prey from the centre of the pit when the antlion begins tossing sand as a reaction to the presence of prey. The mean reaction distance is 3.3 cm in the finest sand (particle size ▫$le 0.23 mm$▫) and 12.3 cm in coarser sand (particle size 1-1.54 mm). The most convenient sands for prey detection are considered to be medium particle-sized sands.
Ključne besede: biology, zoology, receptors, chordotonal organs, vibrations, vibratory signals, transmission of vibrations, reception of vibrations, electrophysiology, substrate vibration, antlions, Myrmeleontidae, sand, substrate vibration, particle size
Objavljeno v DKUM: 07.06.2012; Ogledov: 2477; Prenosov: 106
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The vibrations of microbeams and nanotubes
Jurij Avsec, 2011, izvirni znanstveni članek

Opis: Temperature variations can significantly change the dynamic characteristics of macro-, micro- and nano-structures. In the presented article we have studied the microbeams and nanotubes under thermal effects. Microbeams and nanotubes will be very important in the future in the fields of MEMS and NEMS. For the physical explanation of vibrations of nanotubes classical mechanics is valid with some limitations. We have taken into account the influence of thermal force, axial force in rotating shaft and also gyroscopic effect. The effect of temperature-dependent material properties was considered primarily with respect to the temperature variations. On the basis of our analytical model it is possible to determine the vibrational characteristics in a very wide range of temperatures. In the presented paper it is shown for the first time in scientific literature the combined influence of temperature, gyroscopic effects and rotor speeds on shaft and beam vibrations.
Ključne besede: vibrations of microbeams, vibrations of nanotubes, rotor vibration, thermomechanics, nanomechanics
Objavljeno v DKUM: 05.06.2012; Ogledov: 2218; Prenosov: 36
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Vibration of beams and rotating shafts under thermal effects
Jurij Avsec, 2009, izvirni znanstveni članek

Opis: Temperature variations can significantly change the dynamic characteristics of structures. In the presented paper we have studied the clamped isotropic rotor under thermal effects. We have studied aluminium and copper beam. Because the expansion is restrained by the clamping, the beam undergoes internal stresses, which induce changes in dynamic characteristics. We have also expanded and developed Marques and Inman model as well as Bahzad and Bastami model. We have taken into account the compound influence of thermal force, axial force in rotating shaft and gyroscopic effect. The behavior of rotating shaft is studied in the temperature range of 250-700 K. The influence of temperature-dependent material properties was considered primarily with respect to temperature variations. The presented research work demonstrates a very good agreement between experimental data and results from our vibrational model. On the basis of the proposed analytical model it is possible to determine the vibrational characteristics in very wide range of temperatures. The reported paper is the first one in scientific literature to consider collectively the combined influence of temperature, gyroscopic effects and rotor speeds on shaft and beam vibrations.
Ključne besede: beam vibrations, rotor vibrations, temperature effects, mechanics, thermomechanics
Objavljeno v DKUM: 05.06.2012; Ogledov: 3853; Prenosov: 70
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