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Onchis D.M.,University of Vienna | Gillich G.-R.,Eftimie Murgu University of Resita
International Conference on Engineering Mechanics, Structures, Engineering Geology, International Conference on Geography and Geology - Proceedings | Year: 2010

We propose an adaptive method for the analysis of the dynamical changes in mechanical structures. Using measurement techniques and the flexible Gabor-wavelet transform, we perform an optimal denoising of slowly variable band-limited signals for an improved mechanical structure fault diagnostics.


Gillich G.-R.,Eftimie Murgu University of Resita | Praisach Z.,Science HYDRO ENGINEERING S.A. | Onchis D.M.,University of Vienna
International Conference on Engineering Mechanics, Structures, Engineering Geology, International Conference on Geography and Geology - Proceedings | Year: 2010

Researches concerning structure's safety improvement have increased significantly in the past decades, structural health monitoring becoming an important support in taking decision regarding the need of these structures' remediation. Various techniques, from visual inspection to nondestructive evaluation or vibration-based damage detection are used. The last ones use information regarding changes of natural frequencies, damping ratios, flexibility or mode shapes. Even if numerous results have been achieved in this field, several problems still have to be solved for specific applications before their implementation in practice. Our study approaches the method's viability of damage detection in simple structures using only one accelerometer by analytical method, finite element method and experimental test. The obtained results by all three methods showed that this approach can be used only in high damage rates cases. Finally, are pointed some issues which have to be further worked out.


Onchis-Moaca D.,Eftimie Murgu University of Resita | Gillich G.-R.,Eftimie Murgu University of Resita | Frunza R.,University of Vienna
European Signal Processing Conference | Year: 2012

It is the purpose of the paper to stimulate a systematic investigation of the time-frequency methods used to extract the natural frequency of mechanical structures. In particular, we are interested in cantilever beams, which are simple structures in mechanics, and therefore most appropriate for detailed comparison. We will compare the refined time-frequency spectra for the short-time Fourier transform, for the Wigner transform and for the non-stationary Gabor transform and extract a set of natural frequencies from the accelerometer data placed at the free end of a cantilever beam. © 2012 EURASIP.


Gillich G.-R.,Eftimie Murgu University of Resita | Minda P.F.,Eftimie Murgu University of Resita | Praisach Z.-I.,Eftimie Murgu University of Resita | Minda A.A.,Eftimie Murgu University of Resita
Romanian Journal of Acoustics and Vibration | Year: 2012

This paper introduces a relation which describes the influence of discontinuities on the dynamic response of prismatic cantilever beams. The relation, providing the natural frequencies of weakaxis bending vibration modes of damaged beams, was contrived by considering the continuous model in a global approach, opposite to researches presented in literature involving mainly models based on two segments linked by a rotational spring and focusing on the local effect of the discontinuity. To demonstrate the validity of the proposed relation, a cantilever beam was analyzed both in undamaged and damaged case, being highlighted the natural frequency changes due discontinuity for various damage scenarios; finally curves representing frequency changes were plotted. Comparing the results obtained by the two approaches, a similarity for the frequency changes of the first bending vibration modes is obvious, while for higher modes the "classical" approach is not able to provide any result due to limitations of the numerical methods. Opposite, the proposed relation is simple to be utilized and offer precise results for all bending vibration modes, fact proved by examples presented at the end of the paper.


Gillich G.-R.,Eftimie Murgu University of Resita | Praisach Z.-I.,Eftimie Murgu University of Resita | Iavornic C.M.,Eftimie Murgu University of Resita
Proceedings of the ASME Design Engineering Technical Conference | Year: 2012

The paper presents a method to detect, locate and evaluate damage severity of Euler-Bernoulli beams, based on how natural frequencies change due to damages. Previous researches that dealt with this issue focused only on quantitative changes, mainly considering a global stiffness reduction in the damaged area. The authors have contrived a correlation between the strain energy stored in a segment of the beam, which is proportional with the mode shape curvature of a considered vibration mode at that location, and the frequency change for this mode if damage appears on that segment. This reveals that for an element of the beam, the stiffness change of a certain mode for a given damage varies between zero and a maximum, depending solely on the location of that element. Moreover, one has to consider different stiffness changes for a damaged element placed on a certain location, depending on the vibration mode. This rule how frequencies of various modes change due to damage are used to create patterns, based on relative frequency shifts, which characterize damaged beams in respect to defect location and severity. The method was validated by numerous experiments, which proved its accuracy and reliability. Copyright © 2012 by ASME.


Gillich G.-R.,Eftimie Murgu University of Resita | Praisach Z.-I.,Eftimie Murgu University of Resita
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2012

The paper presents a method to assess damages in beams, based on how natural frequencies of bending vibration modes change due to damages. The authors have contrived a correlation between the strain energy stored in a segment of the beam, which is proportional with the mode shape curvature of a considered vibration mode at that location, and the frequency change for this mode if damage appears on that segment. For a certain mode, damages placed on inflection points of the mode shape curvature, where the strain energy is null, will not produce changes in frequency, while damages placed on maxima will produce the highest changes in frequency. For other locations of damage, the frequency shift will be proportional with the mode shape curvature of the vibration mode at that location. We worked out a general relation, which gives the frequency shift of all bending modes, with one coefficient depending on the support type. To evaluate damages, we determine analytically the relative frequency shift as ratio between the frequency change and the natural frequency of the undamaged beam, for the first ten vibration modes, considering various damage depths and locations. Comparison of results with that obtained by measurements on the real beam permits detection, location and assessment of damages in beams with high accuracy. The method was validated by experiments. © 2012 SPIE.


Gillich G.-R.,Eftimie Murgu University of Resita | Praisach Z.-I.,Eftimie Murgu University of Resita
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2013

The paper presents a method to locate discontinuities in form of transversal cracks in beams, based on vibration measurements. Patterns characterizing frequency changes of the first ten weak-axis bending vibration modes are determined for all possible locations on the structure, using a relation contrived by the authors. It base on the correlation between the strain energy stored in a segment of the beam, which is proportional with the square of the mode shape curvature of the considered vibration mode at that location, and the frequency change for this mode by the occurrence of a discontinuity on that segment. The patters consist from a series of ten values representing the normalized relative frequency shifts for the first ten vibration modes. For a structure similarly supported, by continuous or periodical measurements, potential frequency changes can be detected. By processing these data the so-called damage location index for that crack is found out, also as a series of ten values representing the relative frequency shifts of the ten vibration modes. To precise locate the crack a pattern matching method involving the database with all possible patterns and the damage location index is used. Knowing the location, it is easy to determine by analytic calculus the crack depth. The method is easy to be used, provide accurate results, demands modest computational effort and has the advantage that the measurements may be carried out in situ with rather simple equipment. The method was validated by experiments. © 2013 SPIE.


Gillich G.-R.,Eftimie Murgu University of Resita | Praisach Z.-I.,Eftimie Murgu University of Resita
Key Engineering Materials | Year: 2013

This paper is concerned with vibration based non-destructive evaluation of structures, with a focus on quantitative assessment of damage. In previous works, a reliable method to locate open cracks in beams has been proposed and tested using both data from numerical simulations and laboratory experiments. It bases on the fact the natural frequency of a bending vibrations mode attend different changes, depending on the loss of stored energy for the slice on which the damage is located. As bigger the mode shape curvature value on that location, so bigger the loss of stored energy and consequently the natural frequency decrease in that mode. Analyzing the natural frequency changes for a larger series of vibration modes, it's possible to precisely locate damages. The authors succeed to find a single mathematical relation describing the frequency changes for all bending vibration modes, involving one term defining damage's location and one defining its depth. While the first term changes for different modes, being defined by the mode shape curvature, the second maintain its value for all modes, being affected just by damage depth. This finding permits decoupling the location issue with that of quantitative assessment of damage. Latest researches, presented in this paper, succeed by finding the relation between the second term of the relation and some mechanical characteristics of the beam, i.e. extending the proposed method by including evaluation of damage severity. The approach is illustrated on a cantilever beam, modeled with 3D elements. © (2013) Trans Tech Publications.


Gillich G.-R.,Eftimie Murgu University of Resita | Praisach Z.-I.,Eftimie Murgu University of Resita
Signal Processing | Year: 2014

This paper presents a new method, based on natural frequency changes, able to detect damages in beam-like structures and to assess their location and severity, considering the particular manner in which the natural frequencies of the weak-axis bending vibration modes change due to the occurrence of discontinuities. The problem is to accurately determine frequencies, because their changes present low sensitivity to damage; moreover, in some modes the damping effect is significant, and thus the analyzed signal has to be precisely identified in time. To overcome these difficulties, we propose a three-step approach. In the first stage a time-frequency analysis is performed, to roughly determine the frequency range for the first ten weak-axis bending vibration modes and to locate in time the relevant signal segment for each of the harmonics. Afterwards, filters are applied to the signal, in order to visualize the individual components corresponding to these ten vibration modes. Finally, the power spectrum of each signal component is calculated for the relevant signal segment and for integer periods of the respective harmonic. Having the precise frequencies for the healthy and damaged structure, one can calculate the frequency shifts, and thus damage location and assessment become a pattern recognition problem. It consists in comparing the measured frequency changes with values obtained analytically, using a relation which defines these changes contrived by the authors. The method's performance was experimentally proven on steel beams, for various damage scenarios and boundary conditions. © 2013 Elsevier B.V.


Praisach Z.,Science HYDRO ENGINEERING S.A. | Gillich G.-R.,Eftimie Murgu University of Resita | Birdeanu D.E.,Eftimie Murgu University of Resita
International Conference on Engineering Mechanics, Structures, Engineering Geology, International Conference on Geography and Geology - Proceedings | Year: 2010

In the paper, the authors try to evaluate the possibility of detecting damages in different types of beams, using the vibrations by finite element method. The assumption was that for beams with the same value of transversal area, same length and different geometries, the efficiency of the procedure will be different. Relevant information obtained by calculation is the natural frequencies of the beams; common variations should indicate changes in its structure. Our first approach was to test the method on a cantilever beam, using the finite element method. The first eight natural frequencies for a steel beam were deduced, for both the undamaged and damaged beam. The results were compared on three types of beams. We concluded that the natural frequency cantilever beams change due to damage, for the same amount of area bars take into account the cross section bar geometry and is significant.

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