Using Roving Disc and Natural Frequency Curves for Crack Detection in Rotors

Since long it has been observed that the size of the crack in structures increases with time, and finally, it may lead to its catastrophic failure. Hence, it is crucial to do the vibration study of cracked structures with regard to vibration-based crack detection and the classification of cracks. So far, vibration-based non-destructive testing method is applied to many spring steel cracked cantilever beams for its possible crack detection. However, the effect of various kinds of practical cracks, that is, V-shaped and U-shaped, on the applicability of these methods has been overlooked. To investigate this issue, artificially cracks are made on the cantilever beam. By free vibration analysis, the effect of crack geometry, crack depth, and crack location on natural frequency is investigated. The natural frequency results obtained from V-shaped and U-shaped models for the same crack configurations are compared with each other and it is revealed that the results are not much sensitive for the change of crack geometry. Hence, it is clear that free vibration-based crack detection method approximately predicts the crack parameters, that is, crack location and crack depth, in structures irrespective of the crack geometry. It is also found that for the same configuration, results of natural frequency are comparatively on the lower side for U-shaped crack models than V-shaped crack models. In this study, the natural frequency of each cracked case is computed by a theoretical method and numerical method and shows good agreement. Finally, it is also observed that structural integrity of a cracked cantilever beam is a function of crack location.

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Experimental verification of detection and prediction of multiple cracks by vibrations, FEM and ANN

Multidiscipline Modeling in Materials and Structures ◽

10.1108/mmms-06-2013-0040 ◽

2014 ◽

Vol 10 (3) ◽

pp. 290-303

Author(s):

Prasad Ramchandra Baviskar ◽

Vinod B. Tungikar

Keyword(s):

Modal Analysis ◽

Natural Frequency ◽

Crack Detection ◽

Vertical Plane ◽

Multiple Cracks ◽

Transverse Cracks ◽

Content Type ◽

First Case ◽

Natural Frequency Analysis ◽

Good Agreement

Purpose – The purpose of this paper is to address the determination of crack location and depth of multiple transverse cracks by monitoring natural frequency and its prediction using Artificial Neural Networks (ANN). An alternative to the existing NDTs is suggested. Design/methodology/approach – Modal analysis is performed to extract the natural frequency. Analysis is performed for two cases of cracks. In first case, both cracks are perpendicular to axis. In second case, both cracks are inclined to vertical plane and also inclined with each other. Finite element method (FEM) is performed using ANSYSTM software which is theoretical basis. Experimentation is performed using Fast Fourier Transform (FFT) analyzer on simply supported stepped rotor shaft and cantilever circular beam with two cracks each. Findings – The results of FEM and experimentation are validated and are in good agreement. The error in crack detection by FEM is in the range of 3-15 percent while 5-20 percent by experimentation. The database obtained by modal analysis is used to train the network of ANN which predicts crack characteristics. Validity of method is investigated by comparing the predictions of ANN with FEM and experimentation. The results are in good agreement with error of 7-16 percent between ANN and FEM while 9-21 percent between ANN and experimental analysis. Originality/value – It envisages that the method is capable. It is an effective as well as an alternate method of fault detection in beam/rotating element to the existing methods.

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CRACK DETECTION USING NATURAL FREQUENCY REDUCTION DUE TO CHANGE IN COMPLIANCE

10.1063/1.3114332 ◽

2009 ◽

Author(s):

M. A. Hussain ◽

M. A. Johnson ◽

Donald O. Thompson ◽

Dale E. Chimenti

Keyword(s):

Natural Frequency ◽

Crack Detection

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Effects of Crack on Vibration Characteristics of Mistuned Rotated Blades

Shock and Vibration ◽

10.1155/2017/1785759 ◽

2017 ◽

Vol 2017 ◽

pp. 1-18 ◽

Cited By ~ 6

Author(s):

Hailong Xu ◽

Zhongsheng Chen ◽

Yongmin Yang ◽

Limin Tao ◽

Xuefeng Chen

Keyword(s):

Natural Frequency ◽

Vibration Amplitude ◽

Crack Detection ◽

Vibration Characteristics ◽

Lumped Parameter Model ◽

Crack Model ◽

Breathing Crack ◽

Vibration Localization ◽

Lumped Parameter ◽

Amplitude Increase

Rotated blades are key mechanical components in turbine and high cycle fatigues often induce blade cracks. Meanwhile, mistuning is inevitable in rotated blades, which often makes it much difficult to detect cracks. In order to solve this problem, it is important and necessary to study effects of crack on vibration characteristics of mistuned rotated blades (MRBs). Firstly, a lumped-parameter model is established based on coupled multiple blades, where mistuned stiffness with normal distribution is introduced. Next, a breathing crack model is adopted and eigenvalue analysis is used in coupled lumped-parameter model. Then, numerical analysis is done and effects of depths and positions of a crack on natural frequency, vibration amplitude, and vibration localization parameters are studied. The results show that a crack causes natural frequency decease and vibration amplitude increase of cracked blade. Bifurcations will occur due to a breathing crack. Furthermore, based on natural frequencies and vibration amplitudes, variational factors are defined to detect a crack in MRBs, which are validated by numerical simulations. Thus, the proposed method provides theoretical guidance for crack detection in MRBs.

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Dynamic Response of Tubular T-Joints Under the Influence of Propagating Cracks

Journal of Offshore Mechanics and Arctic Engineering ◽

10.1115/1.2828804 ◽

1996 ◽

Vol 118 (1) ◽

pp. 71-78 ◽

Cited By ~ 3

Author(s):

D. I. Nwosu ◽

A. S. J. Swamidas ◽

J. Y. Guigne´

Keyword(s):

Natural Frequency ◽

Crack Detection ◽

Crack Depth ◽

Bending Moment ◽

Frequency Change ◽

T Joints ◽

Finite Element Program ◽

Crack Location ◽

Percent Change ◽

Frequency Changes

This paper presents an analytical study on the vibration response of tubular T-joints for detecting the existence of cracks along their intersections. The ABAQUS finite element program was utilized for carrying out the analysis. Frequency response functions were obtained for a joint with and without cracks. The joint was modeled with 8-node degenerate shell elements having 5 degrees of freedom per node. Line spring elements were used to model the crack. The exact crack configuration (semielliptical shape, Fig. 5(b)), as observed from numerous experimental fatigue crack investigations at the critical location, has been achieved through a mapping function, that allows a crack in a planar element to be mapped on to the tube surface. The natural frequency changes with respect to crack depth show little changes, being 4.82 percent for a 83-percent crack depth for the first mode. On the other hand, significant changes have been observed for bending moment and curvature as a function of crack depth. For an 83-percent chord thickness crack, a 97-percent change in bending moment at points around the crack vicinity, and 34.15 to 78 percent change in bending moments, for those locations far away from the crack location, have been observed. Natural frequency change should be combined with other modal parameters such as “bending moment (or bending strain)” and “curvature” changes for crack detection. The presence of the crack can be detected at locations far away from the crack location using such sensors as strain gages.

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704 A Crack Detection Method Based on Natural Frequency Change (2^ Report)

The Proceedings of Ibaraki District Conference ◽

10.1299/jsmeibaraki.2008.167 ◽

2008 ◽

Vol 2008 (0) ◽

pp. 167-168

Author(s):

Tadashi HORIBE ◽

Kuniaki TAKAHASHI ◽

Kiyoshi OHMORI

Keyword(s):

Natural Frequency ◽

Crack Detection ◽

Detection Method ◽

Frequency Change

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Development of a Novel Algorithm for a Crack Detection, Localization, and Sizing in a Beam Based on Forced Response Measurements

Journal of Vibration and Acoustics ◽

10.1115/1.2827357 ◽

2008 ◽

Vol 130 (2) ◽

Cited By ~ 14

Author(s):

M. Karthikeyan ◽

R. Tiwari ◽

S. Talukdar

Keyword(s):

Natural Frequency ◽

Crack Detection ◽

Beam Theory ◽

Crack Size ◽

Forced Response ◽

Crack Model ◽

Beam Model ◽

Crack Location ◽

Forced Responses ◽

Crack Element

The present work aims at the development of a method for the crack detection, localization and sizing in a beam based on the transverse force and response signals. The Timoshenko beam theory is applied for transverse vibrations of the beam model. The finite element method is used for the cracked beam forced vibration analysis. An open transverse surface crack is considered for the crack model, which contains standard five flexibility coefficients. The effect of the proportionate damping is also included. A harmonic force of known amplitude with sine-sweep frequency is used to dynamically excite the beam, up to few flexible modes, which could be provided with the help of an exciter. In practice, linear degrees of freedom (DOFs) can be measured quite accurately; however, rotational DOFs are difficult to measure accurately. All rotational DOFs, except at crack element, are eliminated by a dynamic condensation scheme; for elimination of rotational DOFs at the crack element, a new condensation scheme is implemented. The algorithm is iterative in nature and starts with a presumption that a crack is present in the beam. For an assumed crack location, flexibility coefficients are estimated with the help of forced responses. The Tikhonov regularization technique is applied in the estimation of bounded crack flexibility coefficients. These crack flexibility coefficients are used to obtain the crack size by minimizing an objective function. With the help of the estimated crack size and measured natural frequency, the crack location is updated. The procedure iterates till the crack size and location get stabilized up to the desired level of accuracy. The algorithm has a potential to detect no crack condition also. The crack flexibility and damping coefficients are estimated as a by-product. Numerical examples, with the simply supported and cantilevered beams, are given to justify the applicability and versatility of the algorithm in practice. With the numerically simulated forced responses, which have the noise contamination and the error in the natural frequency measurements, the estimated crack parameters (i.e., the crack location and size) are in good agreement.

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