Fuel assembly vibrations in nuclear reactor cores should not be excessive as these can compromise the lifetime of the assembly and lead to safety hazards. This issue is particularly relevant to new reactor designs that use liquid metal coolants. We therefore demonstrate accurate measurements of the vibrations of a fuel assembly in a lead-bismuth eutectic cooled installation with fibre Bragg grating (FBG) based sensors. The use of FBGs in combination with a dedicated sensor integration approach allows accounting for the severe geometrical constraints and providing for the required minimal intrusiveness of the instrumentation, identifying the vibration modes with required accuracy and observing the differences between the vibration amplitudes of the individual fuel pins as well as evidencing a low frequency fuel pin vibration mode resulting from the supports.
Composite materials are increasingly used in aerospace applications, owing to their high strength-to-mass ratio. Such materials are nevertheless vulnerable to impact damage. It is therefore important to investigate the effects of impacts on composites. Here we embed specialty microstructured optical fiber Bragg grating based sensors inside a carbon fiber reinforced polymer, providing access to the 3D strain evolution within the composite during impact. We measured a maximum strain of -655 με along the direction of impact, and substantially lower values in the two in-plane directions. Such in-situ characterization can trigger insight in the development of impact damage in composites.
Measuring strain at the surface of a structure can help to estimate the dynamical properties of the structure under test. Such a structure can be a fuel assembly of a nuclear reactor consisting of fuel pins. In this paper we demonstrate a method to integrate draw tower gratings (DTGs) in a fuel pin and we subject this pin to conditions close to those encountered in a heavy liquid metal (HLM) reactor. More specifically, we report on the performance of DTGs used as a strain sensor when immersed in HLM during thermal cycles (up to 300_C) for up to 700 hours.
KEYWORDS: Simulation of CCA and DLA aggregates, Signal to noise ratio, Fiber Bragg gratings, Detection and tracking algorithms, Spectral resolution, Sensors, Computer simulations, Algorithm development, Sensing systems, Mechanical sensors
Fiber Bragg grating sensing principle is based on the exact tracking of the peak wavelength location. Several peak detection techniques have already been proposed in literature. Among these, conventional peak detection (CPD) methods such as the maximum detection algorithm (MDA), do not achieve very high precision and accuracy, especially when the Signal to Noise Ratio (SNR) and the wavelength resolution are poor. On the other hand, recently proposed algorithms, like the cross-correlation demodulation algorithm (CCA), are more precise and accurate but require higher computational effort. To overcome these limitations, we developed a novel fast phase correlation algorithm (FPC) which performs as well as the CCA, being at the same time considerably faster. This paper presents the FPC technique and analyzes its performances for different SNR and wavelength resolutions. Using simulations and experiments, we compared the FPC with the MDA and CCA algorithms. The FPC detection capabilities were as precise and accurate as those of the CCA and considerably better than those of the CPD. The FPC computational time was up to 50 times lower than CCA, making the FPC a valid candidate for future implementation in real-time systems.
The interaction of Ultrasound waves with bone material has always been of great interest for the scientific
community. This is due to the fact that ultrasonic waves are non-ionizing, cheap, and easy to generate and
to detect. The use of multi-input interleaved multisine offers new applications for ultrasonic testing in bone
specimens, where identification of material properties by means of ultrasound pulses often suffers from poor S/N
ratio. The research reported here, describes a novel application a of scanning Laser Doppler Vibrometer (LDV)
to the analysis of bone specimens by means of underwater visualization of multisines acoustic fields. The results
demonstrate that this new non-invasive acoustic measurement technique can successfully visualize and measure
reflected acoustic fields, as well as diffraction effects.
Scanning laser Doppler vibrometer measurements are characterized by a high spatial resolution and the fact that the
structure is measured (or scanned) point by point. These measurements can be processed with the exciting generic modal
parameter estimators. However, more accurate modal parameter estimates can be obtained by exploiting the spatial
"smoothness" of high spatial-resolution measurements. To do so, the mode shape will be represented by a generalized
parametric Fourier-based model. In this contribution, this generalized parametric Fourier-based mode-shape smoother
will be integrated inside the modal parameter estimation procedure resulting in a fully (spatial as well as temporal)
parameterized modal model.
With the development of optical measurement techniques it is possible to obtain vast amounts of data. In
vibrometry applications in particular operational deflection shapes are often obtained with very high spatial
resolution. Fortunately, many techniques exist to reduce (approximate) the measurement data. One of the
most common techniques for evaluating optical measurement data is by means of a Fourier analysis. However,
this technique suffers from what is known as leakage when a non-integer number of periods is considered. This
gives rise to non-negligible errors, which will obviously hamper the accuracy of the synthesized shape. Another
technique such as a Discrete Cosine Transform, used in the widely spread -jpeg standard does not suffer these
anomalies but can still prove erroneous at times. One of the more recent approaches is via a so-called Regressive
Discrete Fourier Series (introduced by Arruda) which suffers one disadvantage. The problem statement is non-linear
in the parameters and needs a priori information about the deflection shape. This can be resolved by
using the Optimized Regressive Discrete Fourier Series (ORDFS), introduced in this article, which uses a non-linear
least squares approach. In this article the method will be applied in particular to the reduction of data
for laser vibrometer measurements performed on an Inorganic Phosphate Cement (IPC) beam (1D), as well as
on a car door (2D). The proposed technique will also be validated on simulations to illustrate the properties
concerning compression ration and synthesized mode shape error. The introduced method will be bench marked
for compression ratio and synthesized deflection shape error with all prior mentioned techniques as well as to
the more novel generalized regressive discrete Fourier series (GRDFS).
A non-destructive testing (NDT) technique should be able to calculate the position and the size of any defects, in order to measure quality and safety of materials. Complexity is an issue for most NDT tests, requiring specialist knowledge of the technique. This generally means using qualified staff, and omits the possibility of any kind of automation. In this article an experimental methodology for crack detection using Surface Acoustic Waves (SAWs) and optical laser vibrometer measurements will be introduced. The materials under test are a damaged slat track of an Airbus A320 and a steel beam (because in the zone where critical crack propagation occurs the slat track has a beam-like shape) with slots of known depth (0.2, 0.4, 0.6, 0.8 mm). The goal of this article is to compare these ultrasonic-laser measurements in order to understand how much they are suitable to detect damages and imperfections present on the material itself. For future applications, this methodology can be used to monitor the slat track during a fatigue test, to detect damages in an early stage, before a rapid crack growth.
In the last decade the laser Doppler vibrometer (LDV) has become a widely spread instrument for measuring vibrations. It often offers accurate measurements with a high spatial resolution. However, the measurement time of the LDV and especially for the scanning LDV is long. Therefore reducing the measurement time is an attractive objective. A way to achieve this is to use a single sine excitation (on a resonance frequency). However this technique has two major drawbacks: the inability to provide accurate information on the damping and an operational deflection shape that can differ from the true mode shape. In this article a method will be introduced to reduce measurement time for scanning LDV measurements without these defaults. This is done by using a narrow band multisine excitation signal. Now for uncoupled normal modes the obtained time domain sequences for each scan point are in direct linear relation with each other. Therefore it is possible to estimate the full time domain sequence from the current scan point by using the previous scan point and a limited number of time samples from the current scan point, hence reducing the measurement time. This method is a key benefit for in-line quality control, which can have upwards of 1000 spatial measurement locations. The proposed technique will be validated on both simulations and experiments of varying complexity.
In this article we will present a method to estimate sound absorption coefficients from measurements of the incident and reflected sound fields near the material under test. The sound fields are visualized using a scanning laser Doppler vibrometer (SLDV). By aiming the SLDV at a rigid (non-vibrating) object and letting the beam pass through a sound field the spatial pressure distribution can be made visible. By visualizing both incident and reflected sound field with respect to a material sample in this manner, the acoustic absorption coefficients can be determined (this is the ratio of the absorbed energy and incident energy). Two alternative set-ups are proposed in this article: a one-dimensional set-up where the sound field (i.e. the standing waves) inside a thin glass tube is measured, and a two-dimensional set-up where the propagating and reflecting sound field between two parallel glass plates is visualized. While the former can only be used to obtain normal incidence absorption coefficients the latter can also be used to estimate oblique incidence absorption values. It is shown that the method is accurate at high frequencies where traditional standardized acoustic material characterization techniques mostly fail.
With the development of optical measurement techniques it was possible to obtain vast amounts of data. In vibrometry applications in particular where FRF-matrices with tens of thousands of rows and an equal number of rows are stored, data reduction has become a point of interest. It has long been known that it is possible to reduce (approximate) the measurement data (e.g. mode shapes) by means of a Fourier decomposition. One of the most common techniques for evaluating optical measurement data is by means of a Fourier analysis. It is well known that for periodic and band-limited sequences the Discrete Fourier Transform (DFT) returns the true Fourier coefficients when exactly 1 period (or a multiple) is processed. Leakage will occur when less than 1 period is considered. This gives rise to non-negligible errors, which can be resolved by using a Regressive Discrete Fourier Transform (RDFT), introduced in this article. The measured signal is represented by a model using sines and cosines. The coefficients of those sines and cosines are then estimated on a global scale by means of a frequency domain system identification technique. By making use of the regressive technique proposed in this paper, it is possible to reduce the data in comparison to the classical Fourier decomposition even further by a sizeable factor. In this article the introduced method will be applied in particular to the reduction of data for (1D) laser vibrometer measurements performed on a composite (IPC) beam, as well as on an aluminum plate (2D). The proposed technique will also be validated on both 1D and 2D simulations of varying complexity.
Currently, the scanning laser Doppler vibrometer (SLDV) has become a standard laboratory tool for high spatial resolution vibration measurements. Though the quality of the measurement result is very high in well-controlled experiments, there are still some limitations on the application in harsh industrial environments. Firstly, the user interaction of available SLDV systems is too high, requiring trained personnel. But more important, laser drop-out can cause outliers in the measurements when non-treated (and often dark) surfaces are measured. These limitations prevent the real breakthrough of the SLDV in industry, notwithstanding the fact that a high potential exists for the use of the SLDV for quality control. In this article, a procedure will be developed to make the SLDV measurement process fully automatic and more robust. Firstly, an automatic laser calibration and component recognition method -- that is based on newly developed image processing algorithms -- is described. Secondly, a robust singular value decomposition (SVD) is introduced to eliminate laser drop-out from the measurements. The merits of the proposed methodology (measurement automation and robustification) for quality control purposes will be validated on SLDV measurements of a mask of a picture tube.
Optical measurement techniques are being used more and more for quality control in the production process. An important problem when trying to implement optical measurement systems, and in particular the scanning laser Doppler vibrometer (SLDV) which is considered in this paper, is that user interaction is always required. The need for user interaction implies an increase in testing time and cost and also brings an additional source of variability of the test results. In this article, a completely autonomous test procedure is developed to track the vibration behavior of a product during the production process (this includes automatic laser focusing, position calibration, object recognition, grid generation and mode estimation). The proposed procedure typically requires a few minutes only to automatically determine a high resolution model of the vibration behavior. The method will be validated on measurements of an electronic circuit board.
Because a scanning laser vibrometer can perform vibration measurements with a high spatial resolution it is an ideal instrument to accurately locate damage in a structure. In literature different authors successfully used linear vibration measurements to detect faults with the aid of a scanning vibrometer. In this article we will study the feasibility of using nonlinear vibration features to detect and locate damage in a structure. The nonlinearities will be estimated using both classical and a newly developed technique. A comparison of linear and nonlinear damage detection features will be performed on measurements of a cracked plexiglass panel and a delaminated composite plate.
In order to optimize the vibro-acoustic behavior of panel-like structures in a more systematic way, accurate structural models are needed. However, at the frequencies of relevance to the vibro-acoustic problem, the mode shapes are very complex, requiring a high spatial resolution in the measurement procedure. The large number of required transducers and their mass loading effects limit the applicability of accelerometer testing. In recent years, optical measurement methods have been proposed. Direct electronic (ESPI) imaging, using strobed laser illumination, or more recently, pulsed laser illumination, have lately created the possibility to bring the holographic testing approach to the level of industrial applicability for modal analysis procedures. Therefore an automated ESPI system has been developed for the measurement of frequency response functions using stepped sine testing. A conventional numerical modal analysis procedure is used to obtain the modal information. The present paper discusses the various critical elements of a holographic ESPI modal testing system. Next to the optical parts, the integration with the modal analysis procedures, including the integration of geometry and response measurement, are discussed. The paper furthermore discusses test results obtained on a car panel in a vibro-acoustic setup. The results show, that the quality of the frequency response functions is very good, when compared with acceleration sensor measurements. The measurement data are used to predict interior noise.
Information on the measurement noise can be extracted from repeated measurements or from response measurements without exciting the structure. This process, however, slows down the measurement procedure and therefore few authors report about the use of these approaches in modal analysis applications. Since the availability of a quality measure of both measurements and estimated modal parameters yields promising results in many steps of the modal analysis procedure, we propose in this article a noise extraction procedure for high resolution optical measurements that does not require extra measurement time. The measurement noise variances are estimated from the redundancy in the high spatial resolution images. Furthermore the paper discusses the use of spline regression as a data reduction technique. The proposed technique is validated on simulated data as well as on measured holographical images.
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