Full-field Mode Research Articles

Reference-free detection of minute, non-visible, damage using full-field, high-resolution mode shapes output-only identified from digital videos of structures

Detecting damage in structures based on the change in their dynamics or modal parameters (modal frequencies and mode shapes) has been extensively studied for three decades. The success of such a global, passive, vibration-based method in field applications, however, has long been hindered by the bottleneck of low spatial resolution vibration sensor measurements. The primary reason is that damage typically initiates and develops in local regions that need to be captured and characterized by very high spatial resolution vibration measurements and modal parameters (mode shapes), which are extremely difficult to obtain using traditional vibration measurement techniques. For example, accelerometers and strain-gauge sensors are typically placed at a limited number of discrete locations, providing low spatial resolution vibration measurements. Laser vibrometers provide high-resolution measurements, but are expensive and make sequential measurements that are time- and labor-consuming. Recently, digital video cameras—which are relatively low cost, agile, and able to provide high spatial resolution, simultaneous, pixel measurements—have emerged as a promising tool to achieve full-field, high spatial resolution vibration measurements. Combined with advanced vision processing and unsupervised machine algorithms, a new method has recently been developed to blindly and efficiently extract the full-field, high-resolution, dynamic parameters from the video measurements of an operating, output-only structure. This work studies the feasibility of performing damage detection using the full-field, very high spatial resolution mode shape (of the fundamental mode) blindly extracted from the video of the operating (output-only) structure without any knowledge of reference (healthy) structural information. A spatial fractal dimension analysis is applied on the full-field mode shape of the damaged structure to detect damage-induced irregularity. Additionally, the equivalence between the fractal dimension and the squared curvature (modal strain energy) of the mode shape curve, when of high spatial resolution, is mathematically derived. Laboratory experiments are conducted on bench-scale structures, including a building structure and a cantilever beam, to validate the approach. The results illustrate that using the full-field, very high-resolution mode shape enables detection of minute, non-visible, damage in a global, completely passive sensing manner, which was previously not possible to achieve.

Blind identification of full-field vibration modes from video measurements with phase-based video motion magnification

Experimental or operational modal analysis traditionally requires physically-attached wired or wireless sensors for vibration measurement of structures. This instrumentation can result in mass-loading on lightweight structures, and is costly and time-consuming to install and maintain on large civil structures, especially for long-term applications (e.g., structural health monitoring) that require significant maintenance for cabling (wired sensors) or periodic replacement of the energy supply (wireless sensors). Moreover, these sensors are typically placed at a limited number of discrete locations, providing low spatial sensing resolution that is hardly sufficient for modal-based damage localization, or model correlation and updating for larger-scale structures. Non-contact measurement methods such as scanning laser vibrometers provide high-resolution sensing capacity without the mass-loading effect; however, they make sequential measurements that require considerable acquisition time. As an alternative non-contact method, digital video cameras are relatively low-cost, agile, and provide high spatial resolution, simultaneous, measurements. Combined with vision based algorithms (e.g., image correlation, optical flow), video camera based measurements have been successfully used for vibration measurements and subsequent modal analysis, based on techniques such as the digital image correlation (DIC) and the point-tracking. However, they typically require speckle pattern or high-contrast markers to be placed on the surface of structures, which poses challenges when the measurement area is large or inaccessible. This work explores advanced computer vision and video processing algorithms to develop a novel video measurement and vision-based operational (output-only) modal analysis method that alleviate the need of structural surface preparation associated with existing vision-based methods and can be implemented in a relatively efficient and autonomous manner with little user supervision and calibration. First a multi-scale image processing method is applied on the frames of the video of a vibrating structure to extract the local pixel phases that encode local structural vibration, establishing a full-field spatiotemporal motion matrix. Then a high-spatial dimensional, yet low-modal-dimensional, over-complete model is used to represent the extracted full-field motion matrix using modal superposition, which is physically connected and manipulated by a family of unsupervised learning models and techniques, respectively. Thus, the proposed method is able to blindly extract modal frequencies, damping ratios, and full-field (as many points as the pixel number of the video frame) mode shapes from line of sight video measurements of the structure. The method is validated by laboratory experiments on a bench-scale building structure and a cantilever beam. Its ability for output (video measurements)-only identification and visualization of the weakly-excited mode is demonstrated and several issues with its implementation are discussed.

TheXMM-Newtonserendipitous survey

Thanks to the large collecting area (3 x ~1500 cm$^2$ at 1.5 keV) and wide field of view (30' across in full field mode) of the X-ray cameras on board the European Space Agency X-ray observatory XMM-Newton, each individual pointing can result in the detection of hundreds of X-ray sources, most of which are newly discovered. Recently, many improvements in the XMM-Newton data reduction algorithms have been made. These include enhanced source characterisation and reduced spurious source detections, refined astrometric precision, greater net sensitivity and the extraction of spectra and time series for fainter sources, with better signal-to-noise. Further, almost 50\% more observations are in the public domain compared to 2XMMi-DR3, allowing the XMM-Newton Survey Science Centre (XMM-SSC) to produce a much larger and better quality X-ray source catalogue. The XMM-SSC has developed a pipeline to reduce the XMM-Newton data automatically and using improved calibration a new catalogue version has been produced from XMM-Newton data made public by 2013 Dec. 31 (13 years of data). Manual screening ensures the highest data quality. This catalogue is known as 3XMM. In the latest release, 3XMM-DR5, there are 565962 X-ray detections comprising 396910 unique X-ray sources. For the 133000 brightest sources, spectra and lightcurves are provided. For all detections, the positions on the sky, a measure of the quality of the detection, and an evaluation of the X-ray variability is provided, along with the fluxes and count rates in 7 X-ray energy bands, the total 0.2-12 keV band counts, and four hardness ratios. To identify the detections, a cross correlation with 228 catalogues is also provided for each X-ray detection. 3XMM-DR5 is the largest X-ray source catalogue ever produced. Thanks to the large array of data products, it is an excellent resource in which to find new and extreme objects.

Full-Field Calcium K-Edge X-ray Absorption Near-Edge Structure Spectroscopy on Cortical Bone at the Micron-Scale: Polarization Effects Reveal Mineral Orientation.

Here, we show results on X-ray absorption near edge structure spectroscopy in both transmission and X-ray fluorescence full-field mode (FF-XANES) at the calcium K-edge on human bone tissue in healthy and diseased conditions and for different tissue maturation stages. We observe that the dominating spectral differences originating from different tissue regions, which are well pronounced in the white line and postedge structures are associated with polarization effects. These polarization effects dominate the spectral variance and must be well understood and modeled before analyzing the very subtle spectral variations related to the bone tissue variations itself. However, these modulations in the fine structure of the spectra can potentially be of high interest to quantify orientations of the apatite crystals in highly structured tissue matrices such as bone. Due to the extremely short wavelengths of X-rays, FF-XANES overcomes the limited spatial resolution of other optical and spectroscopic techniques exploiting visible light. Since the field of view in FF-XANES is rather large the acquisition times for analyzing the same region are short compared to, for example, X-ray diffraction techniques. Our results on the angular absorption dependence were verified by both site-matched polarized Raman spectroscopy, which has been shown to be sensitive to the orientation of bone building blocks and by mathematical simulations of the angular absorbance dependence. As an outlook we further demonstrate the polarization based assessment of calcium-containing crystal orientation and specification of calcium in a beta-tricalcium phosphate (β-Ca3(PO4)2 scaffold implanted into ovine bone. Regarding the use of XANES to assess chemical properties of Ca in human bone tissue our data suggest that neither the anatomical site (tibia vs jaw) nor pathology (healthy vs necrotic jaw bone tissue) affected the averaged spectral shape of the XANES spectra.

Hollow cone illumination for fast TEM, and outrunning damage with electrons

We consider the possibility of imaging individual bioparticles using snapshot diffraction from femotsecond pulses, using a 3 MeV electron beam, based on the recent experimental performance of these coherent beams. Assuming that radiation damage can be outrun using 100 fs pulses (or less), we find that a sufficient number of electrons are scattered per particle only if the beam diameter can be matched to that of the particle (e.g. a virus), about three orders of magnitude smaller than has currently been demonstrated (and limited by space-charge effects). We then propose the use of the hollow-cone illumination mode for fast transmission electron microscope imaging, because it can provide full-field atomic resolution imaging despite the use of the large incoherent annular source required for an efficient photocathode, so that coherent illumination is not needed for high-resolution imaging. Reciprocity arguments are used to compare this full-field mode with data aquisition times and source brightness in scanning transmission electron microscopy.

Dual scanning and full-field hard x-ray microscopy with a laboratory source.

We report on the experimental demonstration of a hard x-ray microscopy scheme achieving absorption and phase contrast imaging with a standard laboratory source. The x-ray optical system features two crossed planar waveguides coupled to the primary source. The dual waveguide acts as a secondary micron-sized source, enabling high imaging resolution. Both scanning and full-field imaging modes are demonstrated with the same experimental system, with a resolution of about 2 μm in scanning mode. Examples of absorption, differential phase and retrieved phase depth of thin metal grids and glass micro-spheres are reported as proof of concept of the technique.

Dynamic Sensing Performance of a Point-Wise Fiber Bragg Grating Displacement Measurement System Integrated in an Active Structural Control System

In this work, a fiber Bragg grating (FBG) sensing system which can measure the transient response of out-of-plane point-wise displacement responses is set up on a smart cantilever beam and the feasibility of its use as a feedback sensor in an active structural control system is studied experimentally. An FBG filter is employed in the proposed fiber sensing system to dynamically demodulate the responses obtained by the FBG displacement sensor with high sensitivity. For comparison, a laser Doppler vibrometer (LDV) is utilized simultaneously to verify displacement detection ability of the FBG sensing system. An optical full-field measurement technique called amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) is used to provide full-field vibration mode shapes and resonant frequencies. To verify the dynamic demodulation performance of the FBG filter, a traditional FBG strain sensor calibrated with a strain gauge is first employed to measure the dynamic strain of impact-induced vibrations. Then, system identification of the smart cantilever beam is performed by FBG strain and displacement sensors. Finally, by employing a velocity feedback control algorithm, the feasibility of integrating the proposed FBG displacement sensing system in a collocated feedback system is investigated and excellent dynamic feedback performance is demonstrated. In conclusion, our experiments show that the FBG sensor is capable of performing dynamic displacement feedback and/or strain measurements with high sensitivity and resolution.

A Point-Wise Fiber Bragg Grating Sensor to Measure the Vibration of a Cantilever Plate Subjected to Impact Loadings

In this paper, an out-of-plane point-wise fiber Bragg grating (FBG) displacement sensing system is proposed and the application of the FBG sensor is extended for modal analysis of a cantilever plate with highly accurate dynamic measurement results. From the proposed FBG sensing system, the relationships between mode shapes and transient displacement responses of the cantilever plate subjected to impact loadings are discussed in detail. By comparing with a laser Doppler vibrometer (LDV) simultaneously, the proposed FBG displacement sensing system demonstrates extremely high sensitivity and resolution. A long-period fiber grating (LPFG) filter-based wavelength-optical intensity modulation technique is employed to demodulate the responses of the FBG. By employing the fast Fourier transform on the measured transient displacement responses, the resonant frequencies of the cantilever plate are determined. An optical full-field measurement technique called amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) and FEM numerical calculations are also used to provide full-field vibration mode shapes and resonant frequencies of the cantilever plate. Finally, the amplitude of the frequency spectrums obtained from FBG and LDV are emphasized from the mode shapes and impact locations. The experimental results show that the proposed FBG displacement sensor is able to measure transient out-of-plane point-wise displacement responses and can also be used to determine the resonant frequencies of the cantilever plate.

Finite element model updating from full-field vibration measurement using digital image correlation

The comparison of structural responses, e.g. natural frequencies and mode shapes, between predictions and measurements is an important step in finite element (FE) model updating. Full-field measurement techniques such as high speed cameras with digital image correlation (DIC) algorithms provide detailed, global displacement data. It is necessary to compress huge amounts of full-field data before implementing the comparison procedures. Image processing and pattern recognition techniques offer effective ways of doing this. Image decomposition using integral transformation is one of the most common procedures. It is found that appropriate selection or construction of the transformation kernels usually generates succinct and effective shape feature terms. Thus, the discrepancies between the geometric mode shapes may be assessed by using distance measures between the shape feature vectors. In the present study, vibration mode shapes of a composite panel are measured by a DIC system and predicted by a FE model. Succinct and effective shape features of the full-field mode shapes were obtained by employing the Tchebichef moment descriptor. Mode shape discrepancies are clearly indicated by the resultant Tchebichef features. The FE model was then modified and updated. Results show that including only the shape features results in a better updated model than when natural frequencies only are used. The most improved model was obtained when both natural frequencies and shape features are included in the updating routine.

Investigation of Dynamic Responses for Piezoceramic Plates in Resonance by Using Fiber Bragg Grating

ABSTRACTA fiber Bragg grating (FBG) sensor system which can simultaneously measure the point-wise, out-of-plane and in-plane dynamic displacements is proposed. A demodulation system based on the fiber Bragg grating filter is used. The steady-state responses of particle motions of a piezoceramic plate measured by the FBG out-of-plane and in-plane displacement sensors are simultaneously compared with those obtained by a laser Doppler vibrometer (LDV) and a surface-mounted FBG strain sensor, respectively. The integration of the FBG displacement sensor with a dynamic signal analyzer (FBG-DSA) forms a measurement system which has the ability to acquire the frequency response of a piezoceramic plate. An LDV-DSA system and an impedance analyzer are used to compare the results obtained from the dynamic signal analyzer combined with the out-of-plane FBG sensor (OFBG-DSA) and the in-plane FBG sensor (IFBG-DSA), respectively. The experimental results of the impulse excitation as well as the random excitation of the piezoceramic plate are also presented. To explain the experimental results, an optical full-field measurement technique called amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) and FEM numerical calculations are also used to provide full-field vibration mode shapes of the piezoceramic plate. These results indicate that the proposed displacement sensor system has the multiplexing capability to measure the dynamic displacements up to 45kHz.

Advances in X-ray microscopy at FLASH using transmissive optics

In order to successfully plan and perform experiments at high flux EUV- and X-ray sources, e.g. free electron lasers, it is necessary to know which kind of optics, exposed to the full beam, can be used. Due to the high intensities, it is not clear, whether transmissive diffractive optics are applicable, because these optics are usually fabricated on thin membranes which are therefore subject to additional absorption in the desired energy range. Since diffractive optics, especially zone plates, offer the possibility to achieve small spots when used as a focussing element and can also achieve good image quality in microscopic setups, their usage would facilitate many experiments, especially for their easy handling. As a proof of concept, we set up a zone plate based scanning transmission microscope at the unfocussed beamline BL3 at FLASH (DESY/Hamburg). The operating wavelength was 32 nm and 13.8 nm, respectively. While the first attempt, utilizing a zone plate composed of PMMA on silicon substrate failed due to ablation of the PMMA, a second zone plate (chromium on silicon nitride) was successfully used to focus the beam onto different samples (e.g. nickel-mesh, a silicon nitride structured sample and nickel structures on silicon nitride). The resulting focal spot size was estimated from the acquired images to be in the range of 1 μ–3 μ in diameter. Beside the scanning approach, a full-field transmission microscopy setup was tested. A preliminary image in full field mode was taken with a second zone plate, showing features of a Siemens-star pattern down to approximately 120 nm. After several hours of exposure, no damage was visible to the optics. Beside the optics, different filters (Silicon/Zirconium, Zirconium and Aluminum) have been placed in the beam to evaluate possibilities to further reduce intensity which may be necessary if sensitive detectors are involved. All of the filters withstood the irradiation during the whole experiment.

Multiple interference and spatial frequencies' effect on the application of frequency-domain optical coherence tomography to thin films' metrology

Frequency-domain optical coherence tomography (FD-OCT) was proved to be very useful under two conditions: (i) the layered sample scatters so that multiple interferences do not exist and (ii) the interferometer uses collimated beams so that a single spatial frequency is used. We examine the use of FD-OCT for thin films' metrology when multiple interferences and a range of spatial frequencies exist, such as with interference microscopy where the spatial frequencies' content is provided by the numerical aperture (NA) of the microscope objectives. The effect of multiple interferences is found to be reduced significantly by shortening the coherence length such that it is less than the thinnest layer, and the smaller the number of layers, the higher the accuracy. For the spatial frequency effect, it is found that using microscope objectives with NA up to 0.3, the accuracy is still high with the advantage of providing full field operation and high lateral resolution; however, a correction factor has to be introduced for the determination of the thicknesses. An inverse scattering approach is proposed to improve the accuracy and allows the measurement of both thicknesses and dispersion laws and allows better accuracy than polarized reflectometry, in particular in the full field mode using annular lenses. Some experimental results are presented supporting the theoretical predictions.

Resonant vibration of piezoceramic plates in fluid

In this paper, both experimental measurement and theoretical analysis are used to investigate the out-of-plane resonant characteristics of a cantilevered piezoceramic plate in air and three different kinds of fluid. The experimental method, amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI), is the major technique used in this study to measure the resonant characteristics of the cantilivered piezoceramic plate. Both resonant frequencies and full-field mode shapes are obtained from this experimental technique. Numerical computations based on the finite element analysis are presented for comparison with the experimental results. Good quality of mode shapes for the cantilevered piezoceramic plate in air is obtained from the AF-ESPI technique. However, the quality decreases as the viscosity of fluids increases. From the results provided from experimental measurements and numerical computations, it is indicated that the resonant frequencies of the cantilevered piezoceramic plate in fluid decrease with the increase of the viscosity of fluids. Good agreements between the experimental measured data and the numerical calculated results are found for both resonant frequencies and mode shapes of the cantilevered piezoceramic plate in fluid.

Full-Field Uncertainty Determination for Pressure-Sensitive Paint Measurements

T HE principle behind pressure-sensitive paint (PSP) measurements is by nowwell known: a luminescent coating is applied to a test article, the test article is illuminated with light at a short wavelength (for example, blue), and emission from the paint (for example, red) is imaged by one or more digital cameras. Oxygen quenching of the luminescent signal produces the desired pressure sensitivity [1,2]. Pressure distributions are typically computed from the ratio of images based on a suitable calibration equation. In intensity-based measurements, these are images at a wind-off condition and images at the run condition. In lifetime-based measurements, both images are acquired at the run condition by choosing two different temporal gates with respect to a train of excitation pulses. Three-gate measurements are also being developed [3]. Many studies have been performed to assess the error sources in PSP measurements [4–11]. However, assessment of the measurement uncertainty in practice is typically limited to a comparison of PSP data in the vicinity of pressure taps with readings from these taps. The purpose of this paper is to show that two additional measures of uncertainty can easily be implemented. One is based on pixel-topixel variations within an image from a single camera, the other on camera-to-camera variations for points on the test article that are viewed by two or more cameras. In principle, these additional measures of uncertainty can be computed for every point on the surface of the test article. Both produce an estimate of the precision of the PSPmeasurement andmay be thought of as adding “error bars” to the processed data in full-field mode.

The influence of electrode designs on the resonant vibrations for square piezoceramic plates

In this paper, the resonant vibrations of square piezoceramic plates with four different electrode designs are investigated. Two experimental techniques, the amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) and the impedance analysis, are used to access the influence of the electrode arrangement on the resonant characteristics of square piezoceramic plates. Both the out-of-plane and in-plane resonant frequencies and full-field mode shapes of piezoceramic plates with various electrode designs are obtained from the AF-ESPI method. The impedance analyzer is used to measure the resonant and antiresonant frequencies of piezoceramic plates. The dynamic electromechanical coupling coefficient (EMCC), which relates to the ability of conversion between mechanical and electrical energy, is determined from the measured values of resonant and antiresonant frequencies. Experimental results of the resonant vibration characteristics of the square piezoceramic plates are verified by numerical computations based on the finite-element method. Excellent agreement between the experimental and numerical results is found in resonant frequencies and corresponding mode shapes. It is found that the electrode design has important influence on the resonant characteristics of piezoceramic plates. The effect of different designs of electrode is more significant in the in-plane modes than that in the out-of-plane modes.

Tactile letter recognition under different modes of stimulus presentation

Block capital letters were displayed to experienced and inexperienced Ss, using a 20 x 20 matrix of vibratory tactors placed against the back. In two separate experiments, a total of five modes of stimulus presentation, three of them employing a linear scanning slit, were studied. The poorest method, stationary flashing of the letter, allows performance that is well above chance, implying that a purely spatial presentation does convey information. Performance is improved when the letter is moved horizontally across the display. The best performance is achieved when the amount of simultaneous stimulation is limited by using a linear scanning slit. In one method, the letter moves behind a stationary slit, with the result that its horizontal dimension is portrayed only in time. In the other two methods, the scanning slit moves across the stationary letter, portraying the letter both in time and in space. The results of all five display modes indicate that Ss can use whichever representation, spatial or temporal, is available, although patternings which most closely approximate sequential tracing by a single moving point lead to the highest recognition accuracy. We interpret these results in terms of the limited spatial resolution of the cutaneous sense. While the perception of a letter presented in either full-field condition is limited by the spatial resolution, the best measure being the two-point limen, the perception of a letter traced sequentially is limited by the localization acuity of the cutaneous sense, the best measure being the “error of localization,” which is known to be considerably smaller than the two-point limen. Inasmuch as the slit methods of presentation are a compromise between simultaneous and sequential display, letter-recognition accuracy is better with slit presentation than with the corresponding full-field mode of display.