The passively reconstructed Impulse Response Function (IRF) (or Green’s function) between two points of a medium can return a wealth of information about the dynamic behavior of the system including the existence of defects. For a linear system subjected to an unknown excitation source, the deconvolution operation between two monitored points can properly reconstruct the IRF between the two points, effectively using one of the points as a “virtual” source. This technique has been successfully used for studying the global dynamic behavior of structural systems, such as buildings and bridges, in the low frequency range. In civil engineering this approach is often referred to as Seismic Interferometry (SI). Various studies using SI have discussed the high frequency regime of transient wave propagation. This paper will focus on this high frequency regime and propose some novel implementations of SI to detect local structural damage in multistory buildings. Results will be presented from an aluminum rod and a laboratory-scale multistory aluminum frame.
This work explores the opportunity for utilizing ultrasonic arrays in a passive manner to increase imaging speed and accuracy, compared to conventional ultrasonic beamforming approaches. While conventional beamforming requires the use of many transmitters as well as receivers, the possibility exists to minimize the number of transmitters and exploit the normalized cross-power spectrum operator to extract "virtual" Impulse Response Functions between pairs of (only) receivers. In principle, this possibility would allow, for example, the use of a single transmitter element and processing the remaining elements as receivers (passively) to obtain similar image quality as using each element as both transmitter and receiver (as done, for example, in a Full Matrix Capture mode). By minimizing the number of transmitters, it would be possible to significantly increase imaging speed (ultrafast imaging) and simplify multiplexer hardware (few high-voltage output channels). The normalized cross-power spectrum could eliminate the distortion in the phase of the Impulse Response Functions which is commonly seen in active transmission modalities. These exciting possibilities are explored in the case of imaging defects in structural components.
This keynote presentation will cover passive methods for ultrasonic NDT and SHM. These methods apply to cases where the test structure is subjected to uncontrolled excitations (such as from ambient loads) or to a limited number of controlled excitations. In these cases, proper analysis of the reception signals can reveal surprisingly accurate information from array analysis, such as imaging of internal defects. The effort here is to obtain as much information as possible from the receiver signals while exploiting at a minimum the excitation signals in the beamforming process. Examples will be shown for NDT imaging of holes in an aluminum block via plane-wave beamforming and SHM of a composite wind turbine blade via Matched-Field Processing imaging.
A non-contact (vision-based) output-only technique is explored for monitoring and health assessment of civil structures such as multistory buildings and bridges. A consumer-grade camera is used to record the vibrations under natural ambient excitation. Then, the Kanade-Lucas-Tomasi (KLT) algorithm is utilized to track the displacements of desired target points (i.e., the points representing the location of virtual receivers located along the height of the building) and extract the pixeldomain time histories. In the next step, the extracted time history responses are deconvolved between pairs of target points to extract the Impulse Response Functions (IRFs) of each pair. This process, often known as seismic interferometry, extracts the causal and acausal wave propagation behavior between two points of the structure. Depending on the frequency content, this behavior is of interest to reveal structural damage along wave propagation paths. The technique is conceptualized by a numerical example administrated in ABAQUS. It is followed by a preliminary test conducted on a small-scale multistory frame structure subjected to a random shaker excitation, which is monitored by a cellphone’s camera. The main current challenge of this technique is the low frequency of the image recording from the camera. The next phase of this work will utilize a high-speed camera to extract IRFs at frequencies high enough for localized damage detection.
This paper introduces an enhanced ultrasonic sonar-based ranging technique developed by the Experimental Mechanics, NDE & SHM Laboratory at UC San Diego to estimate the deflections of railroad ties. The deflection profile of the ties can be subsequently inspected to evaluate their performance and prevent unwanted events, like train derailments. The proposed sensing layout is comprised of an array of air-coupled capacitive transducers to perform pulse-echo ultrasonic tests in multiple points along the tie, and a high frame-rate camera to capture images of the objects probed by the array. A machine learning-based image processing technique is developed to classify the tie/ballast images based on the texture signature of the visible objects in the camera’s field of view. Next, the relative deflection profile of the ties is reconstructed by tracking the Time of Flight (ToF) of the received waveforms at the points flagged as a tie. A series of field tests was carried out at the Rail Defect Testing Facility of UC San Diego as well as a BNSF yard in San Diego, CA, by mounting the sensing prototype on a car moving at walking speed. The obtained results confirm the potential of the proposed airborne ranging technique for in-motion measurement of the deflections of railroad ties.
Matched Field Processing (MFP) is a generalized beamforming method which matches the received data to a dictionary of replica vectors to localize wave scattering sources (e.g., acoustic sources) in the complex media. The approach has also been used for passive structural monitoring and defect detection. The MFP requires an accurate model of medium, and this is a challenge in some applications. To tackle this issue, data-driven MFP has been recently introduced. Data-driven approaches are considered as model-free methods, which perform with no prior knowledge of the propagation environment to localize a source. This paper introduces a data-driven MFP approach for localizing the primary (i.e., impact) and secondary (i.e., defect) sources in plates. The replica vectors are made using the Fast Fourier Transform of the time history responses of the pristine plate under a controlled external excitation. Then, the MFP is implemented to localize the source. For defect localization, a subtraction approach under Born approximation is employed to remove or weaken the signature of the primary source and extract a set of data which purely contains the acoustic signature of the defect. The performance of the method for primary and secondary source localization is evaluated by studying a small aluminum plate, excited by a controlled broadband noise imposed by an impact hammer. A comparative study is carried out to evaluate the performance of the conventional Bartlett and adaptive White Noise Constraint processors in forming the ambiguity surfaces.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.