Detection and Characterization of Disbond Damage at Steel-Concrete Interfaces Using Attenuation Characteristics of Guided Waves

This paper presents a frequency-wavenumber (f-k) domain signal processing approach for guided plate wave data. The guided waves propagate in a steel plate bonded to concrete substrate, and the analysis aims to characterize the steel-concrete interface. The modal solutions of guided waves in well-bonded, partially bonded, and dis-bonded interface cases are obtained using the Global Matrix technique. The analytical solutions demonstrate that the attenuation characteristics of the fundamental symmetric (S0) guided wave mode in the steel plate are sensitive to the steel-concrete interface bond condition. The attenuation behavior of the S0 mode are captured and extracted from the complete guided wave signal set obtained by air-coupled ultrasonic tests. Using f-k domain signal analysis, the fundamental anti-symmetric (A0) mode is suppressed and the S0 mode is isolated. The S0 mode attenuation across the scanned spatial points is then estimated and used to characterize the bond condition of the steel-concrete interface. This signal processing approach is verified by a series of numerical simulation and laboratory-scale experiments. The results demonstrate that interface bond condition can be successfully characterized using the proposed f-k domain signal analysis approach

Numerical analysis of ultrasonic multiple scattering for fine dust number density estimation

In this study, a method is presented for estimating the number density of fine dust particles (the number of particles per unit area) through numerical simulations of multiply scattered ultrasonic wavefields. The theoretical background of the multiple scattering of ultrasonic waves under different regimes is introduced. A series of numerical simulations were performed to generate multiply scattered ultrasonic wavefield data. The generated datasets are subsequently processed using an ultrasound data processing approach to estimate the number density of fine dust particles in the air based on the independent scattering approximation theory. The data processing results demonstrate that the proposed approach can estimate the number density of fine dust particles with an average error of 43.4% in the frequency band 1-10 MHz (wavenumber × particle radius ≤ 1) at a particle volume fraction of 1%. Several other factors that affect the accuracy of the number density estimation are also presented

Experimental analysis of ultrasonic multiple scattering attenuation through the air with fine dust

In this study, we experimentally evaluated the application of multiple scattering theory for measuring ultrasonic attenuation. Based on the independent approximation theory, the method adopted for calculating the attenuation of coherent waves through air with fine dust is discussed. To obtain a scattering wavefield, a unique ultrasonic scattering hardware was developed, and signal processing schemes were suggested. Four cases of standard particle doses (0, 0.004, 0.008, and 0.012 g) were investigated inside a chamber. The results obtained from the experiments demonstrate that the proposed signal processing approach successfully calculates the scattering attenuation, which correlates well with the applied doses of fine dust. In addition, we discuss the irregular shape and composition of fine dust relative to the scattering cross-section

Concrete delamination depth estimation using a noncontact mems ultrasonic sensor array and an optimization approach

In this study, we present a method to estimate the depth of near-surface shallow delamination in concrete using a noncontact micro-electromechanical system (MEMS) ultrasonic sensor array and an optimization-based data processing approach. The proposed approach updates the bulk wave velocities of the tested concrete element by solving an optimization problem using reference ultrasonic scanning data collected from a full-depth concrete region. Subsequently, the depth of concrete delamination is estimated by solving a separate optimization problem. Numerical simulations and laboratory experiments were conducted to evaluate the performance of the proposed ultrasonic data processing approach. The results demonstrated that the depth of shallow delamination in concrete structures could be accurately estimated

The Greenland Telescope: Construction, Commissioning, and Operations in Pituffik

In 2018, the Greenland Telescope (GLT) started scientific observation in Greenland. Since then, we have completed several significant improvements and added new capabilities to the telescope system. This paper presents a full review of the GLT system, a summary of our observation activities since 2018, the lessons learned from the operations in the Arctic regions, and the prospect of the telescope.Comment: 26 pages, 11 figures, and 8 tables. This is the version of the article before publication editing, as submitted by an author to Publications of the Astronomical Society of the Pacific. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record will be added when it becomes availabl

A ring-like accretion structure in M87 connecting its black hole and jet

The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation^{1,2}. Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole^3. Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of 8.4_{-1.1}^{+0.5} Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects in addition to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow.Comment: 50 pages, 18 figures, 3 tables, author's version of the paper published in Natur

A ring-like accretion structure in M87 connecting its black hole and jet

The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation1,2. Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole3. Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of [Formula: see text] Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects, in addition\ua0to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow

The persistent shadow of the supermassive black hole of M 87

In April 2019, the Event Horizon Telescope (EHT) Collaboration reported the first-ever event-horizon-scale images of a black hole, resolving the central compact radio source in the giant elliptical galaxy M 87. These images reveal a ring with a southerly brightness distribution and a diameter of ∼42 μas, consistent with the predicted size and shape of a shadow produced by the gravitationally lensed emission around a supermassive black hole. These results were obtained as part of the April 2017 EHT observation campaign, using a global very long baseline interferometric radio array operating at a wavelength of 1.3 mm. Here, we present results based on the second EHT observing campaign, taking place in April 2018 with an improved array, wider frequency coverage, and increased bandwidth. In particular, the additional baselines provided by the Greenland telescope improved the coverage of the array. Multiyear EHT observations provide independent snapshots of the horizon-scale emission, allowing us to confirm the persistence, size, and shape of the black hole shadow, and constrain the intrinsic structural variability of the accretion flow. We have confirmed the presence of an asymmetric ring structure, brighter in the southwest, with a median diameter of 43.3−3.1+1.5 μas. The diameter of the 2018 ring is remarkably consistent with the diameter obtained from the previous 2017 observations. On the other hand, the position angle of the brightness asymmetry in 2018 is shifted by about 30° relative to 2017. The perennial persistence of the ring and its diameter robustly support the interpretation that the ring is formed by lensed emission surrounding a Kerr black hole with a mass ∼6.5 × 109 M⊙. The significant change in the ring brightness asymmetry implies a spin axis that is more consistent with the position angle of the large-scale jet

Contactless Ultrasonic Wavefield Imaging to Visualize Near-Surface Damage in Concrete Elements

We present work to detect and visualize near-surface damage in concrete using contactless ultrasonic wavefield imaging technology. A fully contactless ultrasonic scanning system that utilizes a micro-electro-mechanical systems (MEMS) ultrasonic microphone array is used to collect ultrasonic surface wave data from a concrete sample. The obtained wavefield data sets are processed with a frequency-wavenumber (f-k) domain wavefield filtering approach to extract non-propagating oscillatory fields set up by near-surface concrete cracking damage. The experimental results demonstrate that near-surface concrete damage can be detected and visualized using the proposed ultrasonic wavefield imaging approach

Nondestructive damage characterization of concrete and concrete-steel composites using contactless ultrasonic scanning

Concrete is a widely used construction material for critical infrastructure systems such as bridges and nuclear power plants. During its service life, concrete and concrete-steel composite structures are subject to various deterioration mechanisms including freezing/thawing cycles, alkali-silica reaction (ASR) and sulfate attack. Prolonged and repeated exposure to the mechanisms can lead to the initiation of surface and internal damage that grows during the remaining service life of structure unless appropriately repaired. To repair and retrofit damaged structures effectively, damage needs to be detected and characterized early on in the process. Although existing nondestructive evaluation (NDE) approaches have shown potential for detecting and characterizing some type and extents of damage in concrete and concrete-steel composites, the wide range and size of damage types and natural heterogeneity and material variability of the concrete itself limit the effectiveness of existing NDE methods. In this dissertation, new NDE techniques are developed based on contactless ultrasonic scanning measurements and are applied to detect and characterize specific types of damage in concrete and concrete-steel composite structures that are not well addressed with existing NDE technology. For the case of distributed cracking damage in concrete, contactless ultrasonic wavefield imaging hardware and data processing method is studied. The developed wavefield imaging hardware consists of a multi-channel MEMS ultrasonic microphone array with a signal conditioning circuit and an automated mechanical scanner that enables rapid ultrasonic wavefield data acquisition with high signal-to-noise ratio (SNR). To further accelerate the wavefield data acquisition, a compressed sensing approach is also proposed. Then, a series of analyses, numerical simulations and laboratory-scale experiments are carried out to understand the interaction between incident ultrasonic waves and distributed sub-wavelength cracks. A wavefield data processing technique is developed to extract non-propagating oscillatory fields that are sensitive to sub-wavelength cracks. The developed NDE technique is evaluated through application to large-scale concrete block samples under realistic ASR environments. The experimental results demonstrate that the developed contactless wavefield imaging hardware enables rapid acquisition of high-quality ultrasonic wavefield data and that the proposed wavefield data processing technique successfully detects and characterizes distributed cracking damage in concrete. In a separate effort to develop an NDE technique to characterize interface bonding conditions of concrete-steel composite structures, a contactless ultrasonic scanning approach using the attenuation characteristics of guided waves is developed. Based on an understanding of guided wave propagation in multi-layered media through analytical modeling and numerical simulations, a signal processing technique is proposed to extract an attenuation-related wave feature that is sensitive to steel-concrete interface bonding conditions. The feasibility of the developed NDE approach is then established by experiments on steel-clad concrete specimens with varying interface bond quality. The results demonstrate that steel-concrete interface bonding conditions are quantitatively evaluated using the proposed approach while the technique remains unaffected by variations of test conditions such as transmitter lift-off distances and sensor-specimen coupling.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste