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Acoustic Sensors and Their Applications

A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Intelligent Sensors".

Deadline for manuscript submissions: closed (31 July 2023) | Viewed by 21389

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Special Issue Editors


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Guest Editor
Department of Electrical and Computer Engineering, University of Victoria, Victoria, BC V8P 5C2, Canada
Interests: hydrophones; acoustic signal processing; signal classification; Big Data; acoustic devices; acoustic signal detection; data analysis; filter banks; data mining; ecology; environmental science computing; feature extraction; feature selection; feedforward neural nets; geophysical signal processing
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Co-Guest Editor
School of Electrical Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar 752050, India
Interests: signal and image processing; computer vision; machine learning & deep learning; medical image analysis; biometrics

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Co-Guest Editor
Department of Electrical Engineering, Americal Public University, Charles Town, WV 25414, USA
Interests: machine learning and deep learning applied to signal and image processing; computer vision; brain-computer interface; seizure detection and prediction based on brain signals; neurofeedback applications

Special Issue Information

Dear Colleagues,

Acoustic sensors have been in commercial use for more than 60 years. Acoustic sensing technologies have been studied extensively, and the information, transmission, reception, transformation, processing and application of acoustic signals have been developed, with acoustic sensors as central focus. An acoustic sensor is a device that converts a sound wave signal into an electrical signal.

The Special Issue, entitled “Acoustic Sensors and Their Applications” will present original research and critical review articles covering recent advances in all aspects of the development, production, testing, and application of acoustic sensors. Authors are invited to present on advanced research trends in acoustic sensors technology combined with other physical principles (conductometric, optic, calorimetric, etc.).

Dr. Farook Sattar
Dr. Niladri Bihari Puhan
Dr. Reza Fazel-Rezai
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Sensors is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • hydrophones
  • acoustic signal processing
  • acoustic signal classification
  • acoustic devices
  • acoustic signal detection
  • data analysis
  • channel bank filters
  • feature extraction
  • medical signal detection
  • mixture models
  • patient diagnosis

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Published Papers (13 papers)

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Editorial

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3 pages, 164 KiB  
Editorial
Special Issue on Acoustic Sensors and Their Applications (Vol. 1)
by Farook Sattar, Niladri Bihari Puhan and Reza Fazel-Rezai
Sensors 2023, 23(18), 7726; https://doi.org/10.3390/s23187726 - 7 Sep 2023
Viewed by 935
Abstract
Acoustic sensors have been in commercial use for more than 60 years [...] Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)

Research

Jump to: Editorial

13 pages, 1712 KiB  
Communication
Guided Acoustic Waves in Polymer Rods with Varying Immersion Depth in Liquid
by Klaus Lutter, Alexander Backer and Klaus Stefan Drese
Sensors 2023, 23(24), 9892; https://doi.org/10.3390/s23249892 - 18 Dec 2023
Viewed by 806
Abstract
Monitoring tanks and vessels play an important part in public infrastructure and several industrial processes. The goal of this work is to propose a new kind of guided acoustic wave sensor for measuring immersion depth. Common sensor types such as pressure sensors and [...] Read more.
Monitoring tanks and vessels play an important part in public infrastructure and several industrial processes. The goal of this work is to propose a new kind of guided acoustic wave sensor for measuring immersion depth. Common sensor types such as pressure sensors and airborne ultrasonic sensors are often limited to non-corrosive media, and can fail to distinguish between the media they reflect on or are submerged in. Motivated by this limitation, we developed a guided acoustic wave sensor made from polyethylene using piezoceramics. In contrast to existing sensors, low-frequency Hanning-windowed sine bursts were used to excite the L(0,1) mode within a solid polyethylene rod. The acoustic velocity within these rods changes with the immersion depth in the surrounding fluid. Thus, it is possible to detect changes in the surrounding media by measuring the time shifts of zero crossings through the rod after being reflected on the opposite end. The change in time of zero crossings is monotonically related to the immersion depth. This relative measurement method can be used in different kinds of liquids, including strong acids or bases. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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14 pages, 4759 KiB  
Article
Estimation Method of an Electrical Equivalent Circuit for Sonar Transducer Impedance Characteristic of Multiple Resonance
by Jejin Jang, Jaehyuk Choi, Donghun Lee and Hyungsoo Mok
Sensors 2023, 23(14), 6636; https://doi.org/10.3390/s23146636 - 24 Jul 2023
Cited by 3 | Viewed by 1201
Abstract
Improving the operational efficiency and optimizing the design of sound navigation and ranging (sonar) systems require accurate electrical equivalent models within the operating frequency range. The power conversion system within the sonar system increases power efficiency through impedance-matching circuits. Impedance matching is used [...] Read more.
Improving the operational efficiency and optimizing the design of sound navigation and ranging (sonar) systems require accurate electrical equivalent models within the operating frequency range. The power conversion system within the sonar system increases power efficiency through impedance-matching circuits. Impedance matching is used to enhance the power transmission efficiency of the sonar system. Therefore, to increase the efficiency of the sonar system, an electrical-matching circuit is employed, and this necessitates an accurate equivalent circuit for the sonar transducer within the operating frequency range. In conventional equivalent circuit derivation methods, errors occur because they utilize the same number of RLC branches as the resonant frequency of the sonar transducer, based on its physical properties. Hence, this paper proposes an algorithm for deriving an equivalent circuit independent of resonance by employing multiple electrical components and particle swarm optimization (PSO). A comparative verification was also performed between the proposed and existing approaches using the Butterworth–van Dyke (BVD) model, which is a method for deriving electrical equivalent circuits. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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24 pages, 11517 KiB  
Article
Embedded Ultrasonic Inspection on the Mechanical Properties of Cold Region Ice under Varying Temperatures
by Huimin Han, Li Wei, Nizar Faisal Alkayem and Maosen Cao
Sensors 2023, 23(13), 6045; https://doi.org/10.3390/s23136045 - 29 Jun 2023
Cited by 3 | Viewed by 1090
Abstract
The mechanical properties of ice in cold regions are significantly affected by the variation in temperature. The existing methods to determine ice properties commonly rely on one-off and destructive compression and strength experiments, which are unable to acquire the varying properties of ice [...] Read more.
The mechanical properties of ice in cold regions are significantly affected by the variation in temperature. The existing methods to determine ice properties commonly rely on one-off and destructive compression and strength experiments, which are unable to acquire the varying properties of ice due to temperature variations. To this end, an embedded ultrasonic system is proposed to inspect the mechanical properties of ice in an online and real-time mode. With this system, ultrasonic experiments are conducted to testify to the validity of the system in continuously inspecting the mechanical properties of ice and, in particular, to verify its capabilities to obtain ice properties for various temperature conditions. As an extension of the experiment, an associated refined numerical model is elaborated by mimicking the number, size, and agglomeration of bubbles using a stochastic distribution. This system can continuously record the wave propagation velocity in the ice, giving rise to ice properties through the intrinsic mechanics relationship. In addition, this model facilitates having insights into the effect of properties, e.g., porosity, on ice properties. The proposed embedded ultrasonic system largely outperforms the existing methods to obtain ice properties, holding promise for developing online and real-time monitoring techniques to assess the ice condition closely related to structures in cold regions. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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12 pages, 3074 KiB  
Article
All-Optical, Air-Coupled Ultrasonic Detection of Low-Pressure Gas Leaks and Observation of Jet Tones in the MHz Range
by Kyle G. Scheuer and Ray G. DeCorby
Sensors 2023, 23(12), 5665; https://doi.org/10.3390/s23125665 - 17 Jun 2023
Cited by 3 | Viewed by 1468
Abstract
We used an ultrasensitive, broadband optomechanical ultrasound sensor to study the acoustic signals produced by pressurized nitrogen escaping from a variety of small syringes. Harmonically related jet tones extending into the MHz region were observed for a certain range of flow (i.e., Reynolds [...] Read more.
We used an ultrasensitive, broadband optomechanical ultrasound sensor to study the acoustic signals produced by pressurized nitrogen escaping from a variety of small syringes. Harmonically related jet tones extending into the MHz region were observed for a certain range of flow (i.e., Reynolds number), which is in qualitative agreement with historical studies on gas jets emitted from pipes and orifices of much larger dimensions. For higher turbulent flow rates, we observed broadband ultrasonic emission in the ~0–5 MHz range, which was likely limited on the upper end due to attenuation in air. These observations are made possible by the broadband, ultrasensitive response (for air-coupled ultrasound) of our optomechanical devices. Aside from being of theoretical interest, our results could have practical implications for the non-contact monitoring and detection of early-stage leaks in pressured fluid systems. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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17 pages, 6579 KiB  
Article
Improving APT Systems’ Performance in Air via Impedance Matching and 3D-Printed Clamp
by Liu Liu and Waleed Abdulla
Sensors 2023, 23(11), 5347; https://doi.org/10.3390/s23115347 - 5 Jun 2023
Cited by 1 | Viewed by 919
Abstract
This paper presents a study on improving the performance of the acoustic piezoelectric transducer system in air, as the low acoustic impedance of air leads to suboptimal system performance. Impedance matching techniques can enhance the acoustic power transfer (APT) system’s performance in air. [...] Read more.
This paper presents a study on improving the performance of the acoustic piezoelectric transducer system in air, as the low acoustic impedance of air leads to suboptimal system performance. Impedance matching techniques can enhance the acoustic power transfer (APT) system’s performance in air. This study integrates an impedance matching circuit into the Mason circuit and investigates the impact of fixed constraints on the piezoelectric transducer’s sound pressure and output voltage. Additionally, this paper proposes a novel equilateral triangular peripheral clamp that is entirely 3D-printable and cost-effective. This study analyses the peripheral clamp’s impedance and distance characteristics and confirms its effectiveness through consistent experimental and simulation results. The findings of this study can aid researchers and practitioners in various fields that employ APT systems to improve their performance in air. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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16 pages, 469 KiB  
Article
Using Hybrid HMM/DNN Embedding Extractor Models in Computational Paralinguistic Tasks
by Mercedes Vetráb and Gábor Gosztolya
Sensors 2023, 23(11), 5208; https://doi.org/10.3390/s23115208 - 30 May 2023
Cited by 3 | Viewed by 1342
Abstract
The field of computational paralinguistics emerged from automatic speech processing, and it covers a wide range of tasks involving different phenomena present in human speech. It focuses on the non-verbal content of human speech, including tasks such as spoken emotion recognition, conflict intensity [...] Read more.
The field of computational paralinguistics emerged from automatic speech processing, and it covers a wide range of tasks involving different phenomena present in human speech. It focuses on the non-verbal content of human speech, including tasks such as spoken emotion recognition, conflict intensity estimation and sleepiness detection from speech, showing straightforward application possibilities for remote monitoring with acoustic sensors. The two main technical issues present in computational paralinguistics are (1) handling varying-length utterances with traditional classifiers and (2) training models on relatively small corpora. In this study, we present a method that combines automatic speech recognition and paralinguistic approaches, which is able to handle both of these technical issues. That is, we trained a HMM/DNN hybrid acoustic model on a general ASR corpus, which was then used as a source of embeddings employed as features for several paralinguistic tasks. To convert the local embeddings into utterance-level features, we experimented with five different aggregation methods, namely mean, standard deviation, skewness, kurtosis and the ratio of non-zero activations. Our results show that the proposed feature extraction technique consistently outperforms the widely used x-vector method used as the baseline, independently of the actual paralinguistic task investigated. Furthermore, the aggregation techniques could be combined effectively as well, leading to further improvements depending on the task and the layer of the neural network serving as the source of the local embeddings. Overall, based on our experimental results, the proposed method can be considered as a competitive and resource-efficient approach for a wide range of computational paralinguistic tasks. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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14 pages, 3266 KiB  
Article
Modal Decomposition of Acoustic Emissions from Pencil-Lead Breaks in an Isotropic Thin Plate
by Xinyue Yao, Benjamin Steven Vien, Nik Rajic, Cedric Rosalie, L. R. Francis Rose, Chris Davies and Wing Kong Chiu
Sensors 2023, 23(4), 1988; https://doi.org/10.3390/s23041988 - 10 Feb 2023
Cited by 3 | Viewed by 1350
Abstract
Acoustic emission (AE) testing and Lamb wave inspection techniques have been widely used in non-destructive testing and structural health monitoring. For thin plates, the AEs arising from structural defect development (e.g., fatigue crack propagation) propagate as Lamb waves, and Lamb wave modes can [...] Read more.
Acoustic emission (AE) testing and Lamb wave inspection techniques have been widely used in non-destructive testing and structural health monitoring. For thin plates, the AEs arising from structural defect development (e.g., fatigue crack propagation) propagate as Lamb waves, and Lamb wave modes can be used to provide important information about the growth and localisation of defects. However, few sensors can be used to achieve the in situ wavenumber–frequency modal decomposition of AEs. This study explores the ability of a new multi-element piezoelectric sensor array to decompose AEs excited by pencil lead breaks (PLBs) on a thin isotropic plate. In this study, AEs were generated by out-of-plane (transverse) and in-plane (longitudinal) PLBs applied at the edge of the plate, and waveforms were recorded by both the new sensor array and a commercial AE sensor. Finite element analysis (FEA) simulations of PLBs were also conducted and the results were compared with the experimental results. To identify the wave modes present, the longitudinal and transverse PLB test results recorded by the new sensor array at five different plate locations were compared with FEA simulations using the same arrangement. Two-dimensional fast Fourier Transforms were then applied to the AE wavefields. It was found that the AE modal composition was dependent on the orientation of the PLB direction. The results suggest that this new sensor array can be used to identify the AE wave modes excited by PLBs in both in-plane and out-of-plane directions. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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13 pages, 5089 KiB  
Article
Pipeline Leakage Detection Based on Secondary Phase Transform Cross-Correlation
by Hetao Liang, Yan Gao, Haibin Li, Siyuan Huang, Minghui Chen and Baomin Wang
Sensors 2023, 23(3), 1572; https://doi.org/10.3390/s23031572 - 1 Feb 2023
Cited by 4 | Viewed by 1958
Abstract
Leaks from pipes and valves are a reputational issue in industry. Maintenance of pipeline integrity is becoming a growing challenge due to the serious socioeconomic consequences. This paper presents a secondary phase transform (PHAT) cross-correlation method to improve the performance of the acoustic [...] Read more.
Leaks from pipes and valves are a reputational issue in industry. Maintenance of pipeline integrity is becoming a growing challenge due to the serious socioeconomic consequences. This paper presents a secondary phase transform (PHAT) cross-correlation method to improve the performance of the acoustic methods based on cross-correlation for pipeline leakage detection. Acoustic emission signals generated by pipe leakage are first captured by the sensors at different locations, and are subsequently analyzed using the cross-correlation curve to determine whether leakage is occurring. When leakage occurs, time delay estimation (TDE) is further carried out by peak search in the cross-correlation curve between the two sensor signals. In the analysis, the proposed method calculates the secondary cross-correlation function before the PHAT operation. A sinc interpolation method is then introduced for automatic searching the peak value of the cross-correlation curve. Numerical simulations and experimental results confirm the improved performance of the proposed method for noise suppression and accurate TDE compared to the basic cross-correlation method, which may be beneficial in engineering applications. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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9 pages, 1539 KiB  
Article
Broadband Air-Coupled Ultrasound Emitter and Receiver Enable Simultaneous Measurement of Thickness and Speed of Sound in Solids
by Klaas Bente, Janez Rus, Hubert Mooshofer, Mate Gaal and Christian Ulrich Grosse
Sensors 2023, 23(3), 1379; https://doi.org/10.3390/s23031379 - 26 Jan 2023
Cited by 7 | Viewed by 2177
Abstract
Air-coupled ultrasound sensors have advantages over contact ultrasound sensors when a sample should not become contaminated or influenced by the couplant or the measurement has to be a fast and automated inline process. Thereby, air-coupled transducers must emit high-energy pulses due to the [...] Read more.
Air-coupled ultrasound sensors have advantages over contact ultrasound sensors when a sample should not become contaminated or influenced by the couplant or the measurement has to be a fast and automated inline process. Thereby, air-coupled transducers must emit high-energy pulses due to the low air-to-solid power transmission ratios (10−3 to 10−8). Currently used resonant transducers trade bandwidth—a prerequisite for material parameter analysis—against pulse energy. Here we show that a combination of a non-resonant ultrasound emitter and a non-resonant detector enables the generation and detection of pulses that are both high in amplitude (130 dB) and bandwidth (2 µs pulse width). We further show an initial application: the detection of reflections inside of a carbon fiber reinforced plastic plate with thicknesses between 1.7 mm and 10 mm. As the sensors work contact-free, the time of flight and the period of the in-plate reflections are independent parameters. Hence, a variation of ultrasound velocity is distinguishable from a variation of plate thickness and both properties are determined simultaneously. The sensor combination is likely to find numerous industrial applications necessitating high automation capacity and opens possibilities for air-coupled, single-side ultrasonic inspection. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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19 pages, 2503 KiB  
Article
ResSKNet-SSDP: Effective and Light End-To-End Architecture for Speaker Recognition
by Fei Deng, Lihong Deng, Peifan Jiang, Gexiang Zhang and Qiang Yang
Sensors 2023, 23(3), 1203; https://doi.org/10.3390/s23031203 - 20 Jan 2023
Cited by 3 | Viewed by 1894
Abstract
In speaker recognition tasks, convolutional neural network (CNN)-based approaches have shown significant success. Modeling the long-term contexts and efficiently aggregating the information are two challenges in speaker recognition, and they have a critical impact on system performance. Previous research has addressed these issues [...] Read more.
In speaker recognition tasks, convolutional neural network (CNN)-based approaches have shown significant success. Modeling the long-term contexts and efficiently aggregating the information are two challenges in speaker recognition, and they have a critical impact on system performance. Previous research has addressed these issues by introducing deeper, wider, and more complex network architectures and aggregation methods. However, it is difficult to significantly improve the performance with these approaches because they also have trouble fully utilizing global information, channel information, and time-frequency information. To address the above issues, we propose a lighter and more efficient CNN-based end-to-end speaker recognition architecture, ResSKNet-SSDP. ResSKNet-SSDP consists of a residual selective kernel network (ResSKNet) and self-attentive standard deviation pooling (SSDP). ResSKNet can capture long-term contexts, neighboring information, and global information, thus extracting a more informative frame-level. SSDP can capture short- and long-term changes in frame-level features, aggregating the variable-length frame-level features into fixed-length, more distinctive utterance-level features. Extensive comparison experiments were performed on two popular public speaker recognition datasets, Voxceleb and CN-Celeb, with current state-of-the-art speaker recognition systems and achieved the lowest EER/DCF of 2.33%/0.2298, 2.44%/0.2559, 4.10%/0.3502, and 12.28%/0.5051. Compared with the lightest x-vector, our designed ResSKNet-SSDP has 3.1 M fewer parameters and 31.6 ms less inference time, but 35.1% better performance. The results show that ResSKNet-SSDP significantly outperforms the current state-of-the-art speaker recognition architectures on all test sets and is an end-to-end architecture with fewer parameters and higher efficiency for applications in realistic situations. The ablation experiments further show that our proposed approaches also provide significant improvements over previous methods. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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14 pages, 3675 KiB  
Article
An Infinity Tube with an Expansion Chamber for Noise Control in the Ductwork System
by Rong Xue, Cheuk Ming Mak, Chenzhi Cai and Kuen Wai Ma
Sensors 2023, 23(1), 305; https://doi.org/10.3390/s23010305 - 28 Dec 2022
Cited by 1 | Viewed by 1409
Abstract
This paper proposes a muffler with simple geometry to effectively reduce low-frequency noise in ductwork systems. A muffler named infinity tube with an expansion chamber (ITEC) is developed from the infinity tube (IT). Theoretical and numerical analyses of wave propagation in the ITEC [...] Read more.
This paper proposes a muffler with simple geometry to effectively reduce low-frequency noise in ductwork systems. A muffler named infinity tube with an expansion chamber (ITEC) is developed from the infinity tube (IT). Theoretical and numerical analyses of wave propagation in the ITEC have been conducted in this paper. The transfer matrix method is adopted to predict transmission loss theoretically. The theoretical results are validated by the finite element method simulation. The comparison of the transmission loss between the IT and ITEC illustrates that the ITEC has an advantage in low-frequency noise reduction. The transmission loss results of the ITEC are compared with the Helmholtz resonator system to assess the potential for industrial application. Finally, the geometric parameters of the proposed ITEC on its noise attenuation performance have been analyzed. The proposed ITEC can effectively reduce low-frequency noise, and it is suitable for ductwork systems in constrained spaces. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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13 pages, 3259 KiB  
Article
Ultrasonic Measurement of Axial Preload in High-Frequency Nickel-Based Superalloy Smart Bolt
by Shuang Liu, Zhongrui Sun, Guanpin Ren, Cheng Liao, Xulin He, Kun Luo, Ru Li, Wei Jiang and Huan Zhan
Sensors 2023, 23(1), 220; https://doi.org/10.3390/s23010220 - 25 Dec 2022
Cited by 4 | Viewed by 2033
Abstract
A high-frequency, piezoelectric thin-film sensor was successfully deposited on a nickel-based superalloy bolt by radio frequency magnetron sputtering to develop a smart, nickel-based superalloy bolt. Ultrasonic response characterization, high accuracy, and repeatability of ultrasonic measurement of axial preload in nickel-based superalloy smart bolts [...] Read more.
A high-frequency, piezoelectric thin-film sensor was successfully deposited on a nickel-based superalloy bolt by radio frequency magnetron sputtering to develop a smart, nickel-based superalloy bolt. Ultrasonic response characterization, high accuracy, and repeatability of ultrasonic measurement of axial preload in nickel-based superalloy smart bolts are reported here and were fully demonstrated. The axial preload in the nickel-based superalloy smart bolt was directly measured by the bi-wave method (TOF ratio between transverse and longitudinal-mode waves) without using the traditional integration of a longitudinal and shear transducer. A model concerning the bolt before and after tensioning was established to demonstrate the propagation and displacement distribution of the ultrasonic waves inside a nickel-based superalloy smart bolt. The measured A-scan signal presented significantly favorable features including a mixture of transverse and longitudinal mode waves, a pure and broad frequency spectrum which peaked at 17.14 MHz, and high measurement accuracy below 3% for tension of 4 kN–20 kN. For the temporal ultrasonic signal, the measurement envelopes were narrower than for the counterpart of the simulation, justifying the ‘filtration’ advantage of the high-frequency sensor. Both the TOF change of the single longitudinal-mode wave and the TOF ratio between transverse- and longitudinal-mode waves increased linearly with preload force in the range of 0 kN to 20 kN. Compared with the commercial piezoelectric probe, the proposed probe, based on the combination of a high-frequency, piezoelectric thin-film sensor and a magnetically mounted transducer connector, exhibited high tolerance to temperatures as high as 320 °C and high repeatability free from some interference factors such as bolt detection position change and couplant layer thickness. The results indicate that this system is a promising axial preload measurement system for high-temperature fasteners and connectors, and the proposed sensor is a practical, high-frequency ultrasonic sensor for non-destructive testing. Full article
(This article belongs to the Special Issue Acoustic Sensors and Their Applications)
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