Special Issue "Acoustic and Elastic Waves: Recent Trends in Science and Engineering"

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Acoustics and Vibrations".

Deadline for manuscript submissions: closed (31 December 2015).

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editors

Prof. Dimitrios G. Aggelis
E-Mail Website
Guest Editor
Department of Mechanics of Materials and Constructions, Vrije Universiteit Brussel, Pleinlaan 2, Brussels 1050, Belgium
Tel. 0032 6293541
Interests: structural health monitoring (shm); non-destructive evaluation (nde); acoustic emission (ae); ultrasonic testing (ut); scattering; dispersion; attenuation; material evaluation; concrete
Special Issues and Collections in MDPI journals
Dr. Nathalie Godin
E-Mail Website
Guest Editor
INSA-Lyon, MATEIS laboratory UMR CNRS 5510, 69621 Villeurbanne, France
Interests: acoustic emission; mechanical behavior of composites; durability and lifetime of composites
Special Issues and Collections in MDPI journals

Special Issue Information

Dear Colleagues,

The present Special Issue intends to explore new directions in the field of acoustics and ultrasonics. The interest includes, but is not limited to, the use of acoustic technology for condition monitoring of materials and structures. (The basis of this issue comes from selected papers of the 6th International Conference on Emerging Technologies in Non-destructive Testing, ETNDT6 held in Brussels May 27-29th, 2015).

Topics of interest (among others):
• Acoustic emission in materials and structures (without material limitation)
• Innovative cases of ultrasonic inspection
• Wave dispersion and waveguides
• Monitoring of innovative materials
• Seismic waves
• Vibrations, damping and noise control
• Combination of mechanical wave techniques with other types for structural health monitoring purposes.
Experimental and numerical studies are welcome.

Prof. Dr. Dimitrios G. Aggelis
Dr. Nathalie Godin
Guest Editors

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Keywords

  • structural health monitoring (shm)
  • non-destructive evaluation (nde)
  • acoustic emission (ae)
  • ultrasonic testing (ut)
  • scattering
  • dispersion
  • attenuation
  • material evaluation
  • vibration
  • earthquakes

Published Papers (21 papers)

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Research

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Open AccessArticle
On Site Investigation and Health Monitoring of a Historic Tower in Mantua, Italy
Appl. Sci. 2016, 6(6), 173; https://doi.org/10.3390/app6060173 - 08 Jun 2016
Cited by 7
Abstract
The paper describes the strategy adopted to assess the structural condition of the tallest historic tower in Mantua (Italy) after the Italian seismic sequence of May–June 2012 and exemplifies the application of health monitoring using (automated) operational modal analysis. The post-earthquake survey (including [...] Read more.
The paper describes the strategy adopted to assess the structural condition of the tallest historic tower in Mantua (Italy) after the Italian seismic sequence of May–June 2012 and exemplifies the application of health monitoring using (automated) operational modal analysis. The post-earthquake survey (including extensive visual inspection, historic and documentary research, non-destructive (ND) material testing, and ambient vibration tests) highlighted the poor state of preservation of the upper part of the tower; subsequently, a dynamic monitoring system (consisting of a few accelerometers and one temperature sensor) was installed in the building to address the preservation of the historic structure, and automated modal identification was continuously performed. Despite the low levels of vibration that existed in operational conditions, the analysis of data collected over a period of about 15 months allowed to assess and model the effects of changing temperature on modal frequencies and to detect the occurrence of abnormal behavior and damage under the changing environment. The monitoring results demonstrate the potential key role of vibration-based structural health monitoring, implemented through low-cost hardware solutions and appropriate software tools, in the preventive conservation and the condition-based maintenance of historic towers. Full article
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Open AccessArticle
Opto-Acoustic Method for the Characterization of Thin-Film Adhesion
Appl. Sci. 2016, 6(6), 163; https://doi.org/10.3390/app6060163 - 25 May 2016
Cited by 1
Abstract
The elastic property of the film-substrate interface of thin-film systems is characterized with an opto-acoustic method. The thin-film specimens are oscillated with an acoustic transducer at audible frequencies, and the resultant harmonic response of the film surface is analyzed with optical interferometry. Polystyrene, [...] Read more.
The elastic property of the film-substrate interface of thin-film systems is characterized with an opto-acoustic method. The thin-film specimens are oscillated with an acoustic transducer at audible frequencies, and the resultant harmonic response of the film surface is analyzed with optical interferometry. Polystyrene, Ti, Ti-Au and Ti-Pt films coated on the same silicon substrate are tested. For each film material, a pair of specimens is prepared; one is coated on a silicon substrate after the surface is treated with plasma bombardment, and the other is coated on an identical silicon substrate without a treatment. Experiments indicate that both the surface-treated and untreated specimens of all film materials have resonance in the audible frequency range tested. The elastic constant of the interface corresponding to the observed resonance is found to be orders of magnitude lower than that of the film or substrate material. Observations of these resonance-like behaviors and the associated stiffness of the interface are discussed. Full article
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Open AccessArticle
Detecting the Presence of High Water-to-Cement Ratio in Concrete Surfaces Using Highly Nonlinear Solitary Waves
Appl. Sci. 2016, 6(4), 104; https://doi.org/10.3390/app6040104 - 11 Apr 2016
Cited by 19
Abstract
We describe a nondestructive evaluation (NDE) method based on the propagation of highly nonlinear solitary waves (HNSWs) to determine the excess of water on the surface of existing concrete structures. HNSWs are induced in a one-dimensional granular chain placed in contact with the [...] Read more.
We describe a nondestructive evaluation (NDE) method based on the propagation of highly nonlinear solitary waves (HNSWs) to determine the excess of water on the surface of existing concrete structures. HNSWs are induced in a one-dimensional granular chain placed in contact with the concrete to be tested. The chain is part of a built-in transducer designed and assembled to exploit the dynamic interaction between the particles and the concrete. The hypothesis is that the interaction depends on the stiffness of the concrete and influences the time-of-flight of the solitary pulse reflected at the transducer/concrete interface. Two sets of experiments were conducted. In the first set, eighteen concrete cylinders with different water-to-cement (w/c) ratios were cast and tested in order to obtain baseline data to link the ratio to the time of flight. Then, sixteen short beams with fixed w/c ratio, but subject to water in excess at one surface, were cast. The novel NDE method was applied along with the conventional ultrasonic pulse velocity technique in order to determine advantages and limitations of the proposed approach. The results show that the time of flight detected the excess of water in the beams. In the future, the proposed method may be employed in the field to evaluate rapidly and reliably the condition of existing concrete structures and, in particular, concrete decks. Full article
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Open AccessArticle
Numerical Models for the Assessment of Historical Masonry Structures and Materials, Monitored by Acoustic Emission
Appl. Sci. 2016, 6(4), 102; https://doi.org/10.3390/app6040102 - 08 Apr 2016
Cited by 7
Abstract
The paper reviews some recent numerical applications for the interpretation and exploitation of acoustic emission (AE) monitoring results obtained from historical masonry structures and materials. Among possible numerical techniques, the finite element method and the distinct method are considered. The analyzed numerical models [...] Read more.
The paper reviews some recent numerical applications for the interpretation and exploitation of acoustic emission (AE) monitoring results obtained from historical masonry structures and materials. Among possible numerical techniques, the finite element method and the distinct method are considered. The analyzed numerical models cover the entire scale range, from microstructure and meso-structure, up to full-size real structures. The micro-modeling includes heterogeneous concrete-like materials, but mainly focuses on the masonry texture meso-structure, where each brick and mortar joint is modeled singularly. The full-size models consider the different typology of historical structures such as masonry towers, cathedrals and chapels. The main difficulties and advantages of the different numerical approaches, depending on the problem typology and scale, are critically analyzed. The main insight we can achieve from micro and meso numerical modeling concerns the scaling of AE as a function of volume and time, since it is also able to simulate the b-value temporal evolution as the damage spread into the structure. The finite element modeling of the whole structure provides useful hints for the optimal placement of the AE sensors, while the combination of AE monitoring results is crucial for a reliable assessment of structural safety. Full article
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Open AccessArticle
Correlation of Plastic Strain Energy and Acoustic Emission Energy in Reinforced Concrete Structures
Appl. Sci. 2016, 6(3), 84; https://doi.org/10.3390/app6030084 - 16 Mar 2016
Cited by 16
Abstract
This paper presents a comparison of the acoustic emission (AE) energy and the plastic strain energy released by some reinforced concrete (RC) specimens subjected to cyclic or seismic loadings. AE energy is calculated, after proper filtering procedures, using the signals recorded by several [...] Read more.
This paper presents a comparison of the acoustic emission (AE) energy and the plastic strain energy released by some reinforced concrete (RC) specimens subjected to cyclic or seismic loadings. AE energy is calculated, after proper filtering procedures, using the signals recorded by several AE low frequency sensors (25–100 kHz) attached on the specimens. Plastic strain energy is obtained by integrating the load displacement curves drawn from the measurements recorded during the test. Presented are the results obtained for: (i) two beams (with and without an artificial notch) and a beam-column connection subjected to several cycles of imposed flexural deformations; (ii) a reinforced concrete slab supported by four steel columns, and a reinforced concrete frame structure, both of the latter are subjected to seismic simulations with a uniaxial shaking table. The main contribution of this paper, which is a review of some papers previously published by the authors, is to highlight that, in all cases, a very good correlation is found between AE energy and plastic strain energy, until the onset of yielding in the reinforcing steel. After yielding, the AE energy is consistently lower than the plastic strain energy. The reason is that the plastic strain energy is the sum of the contribution of concrete and steel, while the AE energy acquired with thresholds higher than 35 dBAE captures only the contribution of the concrete cracking, not the steel plastic deformation. This good correlation between the two energies before the yielding point also lends credibility to the use of AE energy as a parameter for concrete damage evaluation in the context of structural health monitoring. Full article
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Open AccessArticle
The Stiffness and Damping Characteristics of a Dual-Chamber Air Spring Device Applied to Motion Suppression of Marine Structures
Appl. Sci. 2016, 6(3), 74; https://doi.org/10.3390/app6030074 - 08 Mar 2016
Cited by 10
Abstract
Dual-chamber air springs are used as a key component for vibration isolation in some industrial applications. The working principle of the dual-chamber air spring device as applied to motion suppression of marine structures is similar to that of the traditional air spring, but [...] Read more.
Dual-chamber air springs are used as a key component for vibration isolation in some industrial applications. The working principle of the dual-chamber air spring device as applied to motion suppression of marine structures is similar to that of the traditional air spring, but they differ in their specific characteristics. The stiffness and damping of the dual-chamber air spring device determine the extent of motion suppression. In this article, we investigate the stiffness and damping characteristics of a dual-chamber air spring device applied to marine structure motion suppression using orthogonal analysis and an experimental method. We measure the effects of volume ratio, orifice ratio, excitation amplitude, and frequency on the stiffness and damping of the dual-chamber vibration absorber. Based on the experimental results, a higher-order non-linear regression method is obtained. We achieve a rapid calculation model for dual-chamber air spring stiffness and damping, which can provide guidance to project design. Full article
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Open AccessArticle
Numerical and Experimental Characterization of Fiber-Reinforced Thermoplastic Composite Structures with Embedded Piezoelectric Sensor-Actuator Arrays for Ultrasonic Applications
Appl. Sci. 2016, 6(3), 55; https://doi.org/10.3390/app6030055 - 23 Feb 2016
Cited by 11
Abstract
The paper presents preliminary numerical and experimental studies of active textile-reinforced thermoplastic composites with embedded sensor-actuator arrays. The goal of the investigations was the assessment of directional sound wave generation capability using embedded sensor-actuator arrays and developed a wave excitation procedure for ultrasound [...] Read more.
The paper presents preliminary numerical and experimental studies of active textile-reinforced thermoplastic composites with embedded sensor-actuator arrays. The goal of the investigations was the assessment of directional sound wave generation capability using embedded sensor-actuator arrays and developed a wave excitation procedure for ultrasound measurement tasks. The feasibility of the proposed approach was initially confirmed in numerical investigations assuming idealized mechanical and geometrical conditions. The findings were validated in real-life conditions on specimens of elementary geometry. Herein, the technological aspects of unique automated assembly of thermoplastic films containing adapted thermoplastic-compatible piezoceramic modules and conducting paths were described. Full article
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Open AccessArticle
The Ultrasonic Polar Scan for Composite Characterization and Damage Assessment: Past, Present and Future
Appl. Sci. 2016, 6(2), 58; https://doi.org/10.3390/app6020058 - 22 Feb 2016
Cited by 13
Abstract
In the early 1980’s, the ultrasonic polar scan (UPS) technique was developed to assess the fiber direction of composites in a nondestructive way. In spite of the recognition by several researchers as being a sophisticated and promising methodology for nondestructive testing (NDT) and [...] Read more.
In the early 1980’s, the ultrasonic polar scan (UPS) technique was developed to assess the fiber direction of composites in a nondestructive way. In spite of the recognition by several researchers as being a sophisticated and promising methodology for nondestructive testing (NDT) and materials science, little advance was made during the following 30 years. Recently however, the UPS technique experienced a strong revival and various modifications to the original UPS setup have been successfully implemented. This revival has exposed several powerful capabilities and interesting applications of the UPS technique for material characterization and damage assessment. This paper gives a short historical overview of the UPS technique for characterizing and inspecting (damaged) fiber-reinforced plastics. In addition, a few future research lines are given, which will further expand the applicability and potential of the UPS method to a broader range of (damaged) materials, bringing the UPS technique to the next level of maturity. Full article
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Open AccessArticle
Dynamic Characterization of Cohesive Material Based on Wave Velocity Measurements
Appl. Sci. 2016, 6(2), 49; https://doi.org/10.3390/app6020049 - 09 Feb 2016
Cited by 8
Abstract
The paper presents a description of the dynamic properties of cohesive material, namely silty clays, obtained by using one of the applied seismology methods, the bender elements technique. The authors’ aim was to present the dynamics of a porous medium, in particular an [...] Read more.
The paper presents a description of the dynamic properties of cohesive material, namely silty clays, obtained by using one of the applied seismology methods, the bender elements technique. The authors’ aim was to present the dynamics of a porous medium, in particular an extremely important passage of seismic waves that travel through the bulk of a medium. Nowadays, the application of the bender element (BE) technique to measure, e.g., small strain shear stiffness of soils in the laboratory is well recognized, since it allows for reliable and relatively economical shear wave velocity measurements during various laboratory experiments. However, the accurate estimation of arrival time during BE tests is in many cases unclear. Two different interpretation procedures (from the time domain) of BE tests in order to measure travel times of waves were examined. Those values were then used to calculate shear and compression wave velocities and elastic moduli. Results showed that the dynamic parameters obtained by the start-to-start method were always slightly larger (up to about 20%) than those obtained using the peak-to-peak one. It was found that the peak-to-peak method led to more scattered results in comparison to the start-to-start method. Moreover, the influence of the excitation frequency, the mean effective stress and the unloading process on the dynamic properties of the tested material was studied. In addition, the obtained results highlighted the importance of initial signal frequency and the necessity to choose an appropriate range of frequencies to measure the shear wave velocity in clayey soils. Full article
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Open AccessArticle
Wavelet Packet Decomposition to Characterize Injection Molding Tool Damage
Appl. Sci. 2016, 6(2), 45; https://doi.org/10.3390/app6020045 - 04 Feb 2016
Cited by 2
Abstract
This paper presents measurements of acoustic emission (AE) signals during the injection molding of polypropylene with new and damaged mold. The damaged injection mold has cracks induced by laser surface heat treatment. Standard test specimens were injection molded, commonly used for examining the [...] Read more.
This paper presents measurements of acoustic emission (AE) signals during the injection molding of polypropylene with new and damaged mold. The damaged injection mold has cracks induced by laser surface heat treatment. Standard test specimens were injection molded, commonly used for examining the shrinkage behavior of various thermoplastic materials. The measured AE burst signals during injection molding cycle are presented. For injection molding tool integrity prediction, different AE burst signals’ descriptors are defined. To lower computational complexity and increase performance, the feature selection method was implemented to define a feature subset in an appropriate multidimensional space to characterize the integrity of the injection molding tool and the injection molding process steps. The feature subset was used for neural network pattern recognition of AE signals during the full time of the injection molding cycle. The results confirm that acoustic emission measurement during injection molding of polymer materials is a promising technique for characterizing the integrity of molds with respect to damage, even with resonant sensors. Full article
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Open AccessArticle
Identification of a Critical Time with Acoustic Emission Monitoring during Static Fatigue Tests on Ceramic Matrix Composites: Towards Lifetime Prediction
Appl. Sci. 2016, 6(2), 43; https://doi.org/10.3390/app6020043 - 03 Feb 2016
Cited by 8
Abstract
Non-oxide fiber-reinforced ceramic-matrix composites are promising candidates for some aeronautic applications that require good thermomechanical behavior over long periods of time. This study focuses on the behavior of a SiCf/[Si-B-C] composite with a self-healing matrix at intermediate temperature under air. Static [...] Read more.
Non-oxide fiber-reinforced ceramic-matrix composites are promising candidates for some aeronautic applications that require good thermomechanical behavior over long periods of time. This study focuses on the behavior of a SiCf/[Si-B-C] composite with a self-healing matrix at intermediate temperature under air. Static fatigue experiments were performed below 600 °C and a lifetime diagram is presented. Damage is monitored both by strain measurement and acoustic emission during the static fatigue experiments. Two methods of real-time analysis of associated energy release have been developed. They allow for the identification of a characteristic time that was found to be close to 55% of the measured rupture time. This critical time reflects a critical local energy release assessed by the applicability of the Benioff law. This critical aspect is linked to a damage phase where slow crack growth in fibers is prevailing leading to ultimate fracture of the composite. Full article
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Open AccessArticle
Proof of Concept of Crack Localization Using Negative Pressure Waves in Closed Tubes for Later Application in Effective SHM System for Additive Manufactured Components
Appl. Sci. 2016, 6(2), 33; https://doi.org/10.3390/app6020033 - 25 Jan 2016
Cited by 1
Abstract
Additive manufactured components have a different metallurgic structure and are more prone to fatigue cracks than conventionally produced metals. In earlier papers, an effective Structural Health Monitoring solution was presented to detect fatigue cracks in additive manufactured components. Small subsurface capillaries are embedded [...] Read more.
Additive manufactured components have a different metallurgic structure and are more prone to fatigue cracks than conventionally produced metals. In earlier papers, an effective Structural Health Monitoring solution was presented to detect fatigue cracks in additive manufactured components. Small subsurface capillaries are embedded in the structure and pressurized (vacuum or overpressure). A crack that initiated at the component’s surface will propagate towards the capillary and finally breach it. One capillary suffices to inspect a large area of the component, which makes it interesting to locate the crack on the basis of the pressure measurements. Negative pressure waves (NPW) arise from the abrupt encounter of high pressure fluid with low pressure fluid and can serve as a basis to locate the crack. A test set-up with a controllable leak valve was built to investigate the feasibility of using NPW to localize a leak in closed tubes with small lengths. Reflections are expected to occur at the ends of the tube, possibly limiting the localization accuracy. In this paper, the results of the tests on the test set-up are reported. It will be shown that the crack could be localized with high accuracy (millimeter accuracy) which proves the concept of crack localization on basis of NPW in a closed tube of small length. Full article
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Open AccessArticle
Lamb Wave Interaction with Adhesively Bonded Stiffeners and Disbonds Using 3D Vibrometry
Appl. Sci. 2016, 6(1), 12; https://doi.org/10.3390/app6010012 - 07 Jan 2016
Cited by 12
Abstract
There are many advantages to adhesively bonding stiffeners onto aircraft structures rather than using traditional mechanical fastening methods. However there is a lack of confidence of the structural integrity of adhesively bonded joints over time. Acousto-ultrasonic Lamb waves have shown great potential in [...] Read more.
There are many advantages to adhesively bonding stiffeners onto aircraft structures rather than using traditional mechanical fastening methods. However there is a lack of confidence of the structural integrity of adhesively bonded joints over time. Acousto-ultrasonic Lamb waves have shown great potential in structural health monitoring applications in both metallic and composite structures. This paper presents an experimental investigation of the use of acousto-ultrasonic Lamb waves for the monitoring of adhesively bonded joints in metallic structures using 3D scanning laser vibrometry. Two stiffened panels were manufactured, one with an intentional disbonded region. Lamb wave interaction with the healthy and disbonded stiffeners was investigated at three excitation frequencies. A windowed root-mean-squared technique was applied to quantify where Lamb wave energy was reflected, attenuated and transmitted across the structure enabling the size and shape of the defect to be visualised which was verified by traditional ultrasonic inspection techniques. Full article
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Open AccessArticle
Acoustic Emission Activity for Characterizing Fracture of Marble under Bending
Appl. Sci. 2016, 6(1), 6; https://doi.org/10.3390/app6010006 - 24 Dec 2015
Cited by 6
Abstract
The present paper occupies with the acoustic emission (AE) monitoring of fracture of marble. The specimens belong to two different material types and were tested in three-point bending after being ultrasonically interrogated. Consequently, they were repaired by means of suitable epoxy agent and [...] Read more.
The present paper occupies with the acoustic emission (AE) monitoring of fracture of marble. The specimens belong to two different material types and were tested in three-point bending after being ultrasonically interrogated. Consequently, they were repaired by means of suitable epoxy agent and mechanically re-loaded. Apart from the well-known correlation of pulse velocity to strength, which holds for the materials of this study as well, AE provides some unique insight in the fracture of the media. Parameters like the frequency content of the waveforms, and their duration among others show a transition in relation to the load. According to their strength class, the specimens exhibit distinct AE characteristics even at low load, enabling to judge their final strength class after having sustained just a small percentage of their ultimate capacity. More importantly, the AE activity during reloading indicates the quality of repair; specimens with good restoration of strength, exhibited similar AE activity to the intact specimens, while specimens with lower repaired capacity exhibited random behavior. This work discusses the passive monitoring of marble fracture and shows that AE parameters that have been used to successfully characterize cementitious materials, provide good results in monolithic materials like marble as well. It is suggested that AE monitoring during a proof loading can provide good information on the potential strength class, which is especially useful for repaired specimens, where the pulse velocity cannot be easily used. Full article
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Open AccessArticle
Electromagnetic Acoustic Transducers Applied to High Temperature Plates for Potential Use in the Solar Thermal Industry
Appl. Sci. 2015, 5(4), 1715-1734; https://doi.org/10.3390/app5041715 - 11 Dec 2015
Cited by 5
Abstract
Concentrated Solar Plants (CSPs) are used in solar thermal industry for collecting and converting sunlight into electricity. Parabolic trough CSPs are the most widely used type of CSP and an absorber tube is an essential part of them. The hostile operating environment of [...] Read more.
Concentrated Solar Plants (CSPs) are used in solar thermal industry for collecting and converting sunlight into electricity. Parabolic trough CSPs are the most widely used type of CSP and an absorber tube is an essential part of them. The hostile operating environment of the absorber tubes, such as high temperatures (400–550 °C), contraction/expansion, and vibrations, may lead them to suffer from creep, thermo-mechanical fatigue, and hot corrosion. Hence, their condition monitoring is of crucial importance and a very challenging task as well. Electromagnetic Acoustic Transducers (EMATs) are a promising, non-contact technology of transducers that has the potential to be used for the inspection of large structures at high temperatures by exciting Guided Waves. In this paper, a study regarding the potential use of EMATs in this application and their performance at high temperature is presented. A Periodic Permanent Magnet (PPM) EMAT with a racetrack coil, designed to excite Shear Horizontal waves (SH0), has been theoretically and experimentally evaluated at both room and high temperatures. Full article
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Open AccessArticle
Correlation between Earthquakes and AE Monitoring of Historical Buildings in Seismic Areas
Appl. Sci. 2015, 5(4), 1683-1698; https://doi.org/10.3390/app5041683 - 08 Dec 2015
Cited by 5
Abstract
In this contribution a new method for evaluating seismic risk in regional areas based on the acoustic emission (AE) technique is proposed. Most earthquakes have precursors, i.e., phenomena of changes in the Earth’s physical-chemical properties that take place prior to an earthquake. [...] Read more.
In this contribution a new method for evaluating seismic risk in regional areas based on the acoustic emission (AE) technique is proposed. Most earthquakes have precursors, i.e., phenomena of changes in the Earth’s physical-chemical properties that take place prior to an earthquake. Acoustic emissions in materials and earthquakes in the Earth’s crust, despite the fact that they take place on very different scales, are very similar phenomena; both are caused by a release of elastic energy from a source located in a medium. For the AE monitoring, two important constructions of Italian cultural heritage are considered: the chapel of the “Sacred Mountain of Varallo” and the “Asinelli Tower” of Bologna. They were monitored during earthquake sequences in their relative areas. By using the Grassberger-Procaccia algorithm, a statistical method of analysis was developed that detects AEs as earthquake precursors or aftershocks. Under certain conditions it was observed that AEs precede earthquakes. These considerations reinforce the idea that the AE monitoring can be considered an effective tool for earthquake risk evaluation. Full article
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Open AccessArticle
Monitoring Techniques of Cerium Stabilized Zirconia for Medical Prosthesis
Appl. Sci. 2015, 5(4), 1665-1682; https://doi.org/10.3390/app5041665 - 08 Dec 2015
Cited by 9
Abstract
The purpose of this paper is to emphasize the improvement of Zr-based ceramics properties as a function of addition of Ce ions in the structure of the original ceramics. The structural investigations proposed in this paper cover X-ray, and neutron diffraction offered the [...] Read more.
The purpose of this paper is to emphasize the improvement of Zr-based ceramics properties as a function of addition of Ce ions in the structure of the original ceramics. The structural investigations proposed in this paper cover X-ray, and neutron diffraction offered the first indication of the variation of the phase composition and the structural parameters, micro-hardness measurements as well as non-destructive evaluations in order to analyze the structural properties of these materials with utmost importance in fields such as medicine, where these composite materials are used in hip-implants or dental implants/coatings. In combination of Resonant Ultrasound Spectroscopy, which makes use of the resonance frequencies corresponding to the normal vibrational modes of a solid in order to evaluate the elastic constants of the materials, we emphasize a unique approach on evaluating the physical properties of these ceramics, which could help in advancing the understanding of properties and applications in medical fields. Full article
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Open AccessArticle
Enhancement of Spatial Resolution Using a Metamaterial Sensor in Nondestructive Evaluation
Appl. Sci. 2015, 5(4), 1412-1430; https://doi.org/10.3390/app5041412 - 27 Nov 2015
Cited by 7
Abstract
The current stage of non-destructive evaluation techniques imposes the development of new electromagnetic methods that are based on high spatial resolution and increased sensitivity. Printed circuit boards, integrated circuit boards, composite materials with polymeric matrix containing conductive fibers, as well as some types [...] Read more.
The current stage of non-destructive evaluation techniques imposes the development of new electromagnetic methods that are based on high spatial resolution and increased sensitivity. Printed circuit boards, integrated circuit boards, composite materials with polymeric matrix containing conductive fibers, as well as some types of biosensors are devices of interest in using such evaluation methods. In order to achieve high performance, the work frequencies must be either radiofrequencies or microwaves. At these frequencies, at the dielectric/conductor interface, plasmon polaritons can appear, propagating between conductive regions as evanescent waves. Detection of these waves, containing required information, can be done using sensors with metamaterial lenses. We propose in this paper the enhancement of the spatial resolution using electromagnetic methods, which can be accomplished in this case using evanescent waves that appear in the current study in slits of materials such as the spaces between carbon fibers in Carbon Fibers Reinforced Plastics or in materials of interest in the nondestructive evaluation field with industrial applications, where microscopic cracks are present. We propose herein a unique design of the metamaterials for use in nondestructive evaluation based on Conical Swiss Rolls configurations, which assure the robust concentration/focusing of the incident electromagnetic waves (practically impossible to be focused using classical materials), as well as the robust manipulation of evanescent waves. Applying this testing method, spatial resolution of approximately λ/2000 can be achieved. This testing method can be successfully applied in a variety of applications of paramount importance such as defect/damage detection in materials used in a variety of industrial applications, such as automotive and aviation technologies. Full article
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Open AccessArticle
Dispersion of Functionalized Silica Micro- and Nanoparticles into Poly(nonamethylene Azelate) by Ultrasonic Micro-Molding
Appl. Sci. 2015, 5(4), 1252-1271; https://doi.org/10.3390/app5041252 - 17 Nov 2015
Cited by 5
Abstract
Ultrasound micro-molding technology has proved useful in processing biodegradable polymers with minimum material loss. This makes this technology particularly suitable for the production of biomedical microdevices. The use of silica (SiO2) nanoparticles is also interesting because of advantages like low cost [...] Read more.
Ultrasound micro-molding technology has proved useful in processing biodegradable polymers with minimum material loss. This makes this technology particularly suitable for the production of biomedical microdevices. The use of silica (SiO2) nanoparticles is also interesting because of advantages like low cost and enhancement of final properties. Evaluation of the capacity to create a homogeneous dispersion of particles is crucial. Specifically, this feature was explored taking into account micro- and nano-sized silica particles and a biodegradable polyester derived from 1,9-nonanodiol and azelaic acid as a matrix. Results demonstrated that composites could be obtained with up to 6 wt. % of silica and that no degradation occurred even if particles were functionalized with a compatibilizer like (3-aminopropyl) triethoxysilane. Incorporation of nanoparticles should have a great influence on properties. Specifically, the effect on crystallization was evaluated by calorimetric and optical microscopy analyses. The overall crystallization rate was enhanced upon addition of functionalized silica nanospheres, even at the low percentage of 3 wt. %. This increase was mainly due to the ability of nanoparticles to act as heterogeneous nuclei during crystallization. However, the enhancement of the secondary nucleation process also played a significant role, as demonstrated by Lauritzen and Hoffmann analysis. Full article
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Open AccessArticle
Design of a Stability Augmentation System for an Unmanned Helicopter Based on Adaptive Control Techniques
Appl. Sci. 2015, 5(3), 575-586; https://doi.org/10.3390/app5030575 - 11 Sep 2015
Cited by 2
Abstract
The task of control of unmanned helicopters is rather complicated in the presence of parametric uncertainties and measurement noises. This paper presents an adaptive model feedback control algorithm for an unmanned helicopter stability augmentation system. The proposed algorithm can achieve a guaranteed model [...] Read more.
The task of control of unmanned helicopters is rather complicated in the presence of parametric uncertainties and measurement noises. This paper presents an adaptive model feedback control algorithm for an unmanned helicopter stability augmentation system. The proposed algorithm can achieve a guaranteed model reference tracking performance and speed up the convergence rates of adjustable parameters, even when the plant parameters vary rapidly. Moreover, the model feedback strategy in the algorithm further contributes to the improvement in the control quality of the stability augmentation system in the case of low signal to noise ratios, mainly because the model feedback path is noise free. The effectiveness and superiority of the proposed algorithm are demonstrated through a series of tests. Full article
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Acoustic Emissions to Measure Drought-Induced Cavitation in Plants
Appl. Sci. 2016, 6(3), 71; https://doi.org/10.3390/app6030071 - 04 Mar 2016
Cited by 6
Abstract
Acoustic emissions are frequently used in material sciences and engineering applications for structural health monitoring. It is known that plants also emit acoustic emissions, and their application in plant sciences is rapidly increasing, especially to investigate drought-induced plant stress. Vulnerability to drought-induced cavitation [...] Read more.
Acoustic emissions are frequently used in material sciences and engineering applications for structural health monitoring. It is known that plants also emit acoustic emissions, and their application in plant sciences is rapidly increasing, especially to investigate drought-induced plant stress. Vulnerability to drought-induced cavitation is a key trait of plant water relations, and contains valuable information about how plants may cope with drought stress. There is, however, no consensus in literature about how this is best measured. Here, we discuss detection of acoustic emissions as a measure for drought-induced cavitation. Past research and the current state of the art are reviewed. We also discuss how the acoustic emission technique can help solve some of the main issues regarding quantification of the degree of cavitation, and how it can contribute to our knowledge about plant behavior during drought stress. So far, crossbreeding in the field of material sciences proved very successful, and we therefore recommend continuing in this direction in future research. Full article
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