Air-filled Voids Research Articles

Elastoacoustic wave propagation in a biphasic mechanical metamateriala).

Humans are sensitive to air-borne sound as well as to mechanical vibrations propagating in solids in the frequency range below 20 kHz. Therefore, the development of multifunctional filters for both vibration reduction and sound insulation within the frequency range of human sensitivity is a research topic of primary interest. In this paper, a high-contrast biphasic mechanical metamaterial, composed of periodic elastic solid cells with air-filled voids, is presented. By opening intercellular air-communicating channels and introducing channel-bridging solid-solid couplings, the frequency dispersion spectrum of the metamaterial can be modified to achieve complete and large bandgaps for acoustic and elastic waves. From a methodological viewpoint, the eigenproblem governing the free wave propagation is solved using a hybrid analytical-computational technique, while the waveform classification is based on polarization factors expressing the fraction of kinetic and elastic energies stored in the solid and fluid phases. Based on these theoretical results, a mechanical metafilter consisting of an array of a finite number of metamaterial cells is conceived to provide a technical solution for engineering applications. The forced response of the metafilter is virtually tested in a computational framework to assess its performance in passively controlling the propagation of broadband sound and vibration signals within solid and fluid environments. Quantitative results synthesized by transmission coefficients demonstrate that the metafilter can remarkably reduce the transmitted response in the frequency band of human sensitivity.

Tailoring the Mass Density of 3D Printing Materials for Accurate X-ray Imaging Simulation by Controlled Underfilling for Radiographic Phantoms.

Additive manufacturing and 3D printing allow for the design and rapid production of radiographic phantoms for X-ray imaging, including CT. These are used for numerous purposes, such as patient simulation, optimization of imaging procedures and dose levels, system evaluation and quality assurance. However, standard 3D printing polymers do not mimic X-ray attenuation properties of tissues like soft, adipose, lung or bone tissue, and standard materials like liquid water. The mass density of printing polymers-especially important in CT-is often inappropriate, i.e., mostly too high. Different methods can be applied to reduce mass density. This work examines reducing density by controlled underfilling either realized by using 3D printing materials expanded through foaming during heating in the printing process, or reducing polymer flow to introduce microscopic air-filled voids. The achievable density reduction depends on the base polymer used. When using foaming materials, density is controlled by the extrusion temperature, and ranges from 33 to 47% of the base polymer used, corresponding to a range of -650 to -394 HU in CT with 120 kV. Standard filaments (Nylon, modified PLA and modified ABS) allowed density reductions by 20 to 25%, covering HU values in CT from -260 to 77 (Nylon), -230 to -20 (ABS) and -81 to 143 (PLA). A standard chalk-filled PLA filament allowed reproduction of bone tissue in a wide range of bone mineral content resulting in CT numbers from 57 to 460 HU. Controlled underfilling allowed the production of radiographic phantom materials with continuously adjustable attenuation in a limited but appropriate range, allowing for the reproduction of X-ray attenuation properties of water, adipose, soft, lung, and bone tissue in an accurate, predictable and reproducible manner.

Plasma Dynamics-Based Modeling and Analysis of Partial Discharge in Voids Inside Insulation of Power Transformer

Partial discharge (PD) modeling in voids is being studied; earlier, however, most studies considered a simplified model with many adjustable parameters. In this article, a comprehensive comparative study is being done with possible void shapes in transformer insulation using a modified drift-diffusion model with single adjustable free parameter of the electron generation rate and considering the secondary ionization process. In simulation studies, plasma-dynamics-based models are used to better understand PD activity inside voids. A drift-diffusion model is used for investigating the PD phenomena inside air-filled voids of commonly found geometries, namely, cylindrical, spherical, and ellipsoidal, bounded by different insulating materials in power transformers. The model is applicable for all void shapes and material of insulation. PD inception field, residual field, space charge density, and other electrical properties are obtained, and all the possible parameters are incorporated in the model thus improving its accuracy. In this article, the effect of void shape and insulation material on the PD activity is obtained. The inception electric field at which the discharge initiates is found to be lowest in case of spherical void resulting in easy formation of plasma discharge. This study thus uncovers the physical mechanism of PD inside insulation thus helping in maintenance and optimum design of transformer insulation.

Evaluation of Void Defects behind Tunnel Lining through GPR forward Simulation.

Voids, a common defect in tunnel construction, lead to the deterioration of the lining structure and reduce the safety of tunnels. In this study, ground-penetrating radar (GPR) was used in tunnel lining void detection. Based on the finite difference time domain (FDTD) method, a forward model was established to simulate the process of tunnel lining void detection. The area of the forward image and the actual void area was analyzed based on the binarization method. Both the plain concrete and reinforced concrete lining with various sizes of air-filled and water-filled voids were considered. The rationality of the model was verified by measured data. It was observed that the response mode of voids can be hyperbolic, bowl-shaped, and strip-shaped, and this depends on the void's width. Compared with the air-filled voids, water filling increases the response range of the voids and produces a virtual image. Although the diffracted wave caused by a steel bar will bring about significant interference to the void response, the center position of the voids can be accurately located using 3D GPR.

Ultrasonic Quality Assurance at Magnesia Shotcrete Sealing Structures.

Engineered barriers are a key element to enable safe nuclear waste disposal. One method currently under research for their construction is magnesia concrete applied in a shotcrete procedure. In this study, the ultrasonic echo method is evaluated as a means for quality assurance. Imaging of internal structures (backwall, boreholes) and defects, such as delamination, has successfully been achieved in the shotcrete. Additionally, detailed information about the potential cause of selected reflectors are obtained by phase analysis. In several test blocks of various sizes, no consistent concrete section boundaries have been found by ultrasonic imaging, which was verified by subsequent drilling and complementary tests. An experiment with artificial defects imitating cracks, air-filled voids, and material with lower density has been challenging and shows the limitations of the current methods. Although significant defects, such as a large delamination, are reliably identified, several smaller defects are not identified. Generally, ultrasonic imaging provides a suitable base as a mean for quality assurance during and after the construction of sealing structures. However, further developments are required to enhance the reliability of the method and a full validation is still pending. Still, the method has potential to increase the safety of nuclear waste repositories.

Seasonal roost selection and activity of a remnant population of northern myotis in Pennsylvania.

The decline in northern myotis (Myotis septentrionalis) populations due to the disease white-nose syndrome (WNS) has led to the species receiving federal protection in the United States and Canada, requiring conservation of critical habitats. However, considerably more is known about summer habitat preferences of northern myotis compared to late summer through winter. Our goal was to describe the seasonal presence and habitat use of a remnant colony of northern myotis in central Pennsylvania. We radio-tagged 31 northern myotis and established 6 acoustic monitoring stations to document activity from 2017–2021. We found that roost trees used during the maternity season by reproductive females were occupied by bats during both summer (21 June–14 August) and autumn (15 August–31 October), indicating similar habitat use patterns between seasons. During this time, both males and females preferred to roost in dead and declining trees. No other variable influenced male use, but females also preferred trees located close to water and in forest stands with higher basal area than randomly located trees. Northern myotis with active transmitters never left the study area and were tracked to roosts until early November. During October and November, a female and male were tracked to an underground network of air-filled voids (the Milieu Souterrain Superficiel) we presume to be a hibernaculum. Northern myotis calls were recorded outside this roost between March and October, and bats were observed emerging from this roost during spring and autumn but not summer. Acoustic activity at this site exhibited a seasonal pattern that differed from acoustic activity near roost trees and foraging areas, with a peak of activity during late summer when northern myotis are known to swarm. These data show that northern myotis maternity roosts are used extensively outside of summer and may be vulnerable to forestry practices that occur even outside of the pup-rearing season. These data also support the growing evidence that some northern myotis hibernate outside of caves and mines.

Sustainable and Inexpensive Polydimethylsiloxane Sponges for Daytime Radiative Cooling.

Radiative cooling is an emerging cooling technology that can passively release heat to the environment. To obtain a subambient cooling effect during the daytime, chemically engineered structural materials are widely explored to simultaneously reject sunlight and preserve strong thermal emission. However, many previously reported fabrication processes involve hazardous chemicals, which can hinder a material's ability to be mass produced. In order to eliminate the hazardous chemicals used in the fabrication of previous works, this article reports a white polydimethylsiloxane (PDMS) sponge fabricated by a sustainable process using microsugar templates. By substituting the chemicals for sugar, the manufacturing procedure produces zero toxic waste and can also be endlessly recycled via methods widely used in the sugar industry. The obtained porous PDMS exhibits strong visible scattering and thermal emission, resulting in an efficient temperature reduction of 4.6 °C and cooling power of 43 W m−2 under direct solar irradiation. In addition, due to the air‐filled voids within the PDMS sponge, its thermal conductivity remains low at 0.06 W (m K)−1. This unique combination of radiative cooling and thermal insulation properties can efficiently suppress the heat exchange with the solar‐heated rooftop or the environment, representing a promising future for new energy‐efficient building envelope material.

Photothermocapillary Method for the Nondestructive Testing of Solid Materials and Thin Coatings.

The photothermocapillary (PTC) effect is a deformation of the free surface of a thin liquid layer on a solid material that is caused by the dependence of the coefficient of surface tension on temperature. The PTC effect is highly sensitive to variations in the thermal conductivity of solids, and this is the basis for PTC techniques in the non-destructive testing of solid non-porous materials. These techniques analyze thermal conductivity and detect subsurface defects, evaluate the thickness of thin varnish-and-paint coatings (VPC), and detect air-filled voids between coatings and metal substrates. In this study, the PTC effect was excited by a “pumped” Helium-Neon laser, which provided the monochromatic light source that is required to produce optical interference patterns. The light of a small-diameter laser beam was reflected from a liquid surface, which was contoured by liquid capillary action and variations in the surface tension. A typical contour produces an interference pattern of concentric rings with a bright and wide outer ring. The minimal or maximal diameter of this pattern was designated as the PTC response. The PTC technique was evaluated to monitor the thickness of VPCs on thermally conductive solid materials. The same PTC technique has been used to measure the thickness of air-filled delaminations between a metal substrate and a coating.

The Milieu Souterrain Superficiel as hibernation habitat for bats: implications for white-nose syndrome.

Recent studies have revealed that western populations of little brown bats (Myotis lucifugus) in North America exhibit different hibernation behavior than their eastern counterparts. Understanding these differences is essential for assessing the risk white-nose syndrome (WNS) poses to western bat populations. We used acoustic monitoring and radiotelemetry to study the overwintering behavior of little brown bats near Juneau, Alaska during 2011–2014. Our objectives were to identify the structures they use for hibernation, measure the microclimates within those structures, and determine the timing of immergence and emergence and the length of the hibernation season. We radiotracked 10 little brown bats to underground hibernacula dispersed along two ridge systems. All hibernacula were ≤ 24.2 km from where the bats were captured. Eight bats hibernated in the “Milieu Souterrain Superficiel” (MSS), a network of air-filled underground voids between the rock fragments found in scree (talus) deposits. Two bats hibernated in holes in the soil beneath the root system of a tree or stump (rootball). At least two hibernacula in the MSS were reused in subsequent years. Average MSS and rootball temperatures were warmer and more stable than ambient temperature and were well below the optimal growth range of the fungus that causes WNS. Temperatures in the MSS dropped below freezing, but MSS temperatures increased with depth, indicating bats could avoid subfreezing temperatures by moving deeper into the MSS. Relative humidity (RH) approached 100% in the MSS and under rootballs and was more stable than ambient RH, which also was high, but dropped substantially during periods of extreme cold. Acoustic monitoring revealed that bats hibernated by late October and began emerging by the second week of April; estimates of minimum length of the hibernation season ranged from 156 to 190 days. The cold temperatures, dispersed nature of the hibernacula, and close proximity of hibernacula to summering areas may slow the spread and reduce the impacts of WNS on local populations of little brown bats.

A cell-centric view of lung alveologenesis.

Lung alveologenesis, formation of the alveolar region, allows sufficient gas exchange surface to be packed inside the chest cavity yet with orderly connection to the trachea. The real-life alveolar region, however, bears little resemblance to idealized cartoons owing to its three-dimensional nature, nonuniform shape, and mostly air-filled void. This morphological complexity is matched by its cellular complexity-comprised of intermixed and often tangled cells of the epithelial, mesenchymal, endothelial, and immune lineages. Modern imaging, genetics, and genomics are shedding light on and updating traditional views of alveologenesis. Accordingly, this review describes a cell-centric 3-phase definition of alveologenesis and discusses its failure in diseases and possible reactivation during regeneration.

Applying HF and VHF/UHF Partial Discharge Detection for Distribution Transformer

This paper represents application of high frequency (HF) and very high frequency/ultrahigh frequency (VHF/UHF) partial discharge (PD) detection for a distribution transformer. A capacitive sensor is used to detect the HF electric field caused by charge transfer inside oil–paper insulation due to PD at the defect site, and an electromagnetic sensor or antenna is used for detecting electromagnetic PD transients in the air outside the investigated transformer in the near-field region. Three types of artificial PD sources in air and insulating liquid, which are corona discharge, surface discharge and air void discharge in pressboard, were investigated. Three identical distribution transformers were rated at 22 kV, 400 V and 50 kVA, and were designed and constructed. The first transformer was filled with mineral oil, the second was filled with natural ester and the third was filled with palm oil. The PD generated by the air-filled voids in the insulating papers and pressboards of these transformers with five different conditions were investigated, i.e., non-impregnated paper, impregnated paper for 3 hours, 6 hours, 9 hours and 12 hours. The impregnation process was done with 65°C liquid temperature, and the pressure in the oven was around 5 mbar. From the experimental results, it can be concluded that the electromagnetic PD transients radiated from the corona discharge of both high-voltage (HV) and low-voltage sides in the air are in the VHF range, and surface discharge frequency is extended up to the UHF range. For the PD in the insulating liquid, the phase resolved PD (PRPD) pattern in the HF range is a valuable tool to characterize the PD sources. The PD in an air-filled void inside the insulating paper of the mineral oil transformer is obviously different compared with those of the natural ester transformer and the palm oil transformer. For the manufacturing of distribution transformers in this research, it is found that after the paper insulation is dried out, the impregnation process for a period of 9 hours is suitable for improving the oil–paper insulation with an acceptable PD level. This paper is the cross-field application by applying the antenna and communication theory for detecting the discharge problems in HV equipment.

Modeling partial discharge phenomena

Measurements of partial discharge (PD) activity are a commonly used technique to diagnose the health of insulation material within high voltage plant. The definition of PD is relatively broad, it is generally defined as a localized or confined discharge within an insulating medium [1]. Due to the high electric breakdown strength of insulation materials, PDs typically occur within low density defects in the system. One family of defects which has received significant research attention is air-filled voids, typically spherical or cylindrical, surrounded by solid dielectric material. Experiments have typically used a ‘sandwich’ technique to fabricate cylindrical voids, using three slabs of material with a circular hole drilled into the central slab. The slabs are then pressed together and placed between parallel plate electrodes. A common dielectric material used to fabricate cylindrical voids is LDPE [2]. By injecting air through a syringe before the curing process spherical voids have been fabricated in epoxy resins [3,4] and silicone rubber [5].

GPR pattern recognition of shallow subsurface air voids

Countless subsurface voids in urban areas of cities threaten people’s lives and property. A workflow for automatically identifying subsurface voids from ground penetrating radar (GPR) data was developed in this study. The workflow consists of 3 stages: locating voids automatically from C-scans, then verifying voids from corresponding B-scans, and finally making judgements based upon the previous 2 sets of results. This study adopted 2 (Lai et al., 2016) approaches: approach 1 quantified the GPR response of air voids using forward modelling, while approach 2 used workflow prototyping and validation with inverse modelling. Forward simulations indicated that different ratios of void size to GPR signal footprint could result in a variety of patterns in B-scans: they can be hyperbolas, cross patterns, bowl shaped patterns and reverberations. With a database of void patterns of both C-scans and B-scans established, in approach 2 the workflow uses a pyramid pattern recognition method – with pixel value or gradient being used for feature identification – to search automatically for air-filled void responses in GPR data. The workflow was tested using 2 laboratory and field experiments and the results were promising. The constraint values proposed by the 2 experiments were validated with another site experiment. Given the huge workload involved in city-scale subsurface health inspections, a standardized workflow can help improve efficiency and effectiveness of subsurface void identification.

Buckling-induced reconfigurability in underwater acoustic scatterers

In this work, we explore switchable acoustic scattering from underwater particles via instability-induced internal pattern transformation in the 50 kHz–80 kHz frequency range. Our wavelength scale aqueous scatterer is designed based on modeling using the finite element method for a square lattice of air-filled voids within a shape memory polymer and is directly 3D printed. The structure undergoes a buckling transformation when subjected to an external deformation while simultaneously being heated. Through computational and experimental results, we demonstrate that the deformation state change leads to programmable acoustic transparency, or opacity, for the scattering particle. Underwater propagation experiments resolved in the near field illustrate that the switchable acoustic characteristics are frozen in the structure with rapid cooling after compression, and the initial acoustic state can be automatically recovered through reheating.

Detecting an underground tunnel by applying joint traveltime and waveform inversion

Underground tunnel and void detection is a challenging geophysical problem, and many methods have been proposed. Seismic techniques are promising because of the large seismic velocity contrasts between the air-filled void and the surrounding sediment and concrete. We apply a joint seismic traveltime and waveform inversion method to image a buried tunnel with concrete walls and a void space inside. The joint inversion images the top of the concrete tunnel as a high-velocity anomaly and the void space as a low-velocity anomaly. The location of the velocity anomalies predicted by the method agrees with the known location of the tunnel. As a comparison, the stand-alone full waveform inversion is also applied to the data. The first-arrival traveltime tomography shows weak nonlinearity but fails to image the hidden low-velocity layer. Full waveform inversion is able to image complex near-surface structures, but fitting waveforms may not honor the traveltime fit, especially when the data contain noise. Synthetic and real data tests show that the joint inversion method retains the advantages of both traveltime inversion and full waveform inversion and overcomes their respective drawbacks at the same time. The field example shows the joint inversion provides a better reconstruction of the high-velocity feature representing the top of the concrete wall in terms of its magnitude and location.

The Effect of Material Present in the Void for Thermography Inspection in Concrete

In the previous study by the author, a numerical model to predict the thermal contrasts resulting from subsurface voids (i.e. delaminations) in concrete under a given set of environmental conditions was developed using the finite element method. The model was verified using the experimental test data, and the results indicated that the model could be an effective tool to support the thermography inspection of the concrete. In this present study, the use of the verified model to evaluate the effects of materials present in the void created by the delamination expected to influence the detectability of the subsurface voids. The effect of this parameter on the thermal contrast developed on the surface above a subsurface delamination was assessed under a specific set of environmental conditions. The results indicated that air-filled void produced a significant thermal contrast compared to water-filled void, ice-filled void, and epoxy adhesive-filled void.

Study of wave propagation through porous lattices with buckling instabilities in an aqueous environment

Elastic lattices with cylindrical air-filled voids may undergo significant topology changes when deformed due to the buckling instabilities of the thin elastic ligaments between voids. Such phononic crystals are of interest to, for example, achieve auxetic behavior and create tunable stop bands that vary as a function of the deformation. Most studies reported in the literature by others focus on lattices made of soft elastomers, and the resulting nonlinear deformation or elastic wave propagation due to low-frequency vibrations in air. The current research focuses on transitioning these previously studied concepts on buckling lattices to an aqueous environment. Initial investigations presented here are for 3-D printed lattices comprised of rigid plastic and deformed by leveraging shape memory effects that result in softening and hardening of materials under heating and cooling, respectively. The use of rigid plastic addresses inherent difficulties that arise with soft elastomers, including high loss, fabrication at scales of interest, and appropriate frequency range for wave propagation. The present work focuses on numerical simulations and experimental measurements of wave propagation through finite, plastic lattices with air-filled voids in water. [Work Supported by the Office of Naval Research.]

Development and Characterisation of Gastroretentive Solid Dosage Form Based on Melt Foaming

Dosage forms with increased gastric residence time are promising tools to increase bioavailability of drugs with narrow absorption window. Low-density floating formulations could avoid gastric emptying; therefore, sustained drug release can be achieved. Our aim was to develop a new technology to produce low-density floating formulations by melt foaming. Excipients were selected carefully, with the criteria of low gastric irritation, melting range below 70°C and well-known use in oral drug formulations. PEG 4000, Labrasol and stearic acid type 50 were used to create metronidazole dispersion which was foamed by air on atmospheric pressure using in-house developed apparatus at 53°C. Stearic acid was necessary to improve the foamability of the molten dispersion. Additionally, it reduced matrix erosion, thus prolonging drug dissolution and preserving hardness of the moulded foam. Labrasol as a liquid solubiliser can be used to increase drug release rate and drug solubility. Based on the SEM images, metronidazole in the molten foam remained in crystalline form. MicroCT scans with the electron microscopic images revealed that the foam has a closed-cell structure, where spherical voids have smooth inner wall, they are randomly dispersed, while adjacent voids often interconnected with each other. Drug release from all compositions followed Korsmeyer-Peppas kinetic model. Erosion of the matrix was the main mechanism of the release of metronidazole. Texture analysis confirmed that stearic acid plays a key role in preserving the integrity of the matrix during dissolution in acidic buffer. The technology creates low density and solid matrix system with micronsized air-filled voids.

Direct in-situ observation of early age void evolution in sustainable cement paste containing fly ash or limestone

This paper uses fast X-ray Computed Tomography (fCT) to study the change in air-filled voids during hydration of portland cement pastes of different water to cement ratios and with fly ash and limestone additives between 30 min and 12 h of hydration with deionized and de-aired water. For the deionized samples, the total volume of the air-filled voids increased during the induction period and decreased as the acceleration period began and finally stayed constant. The samples with 20% replacement of fly ash and 20% replacement of limestone showed the same trends as the OPC samples but with a reduction in the observed air-filled void formation. The air-filled void distribution in these samples changed from a limited number of large voids at the beginning of the measurements to a system with a significantly greater number of uniformly distributed smaller voids. The changes in the air-filled voids were not significant in pastes made with 50% fly ash or with de-aired water in the same testing condition. This might be explained by differences in pore solution chemistry and a reduced amount of dissolved air in the mixing water. A mechanism is proposed for a change in ionic concentration in solution to cause release of the dissolved air within the mixing water.

Testing the effectiveness of an inverse Wenner-Schlumberger array for geoelectrical karst void reconnaissance, on the Swabian Alb high plain, new line Wendlingen–Ulm, southwestern Germany

On the Swabian Alb high plain, southwestern Germany, the new high-speed railway line Wendlingen–Ulm of the German Railway Company (Deutsche Bahn AG) is currently under construction. 2.5D electrical resistivity tomography (ERT) measurements (meaning the use of parallel 2D lines) were performed at the excavation bottom level in addition to the investigation program of the construction company including microgravimetric and seismic methods. The aim was to test the effectiveness of an inverse Wenner-Schlumberger array for exploring geological defective karst voids and other karstic features in a highly heterogeneous karstic environment. A complete 3D ERT survey (meaning the use of additional orthogonal cross-lines) was not carried out due to restrictive circumstances (terrain effect, anisotropic effect). The 2.5D ERT survey (ABEM Terrameter SAS 4000, inverse Wenner-Schlumberger array, six lines, 60 m line length, 0.75 m electrode spacing, lines oriented perpendicular to the long axis of an oval-shaped collapse doline and the most prominent voids, 1.5 m line spacing, smoothness-constrained least-squares/robust inversion, RMS error: 1.60–3.20% for a maximum of 5 iterations) provided resistivity values of marls and marlstones, a limestone bedrock, a collapse doline and of an air-filled cavity. A clear demarcation between the marls and marlstones and the limestone bedrock was only possible outside the area of influence of the collapse doline; the latter could not be exactly delineated from the adjoining geological units. In addition, one of three air-filled voids (open fractures, cavities) could be derived accurately in location and roughly in size and shape from the 2.5D ERT survey; the other two were not detected by means of ERT because of (i) its location beneath a high-resistive anomaly and (ii) its comparatively small maximum horizontal opening width of only 1 time the used inter-electrode spacing. Extensive direct probing investigations served for calibration and discussion. The 2.5D ERT measurements conducted with an inverse Wenner-Schlumberger array complemented the microgravimetric and seismic results of the previously conducted investigation program of the construction company and provided a contribution to karst void reconnaissance in a highly heterogeneous karstic environment like the Swabian Alb high plain, on the new line Wendlingen–Ulm.