Spherical Voids Research Articles

The Effect of Protrusions on the Initiation of Partial Discharges in XLPE High Voltage Cables

This paper is focusing on 3D Finite Elements Analysis (FEA) based modelling of protrusions as defects or imperfections in the XLPE high voltage cable. This study is aiming to examine the impact protrusions have on the initiation of partial discharges. Spherical and ellipsoidal protrusions with different sizes at the conductor screen of the high voltage cable is an essential content of this paper. In addition, a spherical gas-filled void is placed inside and outside the protrusions, and a water tree produced from protrusions is under consideration. The partial discharge influence taking place at the protrusions and the stress enhancement factor is determined for all the variations mentioned to quantify the rise in the inception of partial discharges due to the protrusions.

An investigation into the porosity of extrusion-based 3D printed concrete

Concrete structures additively manufactured by extrusion-based 3D concrete printing are reportedly orthotropic in mechanical behavior and exhibit relative weakness in interfacial regions. Microstructure, including porosity content, 3D porosity distribution and pore morphology presents a physical basis for these phenomena. Here, a first and comprehensive microstructural investigation is reported, using X-ray computed tomography to visualize and quantify porosity, pore sizes, shapes and distributions in extrusion-based 3D printed concrete. 3D printed plastic molds are used to sample specimens from freshly 3D printed concrete filaments, for minimum disturbance. As reference, similar specimens of the exact same concrete mix, but cast without compaction, instead of being 3D printed are included in the study. A fixed dimeter of 20 mm, but varying height is used to include a single filament layer (10 mm), two layers (20 mm) and four layers (40 mm). Both typical horizontal interfaces in multi-layer elements, and vertical interfaces between multilaterally deposited filaments are studied. Whilst a single 3D printable concrete mix are considered, print variables of pass time (0–60 min with 15 min intervals) and print speed (80, 100 and 120 mm/s) are considered to investigate their potential alteration of the microstructure. Findings are significant, indicating tri-axial spheroid shaped air voids present in printed specimens, elongated and flat in the print direction, compared to mostly spherical voids in cast specimens. This prompts for more research to be conducted into the effect of stress concentrations at micro-cracks or voids in 3D printed concrete, which especially impacts mechanical behavior. Furthermore, it is found that vertical and horizontal interlayers comprise of similar porosity, and that it is inaccurate to qualify the homogeneity of typically fissile 3D printed concrete elements based solely on superficial cross-sectional photographic evidence from saw-cut samples.

Strong Shock in the Uniformly Expanding Universe with a Spherical Void

Propagation of strong shock wave in the expanding universe is studied using approximate analytic, and exact numerical solution of self-similar equations. Both solutions have similar properties, which change qualitatively, depending on the adiabatic powers $\gamma$. In the interval $1<\gamma<\gamma_{cr} \sim 1.16$ analytic and numeric solutions fill all the space without any voids and they are rather close to each other. At larger $\gamma>\gamma_{cr}$ a pressure becomes zero at finite radius, and a spherical void appears around the origin in both solutions. All matter is collected in thin layer behind the shock wave front. The structure of this layer qualitatively depends on $\gamma$. At the inner edge of the layer the pressure is always zero, but the density on this edge is jumping from zero to infinity at $\gamma \approx 1.4$ in both solutions.

Modeling the atomic structure in the vicinity of the spherical voids and calculation of void growth rate anisotropy in bcc iron and tungsten

We study the influence of the atomic structure in the vicinity of voids on their growth rate anisotropy. In the first part, we model the atomic structure in the vicinity of nanovoids in α-Fe and W using the advanced Molecular Statics method. In the second part, we use the earlier obtained equations that taking the influence of elastic fields into account to calculate the shifting rate of the void surface elements, and to evaluate the components of the strain tensor we use the atomic structure modeling results from the first part. The calculations are performed for voids of several sizes at certain oversaturation’s in a wide temperature range. The simulation results for the mentioned metals with a bcc structure show that displacements of atoms located along the crystallographic directions of the <100>, <110>, <111> types in the vicinity of the voids are significantly different, and this anisotropy of atom displacements leads to a reduction of spherical symmetry for the shifting rate of the surface elements. As the result, the initially spherical void shape becomes faceted.

On the Mechanical Behaviour of Biosourced Cellular Polymer Manufactured Using Fused Deposition Modelling.

The aim of this study is to investigate on the compression performance of cellular Polylactic Acid (PLA) manufacturing while using Fused Deposition Modelling. Computer Aided Design (CAD) models of cellular structures are designed using the sequential addition of spherical voids with porosity content varying from 10% to 60%. The three-dimensional (3D) microstructures of cellular PLA are characterised using X-ray micro-tomography to retrieve the correlation between the process-induced defects and the cellular geometrical properties. Mechanical testing is performed under severe compression conditions allowing for the reduction in sample height up to 80%. Finite element computation that is based on real microstructures is used in order to evaluate the effect of defects on the compression performance. The results show a significant drop of the process-induced defects thanks to the use of small layer thickness. Both mechanical anisotropy and performance loss are reduced due to vanishing process-induced defects more significantly when the amount of intended porosities is large. The compression behaviour of 3D printed PLA cellular structures is then found to be only guided by the amount and distribution of the intended porosity.

Voronoi Polyhedra as a Tool for the Characterization of Inhomogeneous Distribution in 1-Butyl-3-methylimidazolium Cation-Based Ionic Liquids.

The inhomogeneity distribution in four imidazolium-based ionic liquids (ILs) containing the 1-butyl-3-methylimidazolium (C4mim) cation, coupled with tetrafluoroborate (BF4), hexafluorophosphate (PF6), bis(trifluoromethanesulfonyl)amide (TFSA), and trifluoromethanesulfonate (TfO) anions, was characterized using Voronoi polyhedra. For this purpose, molecular dynamic simulations have been performed on the isothermal-isobaric (NpT) ensemble. We checked the ability of the potential models to reproduce the experimental density, heat of vaporization, and transport properties (diffusion and viscosity) of these ionic liquids. The inhomogeneity distribution of ions around the ring, methyl, and butyl chain terminal hydrogen atoms of the C4mim cation was investigated by means of Voronoi polyhedra analysis. For this purpose, the position of the C4mim cation was described successively by the ring, methyl, and butyl chain terminal hydrogen atoms, while that of the anions was described by their F or O atom. We calculated the Voronoi polyhedra distributions of the volume, the density, and the asphericity parameters as well as that of the radius of the spherical intermolecular voids. We carried out the analysis in two steps. In the first step, both ions were taken into account. The calculated distributions gave information on the neighboring ions around a reference one. In the second step, to distinguish between like and oppositely charged ions and then to get information on the inhomogeneity distribution of the like ions, we repeated the same calculations on the same sample configurations and removed one of the ions and considered only the other one. Detailed analysis of these distributions has revealed that the ring hydrogen atoms are mainly solvated by the anions, while the methyl and butyl terminal H atoms are surrounded by like atoms. The extent of this inhomogeneity was assessed by calculating the cluster size distribution that shows that the dimers are the most abundant ones.

Quasi-brittle porous material: Simulated effect of stochastic air void structure on compressive strength

The effect of porosity comprised of spherical air voids on the compressive strength of quasi-brittle material was studied via simulations. The simulated porous structures were based on pore size distributions of two mortar samples measured by X-ray microtomography. While the simulation method set practical limits on the size of sample, the base of the statistics was established by simulating 128 small structures generated by sampling from pore structures of two mortars. By studying the application of the classical strength-porosity formulas to the simulated data, a new simple model was formed. A linear relationship was achieved between the cubic root of air void fraction (porosity) and the simulated compressive strength. The reasons for scattering of simulated strength around fitted trend remained unresolved in this study; no clear dependence on pore number or other distribution properties was observed. With the presented simulation approach, the dependence of compressive strength on porosity is achieved independently of disturbances that occur in experimental studies creating understanding of compressional behavior of low porosity materials.

Experimental and computational analysis of structure-property relationship in carbon fiber reinforced polymer composites fabricated by selective laser sintering

A fundamental understanding of structure-property relationship of carbon fiber reinforced polymer (CFRP) composites fabricated by selective laser sintering (SLS) is essential to provide guidance to improve the mechanical properties in this promising additive manufacturing process. In the present study, we propose a new computational framework to evaluate the mechanical behavior of SLSed CFRP based on the representative volume element (RVE) model. Firstly, a new voxel-based fiber packing algorithm is proposed with the three-dimensional fiber orientation tensor to describe the fiber distribution. The porosity generated in the manufacturing process is also considered by adding spherical voids in the matrix region. In order to remove the artificial stress concentration induced by the voxel-based algorithm, a python script is developed to regenerate the geometries of different phases with conforming mesh. Meanwhile, the constitutive models of different phases in SLSed CFRP, i.e., matrix, fiber and interface, are established according to the material characteristics. Next, the proposed computational framework of SLSed CFRP is validated by the X-ray computed tomography (XCT) measurements and tensile tests of a representative SLSed carbon fiber/polyamide 12 (CF/PA12) composite. The effects of fiber volume fraction and fiber orientation distribution on the mechanical behavior of SLSed CF/PA12 composite are further quantitatively explored and ranked with regards to their influence on stiffness and failure strength.

The predicted shapes of voids and Xe bubbles in [formula omitted

Morphology is a fundamental attribute when investigating voids and bubbles in UO2. This study uses molecular dynamics and Monte Carlo simulations to predict the lowest energy shapes for voids and bubbles in UO2. The energies of the {100},{110} and {111} surfaces have been calculated and used to predict the equilibrium void shape from Wulff construction. This equilibrium shape is compared to low energy faceted voids exhibiting different relative proportions of each family of terminating surfaces. It is found that the equilibrium Wulff shape does not represent the lowest energy morphology for nm void sizes at temperatures between 300 K and 1200 K. Furthermore, the lowest energy faceted voids are slightly more energetically favourable than spherical voids, and as Xe is added, and bubble pressure increases, the faceted morphology becomes even more favourable than the spherical shape.

Redshift-space effects in voids and their impact on cosmological tests. Part I: the void size function

ABSTRACT Voids are promising cosmological probes. Nevertheless, every cosmological test based on voids must necessarily employ methods to identify them in redshift space. Therefore, redshift-space distortions (RSD) and the Alcock–Paczyński effect (AP) have an impact on the void identification process itself generating distortion patterns in observations. Using a spherical void finder, we developed a statistical and theoretical framework to describe physically the connection between the identification in real and redshift space. We found that redshift-space voids above the shot noise level have a unique real-space counterpart spanning the same region of space, they are systematically bigger and their centres are preferentially shifted along the line of sight. The expansion effect is a by-product of RSD induced by tracer dynamics at scales around the void radius, whereas the off-centring effect constitutes a different class of RSD induced at larger scales by the global dynamics of the whole region containing the void. The volume of voids is also altered by the fiducial cosmology assumed to measure distances, this is the AP change of volume. These three systematics have an impact on cosmological statistics. In this work, we focus on the void size function. We developed a theoretical framework to model these effects and tested it with a numerical simulation, recovering the statistical properties of the abundance of voids in real space. This description depends strongly on cosmology. Hence, we lay the foundations for improvements in current models of the abundance of voids in order to obtain unbiased cosmological constraints from redshift surveys.

A model for the conduction shape factor in spherical void phase porous materials

This article presents a method for determining the conduction shape factor for complex porous media by performing pore-level simulations on the solid phase of mathematically-generated geometric models. The shape factor is a geometric property and does not depend on the thermophysical properties of the material or the interstitial fluid. The shape factor is established for several geometric variations of permeable, spherical-void-phase porous materials using a direct computational approach and it is shown that there is a dependency on both the porosity and pore diameter. We also show that the shape factor can be characterized by a quadratic expression that is in terms of porosity and mean pore diameter. The results are validated directly by comparing computations of conjugate heat transfer done using pore-level and volume-averaged approaches.

Yield criterion and finite strain behavior of random porous isotropic materials

The mechanical response of isotropic elastoplastic materials containing random distributions of initially spherical voids is investigated computationally based on Fast Fourier Transform simulations. Numerical limit-analysis simulations at constant stress triaxiality allow to determine the yield surfaces, leading in particular to the determination of a Representative Volume Element size for the onset of coalescence/inhomogeneous yielding. Moreover, two different coalescence regimes are observed that differ by the presence of shearing. The yield surfaces are found to be consistent with the combination of two models proposed in the literature, a GTN-type model calibrated for homogeneous yielding of random porous materials and an inhomogeneous yielding model accounting for both coalescence with or without shear. Finite strain simulations performed for different hardening moduli and stress triaxialities under axisymmetric loading conditions confirm the existence of a RVE up to the onset of inhomogeneous yielding. Coalescence strains are found to be significantly smaller for random porous materials than for periodic distribution of voids. A homogenized model is finally proposed that reproduces quantitatively the behavior of isotropic elastoplastic materials containing random distributions of voids under finite strains.

Metrological assessment of image quality in ultrasonic medical diagnostic equipment

This paper presents a model for estimating uncertainty of measurement of performance parameters commonly used in the evaluation of ultrasound images. These are axial and lateral resolutions, contrast detail, spherical void, dead zone, image uniformity, measurement system accuracy, display and recording errors, vertical and horizontal distance accuracy, and anechoic object perception. These parameters were measured by two technicians following the recommendations of document IEC/TS 61390:1996, using a phantom, and the model of uncertainty proposed based on the guide to the expression of uncertainty in measurement (BIPM JCGM 100:2008). Moreover, an evaluation was carried out after 1 year to assess changes in the equipment image quality tested. In the assessment of operator influence on measurements, it was found that there is no statistical evidence that measurement results are different among most of the parameters. The results of three parameters were considered different between the two operators: contrast-detail resolution, display and recording errors, and measurement system accuracy. When considering the normalized error in comparison to the measurements made by the same operator under the same conditions in two consecutive years, no parameter showed a difference, given the level of uncertainty practiced. It is noteworthy that the proposed model of uncertainty contributes as an unprecedented way for metrological evaluation of diagnostic imaging by ultrasound.

Effect of Void on Yielding Behaviors in a Bicrystalline Copper

Nanovoids make a big contribution to the plasticity and failure of nanocrystalline metallic materials. To develop a further understanding of the role played by spherical voids in governing the plasticity of nanocrystalline metals, molecular dynamics simulations are performed on bicrystalline copper models with a spherical void at room temperature under 1 × 108 s−1 strain rate. The simulation results indicate that the introduction of void lead to the reduction in yield stress with increasing void diameter. The spherical void acts as either a barrier or a source for dislocations. For intergranular spherical void, the dislocations are emitted from the intersections between the void and grain boundary (GB). However, the initial dislocation nucleation site transits from GBs to spherical void surface at a critical diameter of 9.5 nm in the models with intragranular void. According to the Lubarda model, the dislocation emission critical stress at intermediate void diameters is effectively predicted, but it is not applicable for the model with extremely large and small void since the factors of GB and grain orientation are not considered. Some more suitable models need to be developed.

Effect of sintering germanium epilayers on dislocation dynamics: From theory to experimental observation

High-quality germanium epilayers on Si with low threading-dislocation density were achieved by sintering of porous Ge/Si films. The process consists in the formation of porous Ge nanostructures by dislocation-selective electrochemical etching of Ge/Si films, followed by high-temperature treatment to generate a monocrystalline, voided Ge layer that intercepts dislocations and prevents them from propagating to the sample surface. In this work, we model the morphological changes that occur during the thermal treatment-induced surface diffusion of an axially symmetric hole. Simulations and experiments show individual large spherical voids, aligned along the dislocation core. The creation of voids could facilitate interactions between dislocations, enabling the dislocation network to change its connectivity in a way that facilitates the subsequent annihilation of dislocation segments. This confirms that thermally activated processes such as state diffusion of porous materials provide mechanisms whereby the defects are removed or arranged in configurations of lower energy. This demonstration paves the way to develop a virtual substrate on many other heterostructures for potential applications in integrated photonic and optoelectronic devices.

Void growth and morphology evolution during ductile failure in an FCC single crystal

Void growth and morphology evolution are studied using a 3D representative volume element with a spherical void embedded in an FCC single crystal. The plastic flow contours are studied to determine the scenarios leading to fully plastic flow and plastic flow with elastic region. Further, the effect of anisotropy on void growth is studied through three initial crystallographic orientations (ICOs) [100], [110], & [111] with respect to loading direction. Void growth and macroscopic stress variations with applied strain are obtained from our simulations. It is observed that the peak stress corresponds to rapid void growth initiation. The peak stress is found to be dependent on void volume fraction and ICO. Furthermore, an additional geometrical parameter, diagonal distortions $$(D_{d_{i}})$$ is introduced to classify the non-spheroidal void shapes observed in deformed anisotropic crystal.

Heat treatment possibilities for an in situ βTi-TiC composite made by laser powder bed fusion

After laser powder bed fusion (LPBF) of an ultra-strong in situ TiC whisker reinforced β-Ti composite, this paper investigates the evolution of microstructure and mechanical properties in response to heat treatment at different temperatures. Using in depth nano-SEM and TEM analyses, it is shown that ageing at 400 °C rounds the whiskers, annihilates the strain fields and grows Mo segregated nano-cells, but without improving the ductility. In contrast, ageing at 600 °C enables the transformation of metastable β to a lamellar β + α, leading to a dual phase matrix embedding TiC particles. This is in such a manner that extra ageing at 600 °C coalesces the nano-lamellar α + β microstructure to form a coarser micro-lamellar α + β matrix. This microstructure achieves 66 % of the compressive deformation of Cp-Ti, and over 1400 MPa compressive strength after 1 h of ageing at 600 °C. Despite this success under compression, hard and stiff TiC particles may still cause large spherical fractured voids, severely limiting the plastic deformation under tension.

Development process and testing of partial discharge detection device on medium voltage XLPE cable

High voltage assets play a vital role in providing uninterrupted power to the consumers and any slight problems experienced by the assets may cause losses in millions of dollars to businesses. Therefore it is of utmost importance to monitor the health of high voltage assets. This research presents the development process of a Partial Discharge (PD) device that is able to detect PD acoustic waves for monitoring high voltage assets purposes. Medium voltage Cross-Linked Polyethylene (XLPE) cable was used which was introduced with spherical void defects at the joints of the cable that functioned to produce PD acoustic waves. Outcome of the development processes provides the finished design of the PD sensing device, known as Partial Discharge Detection (PDD) device. The functionality of the PDD device was also assessed through controlled experimentations, and they proved to be successful. Pure PD waveform captured by the ultrasonic sensor was similar when compared to a HFCT sensor’s pure PD waveform. The PDD device is a small and affordable, and is opened to various improvements such as integrating Artificial Intelligence (AI) unto the device, and one day may replace most existing bulky and expensive PD sensing devices that are readily available in the market.

Prediction of forming limits for porous materials using void-size dependent model and bifurcation approach

The scientific literature has shown the strong effect of void size on material response. Several yield functions have been developed to incorporate the void size effects in ductile porous materials. Based on the interface stresses of the membrane around a spherical void, a Gurson-type yield function, which includes void size effects, is coupled with the bifurcation theory for the prediction of plastic strain localization. The constitutive equations as well as the bifurcation-based localization criterion are implemented into the finite element code ABAQUS/Standard within the framework of large plastic deformations. The resulting numerical tool is applied to the prediction of forming limit diagrams (FLDs) for an aluminum material. The effect of void size on the prediction of FLDs is investigated. It is shown that smaller void sizes lead to an increase in the ductility limits of the material. This effect on the FLDs becomes more significant for high initial porosity, due to the increase of void-matrix interface strength within the material.

Influence of pore size on the plastic deformation of c-axis-compressed magnesium single crystals

The molecular dynamics method is used to establish a single crystal model of magnesium with different void sizes. Uniaxial compression along the c-axis is carried out at 300 K. Combined with the stress–strain curve, potential energy curve and dislocation density curve of the four models, the compression mechanical energy and structural evolution process of a single crystal of magnesium with different hole sizes are analysed. Results show that when the radius of the single spherical void is large, the elastic modulus is small, the yield stress is low, the potential energy value is large, and the absolute value is small, such conditions facilitate deformation. When the hole radius is small, complete closure under c-axis compression requires minimal time and deformation.