Void Configuration Research Articles

An algorithm for virtual fabrication of air voids in asphalt concrete

Motivated by the virtual testing of asphalt concrete, the North Carolina State University research team has developed an algorithm to computationally generate air voids. After examining the X-ray tomographic images of real asphalt concrete microstructure, we concluded that the air void's shape and size are affected primarily by the surrounding local aggregate structure. Building on this observation, we developed an algorithm to generate random but representative air void configurations inside a given microstructure. By applying the algorithm to scanned aggregate structures, we show that the generated air voids not only look visually similar to actual air voids, but also are effective in capturing modulus reduction. The algorithm is included in a virtual aggregate structure generation framework, resulting in a streamlined virtual fabrication procedure for asphalt concrete that can qualitatively capture the effects of accelerated degradation due to the presence of air voids.

Gravitational entropy of cosmic expansion

AbstractWe apply a recent proposal to define “gravitational entropy” to the expansion of cosmic voids within the framework of nonperturbative General Relativity. By considering CDM void configurations compatible with basic observational constraints, we show that this entropy grows from post‐inflationary conditions towards a final asymptotic value in a late time fully nonlinear regime described by the Lemaître‐Tolman‐Bondi (LTB) dust models. A qualitatively analogous behavior occurs if we assume a positive cosmological constant consistent with a Λ‐CDM background model. However, the Λ term introduces a significant suppression of entropy growth with the terminal equilibrium value reached at a much faster rate. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evaluation of ductile fracture in sheet metal forming using the ellipsoidal void model

The ductile fracture in the simulation of sheet-metal-forming processes is evaluated by the ellipsoidal void model previously proposed by the author. In the present study, the simulation and experiment of the hole expansion test are performed using six types of metals. For an alloy, the relationship between prestrain and hole expansion ratio calculated using the ellipsoidal void configuration and ellipsoidal void shape and that calculated using the ellipsoidal void configuration and circular void shape agree with the relationship obtained experimentally. For a pure metal, the relationship between prestrain and hole expansion ratio calculated using the average void configuration and average void shape agrees with that obtained experimentally. Furthermore, the method of introducing prestrain to an as-rolled sheet is proposed, and the prestrain in this sheet is estimated.

Detection of multiple flaws in piezoelectric structures using XFEM and level sets

An iterative procedure to solve the inverse problem of detecting multiple voids in piezoelectric structure is proposed. In each iteration the forward problem is solved for various void configurations, and at each iteration, the mechanical and electrical responses of a piezoelectric structure is minimized at known specific points along the boundary to match the measured data. The Extended Finite Element method (XFEM) is employed for determining the responses as it allows the use of a fixed mesh for varying void geometries. The numerical method based on combination of classical shape derivative and of the level-set method for front propagation used in structural optimization is utilized to minimize the cost function. The results obtained show that this method is effectively able to determine the number of voids in a piezoelectric structure and its corresponding locations and shapes.

Numerical assessment of the effect of void morphology on thermo-mechanical performance of solder thermal interface material

The presence of voids in solder thermal interface material (STIM) layers affects the reliability and mechanical performance of formed solder joint. Previous studies suggest that the level of void effect depends not only on the size of the voids but also on the distribution and location of voids. In this work, a void morphology generating algorithm was used to generate representative volume elements (RVEs) depicting the seeming randomness of voids in a given STIM layer. The generated 2D RVEs were converted to 3D RVEs within a finite element modelling (FEM) environment and subsequently incorporated into the chip and heat spreader to complete the geometric model. The maximum damage site in the Sn-3Ag-0.5Cu (SAC305) solder joint as predicted by finite element analysis (FEA) of the geometric model showed a good qualitative agreement with experimental observations elsewhere. Further numerical assessment of the thermo-mechanical performance of SAC305 alloy as STIM layer due to the different generated void morphology was carried out. Results showed that solder voids can either influence the initiation or propagation of damage in the STIM layer, depending on the configuration, size and location of voids. While the small voids around the critical region of the solder joints appeared to enhance stress and strain localisation around the maximum damage site thus facilitating damage initiation; small voids also showed potentials of arresting damage propagation. In addition, results from this study indicated that void located in the surface of the solder joint, particularly voids at the solder/silicon die interface are more detrimental compared to void embedded in the middle of the solder layer. The innovative technique employed in this study to numerically generate realistic solder void morphologies would be beneficial to the solder voids modelling research community.

Plane stress local failure criterion for polycarbonate containing laser drilled microvoids

The deformation and fracture mechanisms of voided polymers and rubber-toughened polymers are an active research area where accurate fracture models still need to be proposed. The reason for the lack of accurate models stems from a lack of controlled experiments at the microscale, the scale of voids and cracks forming in polymers. In this paper, controlled experiments were carried out on thin polycarbonate sheets containing controlled arrays of laser microvoids, loaded in-situ under an optical microscope. Void dimensions were extracted experimentally and compared to a simple finite element model that was able to accurately predict macroscopic stress strain curves as well as the microscopic behavior in terms of void growth. It is shown that the local equivalent plastic strain at failure is independent of the void configuration studied and can therefore be used as a criterion for predicting fracture in voided polycarbonate under plane stress. We propose that it is not the intervoid spacing that controls fracture but the competition between intervoid strain localization and the spread of plasticity through the propagation of shear bands.

An ellipsoidal void model for the evaluation of ductile fracture in sheet metal forming

A void model that can be used for the evaluation of ductile fracture in the simulation of sheet metal-forming processes is proposed. This void model is an extension of the Thomason model of void coalescence, which is based on the internal necking of the matrix between voids. In sheet metal-forming processes, the material is subjected to a complex strain path. Hence, the prestrain should be considered for the evaluation of ductile fracture in the simulation of sheet metal-forming processes. In the present paper, the prestrain is first introduced in the proposed void model. The simulation of the hole expansion test, which is a fundamental material test in sheet metal forming, is performed while paying special attention to the void configuration and the void shape. Finally, the validity of the proposed void model is confirmed by comparing the simulation results with the experimental results.

Gender myth and the mind-city composite: from Plato’s Atlantis to Walter Benjamin’s philosophical urbanism

In the early twentieth century Walter Benjamin introduced the idea of epochal and ongoing progression in interaction between mind and the built environment. Since early antiquity, the present study suggests, Benjamin’s notion has been manifest in metaphors of gender in city-form, whereby edifices and urban voids have represented masculinity and femininity, respectively. At the onset of interaction between mind and the built environment are prehistoric myths related to the human body and to the sky. During antiquity gender projection can be detected in western perceptions linking natural and built environments, commencing with Plato’s Atlantis and his Myth of Er, and later as a likely import of the Chinese yin-yang mythology. Culminating with the Age of Discovery, alongside advances in experiential awareness of the Earth’s sphericity, respective feminine and masculine earmarks can be detected in early modern perspicacity of the Earth’s southern and northern hemispheres. Our conceptions of natural and built environments inherently continue to contain gender traits. Yet urban voids, as the feminine face of city-form, have been severely understated in the built environment. Through design and configuration of urban voids, allegories of femininity in city-form ought to be celebrated, not discarded.

Numerical study on thermal impacts of different void patterns on performance of chip-scale packaged power device

Chip scale package (CSP) technology offers promising solutions to package power device due to its relatively good thermal performance among other factors. Solder thermal interface materials (STIMs) are often employed at the die bond layer of a chip-scale packaged power device to enhance heat transfer from the chip to the heat spreader. Nonetheless, the presence of voids in the solder die-attach layer impedes heat flow and could lead to an increase in the peak temperature of the chip. Such voids which form easily in the solder joint during reflow soldering process at manufacturing stage are primarily occasioned by out-gassing phenomenon and defective metallisation. Apparently, the thermal consequences of voids have been extensively studied, but not much information exist on precise effects of different patterns of solder die-attach voids on the thermal performance of chip-level packaged power device. In this study, three-dimensional finite element analysis (FEA) is employed to investigate such effects. Numerical studies were carried out to characterise the thermal impacts of various voids configurations, voids depth and voids location on package thermal resistance and chip junction temperature. The results show that for equivalent voiding percentage, thermal resistance increases more for large coalesced void type in comparison to the small distributed voids configuration. In addition, the study suggests that void extending through the entire thickness of solder layer and voids formed very close to the heat generating area of the chip can significantly increase package thermal resistance and chip junction temperature. The findings of this study indicate that void configurations, void depth and void location are vital parameters in evaluating the thermal effects of voids.

Energetics of small hydrogen–vacancy clusters in bcc iron

Hydrogen may be trapped in voids in iron, leading to undesirable material properties. In this paper, the energetics of small hydrogen–vacancy clusters in body centered cubic iron are investigated. Results from two interatomic potentials are compared. We use molecular dynamics and Monte Carlo methods to find the minimum energy configurations of voids of up to ten vacancies containing up to 50 hydrogen atoms with ratios of hydrogen to vacancy of 10 or less. The formation energies and binding energies of defects to these clusters are calculated. Our results indicate that the hydrogen stabilizes bubbles by causing vacancies to be more tightly bound to clusters, while neighboring irons are less tightly bound. Hydrogen itself becomes less well bound to clusters as the inventory increases. The more physically relevant potential indicates a maximum supported ratio of hydrogen atoms to vacancies of about 4.

Stochastic micromechanical model of the deterioration of asphalt mixtures subject to moisture diffusion processes

AbstractThe deleterious effect of moisture in the microstructure of asphalt mixtures, usually referred to as moisture damage, has been recognized as a main cause of early deterioration of asphalt pavements. The initiation and evolution of moisture‐related deterioration is strongly influenced by the internal air void structure of asphalt mixtures. Despite its importance, the majority of works conducted on the micromechanical modeling of asphalt mixtures overlook the role of the air void structure, mainly because of its high complexity and variability. This paper explores the influence of air void variability on the performance of asphalt mixtures subjected to moisture diffusion processes. A stochastic modeling technique based on random field theory was used to generate internal distributions of physical and mechanical properties of the asphalt matrix of the mixture that depend on probable air voids distributions. The analysis was conducted by means of a coupled numerical micromechanical model of moisture damage. The results showed that the variability and distribution of air voids are decisive in determining the moisture‐dependent performance of asphalt mixtures. Furthermore, it was also shown that a stochastic characterization of the diverse air void configurations is a feasible and useful approach to better represent and understand mechanically related deterioration processes in asphalt mixtures. Copyright © 2010 John Wiley & Sons, Ltd.

A 3D stylized half-core CANDU benchmark problem

A 3D stylized half-core Canadian deuterium uranium (CANDU) reactor benchmark problem is presented. The benchmark problem is comprised of a heterogeneous lattice of 37-element natural uranium fuel bundles, heavy water moderated, heavy water cooled, with adjuster rods included as reactivity control devices. Furthermore, a 2-group macroscopic cross section library has been developed for the problem to increase the utility of this benchmark for full-core deterministic transport methods development. Monte Carlo results are presented for the benchmark problem in cooled, checkerboard void, and full coolant void configurations.

Assembled modules technology for site-specific prolonged delivery of norfloxacin

The aim of this research was to design and study norfloxacin (NFX) release in floating conditions from compressed hydrophilic matrices of hydroxypropylmethylcellulose (HPMC) or poly(ethylene oxide) (PEO). Module assembling technology for drug delivery system manufacturing was used. Two differently cylindrical base curved matrix/modules, identified as female and male, were assembled in void configuration by friction interlocking their concave bases obtaining a floating release system. Drug release and floatation behavior of this assembly was investigated. Due to the higher surface area exposed to the release medium, faster release was observed for individual modules compared to their assembled configuration, independently on the polymer used and concentration. The release curves analyzed using the Korsmeyer exponential equation and Peppas & Sahlin binomial equation showed that the drug release was controlled both by drug diffusion and polymer relaxation or erosion mechanisms. However, convective transport was predominant with PEO and at low content of polymers. NFX release from PEO polymeric matrix was more erosion dependent than HPMC. The assembled systems were able to float in vitro for up to 240 min, indicating that this drug delivery system of norfloxacin could provide gastro-retentive site-specific release for increasing norfloxacin bioavailability.

Assemblage of drug release modules: Effect of module shape and position in the assembled systems on floating behavior and release rate

The aim of this work was to study the clindamycin release kinetics from floating delivery systems consisting of two modules assembled in void configuration, according to the modified release technology platform known as Dome Matrix®. Two modules differently shaped, i.e., female and male, formulated as swellable matrices and containing clindamycin, were assembled by friction interlocking. Then, by stacking additional female modules without drug on the assembled two-module floating system, modulation of clindamycin release rate and kinetics was attained. The additional modules stacked on the assembled system acted as a transient barrier to clindamycin release from the void configuration. Inertness, dissolution/erosion or swelling behavior characterized their performance as matrices in simulated gastric fluid. In particular, we found that stacking additional barrier modules on the bases of void configuration, the drug release rate and kinetics of the assembled system were modified in dependence on the composition of module added. In particular, the quickly soluble module exerted an influence on the release rate in the late time of delivery. The swellable module produced a significant reduction in release rate of void assembly, but the release mechanism remained the same. Finally, the inert module led to a substantial linearization of the release profile with a minimal reduction in release rate.

Tapioca starch graft copolymers and Dome Matrix® modules II. Effect of modules assemblage on Riboflavin release kinetics

This paper studies the Riboflavin release from systems made of assembled modules of Dome Matrix® technology using tapioca starch-ethylmethacrylate (TSEMA) and tapioca hydroxypropylstarch-ethylmethacrylate (THSEMA) graft copolymers produced by two different drying methods. Two different shape modules were manufactured for this study, i.e., female and male modules, in order to facilitate their assemblage in “void configuration”, a system with an internal void space. Drug release studies on void configurations based on THSEMA show faster releases than TSEMA; HPMC systems used as a comparative reference showed intermediate release. Moreover, using void configurations made with one module of TSEMA and the other of THSEMA is possible to average the drug release, without difference between the drying methods used for the polymers. With respect to the floatation characteristics, all the void configurations floated immediately and, due to the mass center of the system, the floatation position of the system was always axial with the female module up and the male down. The drug release studies performed with a sinker to force the immersion of the systems in the medium did not show differences with respect to the dissolution test without a sinker. The combination of floatation capability of the assembled modules and the prolonged drug release provided with the graft copolymers make these assembled modules candidates as controlled release gastro-retentive dosage forms.

Void shape control in GaN re-grown on hexagonally patterned mask-less GaN

We present the results of GaN re-growth on hexagonally patterned GaN templates. Sapphire was used as the original substrate and the samples were grown by metalorganic vapor phase epitaxy (MOVPE). The re-growth on the patterned templates results in the formation of voids at the GaN/sapphire interface. Our extensive scanning electron microscopy (SEM)-based experimental investigations show that the void shape can be controlled from nearly vertical to fully inclined configurations. It was found that the initial hexagon hole diameter plays a key role in determining the final profile of the void sidewalls. X-ray diffraction analysis of the GaN layers indicates that the layers with inclined sidewall voids have an improved crystalline quality. Knowledge of the void configurations in the GaN layers and a possibility to control their shape can help in enhancing light extraction from the light emitting structures.

Artesunate-clindamycin multi-kinetics and site-specific oral delivery system for antimalaric combination products

The aim of this work was to study a multi-kinetics and site-specific oral antimalaria drug delivery system (MKS_DDS), containing artesunate and clindamycin, based on the Dome Matrix® module assembly technology. The MKS_DDS assembled system comprises of four modules, i.e., two controlled release (CR) modules for delivery of 160 mg of clindamycin phosphate, one immediate release module containing 50 mg of artesunate and one immediate release module containing 80 mg of clindamycin phosphate. These modules have been assembled in stacked and void configurations. The void configuration is able to float and showed gastro-retentive behavior. The MKS_DDS was investigated for its mechanical characteristics, system behavior during release, drug release rate and mechanism. A bioavailability study (dogs) showed that the clindamycin plasma curve of the MKS_DDS system exhibited a quasi constant release rate, up to 8 h. The MKS_DDS system containing clindamycin and artesunate allows the use of one tablet containing one immediate release dose of artesunate and of clindamycin and a portion of clindamycin released over a prolonged time, by exploiting the gastro-retentive properties of a floating system.

The degenerate scale problem for the Laplace equation and plane elasticity in a multiply connected region with an outer circular boundary

This paper investigates the degenerate scale problem for the Laplace equation and plane elasticity in a multiply connected region with an outer circular boundary. Inside the boundary, there are many voids with arbitrary configurations. The problem is analyzed with a relevant homogenous BIE (boundary integral equation). It is assumed that all the inner void boundary tractions are equal to zero, and tractions on the outer circular boundary are constant. Therefore, all the integrations in BIE are performed on the outer circular boundary only. By using the relation z * conjg( z) = a * a, or conjg( z) = a * a/ z on the circular boundary with radius a, all integrals can be reduced to an integral for complex variable and they can be integrated in closed form. The degenerate scale a = 1 is found in the Laplace equation and in plane elasticity regardless of the void configuration.

Microscopic mechanism of void coalescence under shock loading

The influence of void configuration on void coalescence in single crystal copper under shock loading along ［100］ direction has been investigated with molecular dynamics （MD） simulation. The results reveal that the voids collapse and grow by means of emission of shear dislocation loops. In the tension stage, the voids first grow independently, then the plastic zone around the voids begin to interact and overlap, leading to the void coalescence. The pattern of void coalescence observed in our simulations coincides with the microscopic experimental results. We calculated for four different configurations, characterized by the angle （θ） formed by shock direction and the line connecting the two centers of voids, and found that among the four configurations，the coalescence of the voids prefers to occur where θ is 60°. Based on the resolved shear stress model around the void, we can clearly explain the simulation results.

Module assemblage technology for floating systems: In vitro flotation and in vivo gastro-retention

The aim of this research was to study, in vitro by resultant-weight measurement and in vivo by γ-scintigraphy experiments in humans, the floatation behavior of systems obtained by modules assembled in void configuration. The assembled system technology allowed the manufacturing of a system characterized by the presence of an internal void space that provided an apparent density lower than water. The gastro-retention times of floating assembled systems were determined in comparison with non-floating systems having the same mass and composition. In vitro the floatation of the system started immediately after immersion in water and lasted for more than 5 h. The in vivo studies confirmed that the in vitro floating ability of void configuration was maintained also in the human stomach where the system stayed for periods of time ranging from 2.5 to 5.0 h, depending on the food regimen and the sex of the subject. Reiterate eating and drinking further prolonged the stomach residence time.