Void Size Distribution Research Articles

Microstructural analysis of W irradiated at different temperatures

Pure W was neutron irradiated at 600 °C, 900 °C, 1000 °C, 1100 °C, and 1200 °C with a dose of ∼1 dpa, and its microstructure was subsequently analyzed using transmission electron microscopy (TEM). Three types of defects were observed and analyzed in detail: (i) voids, (ii) dislocation loops with a diameter of up to 20 nm and (iii) W-Re-Os containing precipitates. The TEM study includes a detailed examination of the defect structure, e.g., determination of the size and spatial distribution of voids, precipitates and dislocation loops. Application of g-imaging shows the formation of the interstitial loops with both ½<111> and 〈100〉 Burgers vectors. In addition, local chemical and structural analysis revealed the formation of χ-type precipitates, which are attached to the voids at 1100 °C. It was also found that voids, loops and precipitates are surrounded by a solid solution cloud enriched with Re and Os. The use of highly sensitive nanoscale chemical analysis allows the identification of differences in Re/Os segregation at and in the vicinity of the three defect types. A precise analysis of the chemical composition of the precipitates and clouds around the voids was achieved by a special preparation of their replicates.

Determination of as-built properties of fiber reinforced polymers in a wind turbine blade using scanning electron and high-resolution X-ray microscopy

The fiber volume fraction (FVF) and porosity in fiber reinforced polymers (FRPs) depends strongly on the manufacturing process. These parameters influence the mechanical properties and thus the performance of an FRP. For this research, an epoxy-pre-impregnated glass FRP was investigated to determine the FVF and matrix mass fraction taking into consideration all material constituents, including sizing, stitching thread, as well as the porosity, the area density and the fiber orientations of each lamina, the filament fiber diameter, and the inter-laminar void size and shape. Therefore, samples from a commercially manufactured wind turbine rotor blade were experimentally investigated using scanning electron (SEM) and high-resolution X-ray microscopy (micro-CT), as well as a standardized calcination method and geometric measurements. Post-processing techniques such as thresholding and edge detection were used to analyze the images. There was good FVF agreement between SEM and the method of calcination. Micro-void cross-sectional shapes were well captured by SEM while meso- and macro-voids were volumetrically resolved with a reproducible void size distribution for two sample volumes by micro-CT.

Investigating the transfer of toughness from rubber modified bulk epoxy polymers to syntactic foams

Syntactic foams are lightweight, high specific strength materials used in the aerospace and naval industries. Their utility is limited by their brittleness. The epoxy polymer matrix in an epoxy/hollow glass microsphere (GMS) syntactic foam was modified using carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber with the aim to increase fracture toughness. The microstructure and fracture properties were investigated, and compared to CTBN modified bulk epoxy polymers. The formation of complex CTBN microstructures was responsible for the increase in fracture energy, from 193 J/m2 for the unmodified syntactic foam, to 296 J/m2 at 12 wt% CTBN modification. However, this increase is much smaller than for the CTBN modification of bulk epoxy polymers, where an increase from 101 J/m2 to 1112 J/m2 was measured for the same CTBN concentration. There is little toughness transfer from the bulk epoxy polymers to the syntactic foams, attributable to small interstitial regions between the GMS, restricting plastic zone size. A statistical approach to the analytical modelling of fracture energy in the bulk epoxy polymers highlights the importance of considering the underlying distribution of rubber particle and void sizes. The increase in fracture energy achieved in this work can increase the overall usefulness of syntactic foams in structural applications.

Toward a machine-guided approach to energetic material discovery

In this article, we trained a machine learning (ML) model to connect microstructural details of an energetic material formulation to its performance for the purpose of guiding the discovery of new explosive formulations. Our hypothesis was that the algorithm would robustly learn the training data and produce an accurate surrogate model. Specifically, the algorithm learned the relationship between details of the void size distribution (VSD), initiating shock pressure, and the energetic material performance. We used realistic constraints on the VSD and a range of cases were ingested by a physically informed reactive flow model working within a hydrodynamic solver running on high-performance computing resources. The ML algorithm produced a surrogate model that accurately predicted known test points around the parameter space. In addition to the utility of the model and the process used for its development, we noted interesting comparisons between what we, the authors—subject matter experts, would heuristically conclude from the training data and the surrogate model predictions. We detected nuanced details that were missed by the surrogate model; however, these details are not important to an energetic material formulator. We concluded that the algorithm did indeed robustly learn the training data and produce an accurate surrogate model. We further concluded that the surrogate model is a powerful tool to guide the formulator in the absence of subject matter experts and limited-access computing resources.

Effects of geometric array and size of internal voids on tensile ductility of AZ31 magnesium alloy sheets

This study aims to investigate the contributions of the geometric array and size distribution of perforated cylindrical voids to the tensile strain of a metallic sheet through the numerical correction of the plastic constraint factor with regard to the geometric array of internal voids. In AZ31 alloy sheets fabricated via strip casting, internal voids were formed via mechanical drilling in the form of perforated cylindrical holes. The tensile strain decreased significantly (from 0.18 to 0.01) as the void area fraction increased to 40%, and its nominal value decreased with an increase in the void size, even when the geometric array (ratio of void size to ligament distance between voids, a/l) was maintained. A plastic constraint factor describing the geometric array and size distribution of perforated cylindrical voids was proposed through the modification of an original form for isolated spherical voids, and the theoretical prediction using a modified constitutive model agreed well with the experimental results. The tensile ductility of a metallic sheet with perforated cylindrical voids depends more sensitively than isolated spherical voids on the variation in the void area fraction, because the effective void area fraction is overestimated by the exclusion of the strain-hardening exponent term in the modified plastic constraint factor. Additionally, increases in the void size and void area fraction at a fixed a/l ratio lead to a reduction in the tensile strain, by the increase of the plastic zone and the intensification of stress concentration around a void.

Relationship between fine aggregate size and the air void system of six mortars: I. Air void content and diameter distribution

Precisely determining the characteristics of the air void system in cement-based materials is important in material design and freeze-thaw durability evaluation. X-ray computed tomography (XCT) has been used to characterize air void systems, since it is able to measure the 3D spatial arrangement of the voids. Though the factors influencing air void formation and arrangement have been well-studied, the role of fine aggregate size and gradation has mainly been ignored. In this paper, six mortars were prepared with varying sand size distributions and their air void system quantitatively characterized via XCT. The role of sand size distribution and blended proportions were combined via calculation of the fine aggregate specific surface area (SSA). A linear relationship between SSA and global air void volume fraction was observed, with increasing air void volume fraction as SSA increased (and sand size decreased). The 2D circle diameter distribution was derived from 2D analysis of each XCT slice and the 3D sphere diameter distribution was derived from 3D analysis of the stacked 3D XCT microstructure. The distributions for 2D and 3D were calculated from each other using stereological methods and compared with the original experimental results. All three methods used were in reasonable agreement. The air void size distributions obtained via high resolution (small voxel size, small scan volume) and low resolution (large voxel size, large scan volume) XCT scans compared favorably in a compatible air void size range, which illustrates how the problems of differing XCT resolutions can be resolved.

Relative entropy as an index of soil structure

AbstractSoil structure controls key soil functions in both natural and agro‐ecosystems. Thus, numerous attempts have been made to develop methods aiming at its characterization. Here we propose an index of soil structure that uses relative entropy to quantify differences in the porosity and pore(void)‐size distribution (VSD) between a structured soil derived from soil water retention data and the same soil without structure (a so‐called reference soil) estimated from its particle‐size distribution (PSD). The difference between these VSDs, which is the result of soil structure, is quantified using the Kullback–Leibler Divergence (KL divergence). We applied the method to soil data from two Swedish field experiments that investigate the long‐term effects of soil management (fallow vs. inorganic fertilizer vs. manure) and land use (afforested land vs. agricultural land dominated by grass/clover ley) on soil properties. The KL divergence was larger for the soil receiving regular addition of manure compared with the soils receiving no organic amendments. Furthermore, soils under afforested land showed significantly larger KL divergences compared to agricultural soils near the soil surface, but smaller KL divergences in deeper soil layers, which closely mirrored the distribution of organic matter in the soil profile. Indeed, a significant positive correlation (r = 0.374, p &lt; 0.001) was found between soil organic carbon concentrations and KL divergences across all sites and treatments. Despite challenges related to modelling the VSD of the reference soil without structure, the proposed index proved useful for evaluating differences in soil structure in response to soil management and land‐use change and reflected the expected effects of soil organic matter on soil structure. We conclude that relative entropy shows great potential to serve as an easy‐to‐use index of soil structure, as it only requires widely available data on soil physical and hydraulic properties.Highlights A new index of soil structure is proposed based on relative entropy A method is developed that separates the effects of soil texture and structure on the pore space The index identified soil structural differences in response to land use and soil organic carbon concentrations (SOC) The index shows the potential to serve as an easy‐to‐use metric of soil structure

Understanding Relationships between Free Volume and Oxygen Absorption in Ionic Liquids.

Understanding the factors that govern gas absorption in ionic liquids is critical to the development of high-capacity solvents for catalysis, electrochemistry, and gas separations. Here, we report experimental probes of liquid structure that provide insights into how free volume impacts the O2 absorption properties of ionic liquids. Specifically, we establish that isothermal compressibility─measured rapidly and accurately through small-angle X-ray scattering─reports on the size distribution of transient voids within a representative series of ionic liquids and is correlated with O2 absorption capacity. Additionally, O2 absorption capacities are correlated with thermal expansion coefficients, reflecting the beneficial effect of weak intermolecular interactions in ionic liquids on free volume and gas absorption capacity. Molecular dynamics simulations show that the void size distribution─in particular, the probability of forming larger voids within an ionic liquid─has a greater impact on O2 absorption than the total free volume. These results establish relationships between the ionic liquid structure and gas absorption properties that offer design strategies for ionic liquids with high gas solubilities.

Multiscale investigation of the microstructural mechanisms driving ratchet growth in PBX 9502

ABSTRACT The high explosive PBX 9502 undergoes irreversible expansion during thermal cycling (“ratchet growth”). Recent innovations in thermomechanical modeling via homogenization strategies are beginning to incorporate mesoscale information such as grain size, total porosity, and spatial distribution of voids and cracks. To generate a complete experimental data set to challenge and inform these models, PBX 9502 pellets were thermally cycled, cross-sectioned using ion polishing, and imaged in high resolution with scanning electron microscopy. Ratchet growth was found to drive expansion through microcracking. Microcracks were affected by agglomeration of crystals within the PBX. Virgin material showed greater ratchet growth than recycled material.

Cosmic voids in GAN-generated maps of large-scale structure

A Generative Adversarial Network (GAN) was used to investigate the statistics and properties of voids in a ΛCDM universe. The total number of voids and the distribution of void sizes is similar in both sets of images and, within the formal error bars, the mean void properties are consistent with each other. However, the generated images yield somewhat fewer small voids than do the simulated images. In addition, the generated images yield far fewer voids with central density contrast ∼−1. Because the generated images yield fewer of the emptiest voids, the distribution of the mean interior density contrast is systematically higher for the generated voids than it is for the simulated voids. The mean radial underdensity profiles of the largest voids are similar in both sets of images, but systematic differences are apparent. On small scales (r<0.5rv), the underdensity profiles of the voids in the generated images exceed those of the voids in the simulated images. On large scales (r>0.5rv), the underdensity profiles of the voids in the simulated images exceed those of the voids in the generated images. The discrepancies between the void properties in the two sets of images are attributable to the GAN struggling to capture absolute patterns in the data. In particular, the GAN produces too few pixels with density contrasts ∼−1 and too many pixels with density contrasts in the range ∼−0.88 to ∼−0.63.

Ion-binding ameliorates the organic solvents nanofiltration performance of poly (butyl acrylamide-co-divinylbenzene) composites

• Poly (butyl acrylamide-co-divinylbenzene) nanofiltration membrane was synthesized. • The toplayer regularly distributed oxygen and nitrogen groups to anchor metal ions. • Fe-binding treatment enhanced the permeance and dye rejection in organic solvents. • Changes of the void size and the charge density in the toplayer were disclosed. A new nanofiltration composite membrane poly (butyl acrylamide-co-divinylbenzene) (poly-TaDb) was synthesized via a facile photopolymerization on polyetherimide using the monomers of n- tert -butyl acrylamide and divinylbenzene, and evaluated the separation performance in organic solutions. The unique poly-TaDb toplayer has a feature of branch structure containing regularly distributed oxygen and nitrogen groups that work as the interacting sites with metal ions (Fe 3+ , Zn 2+ , and Cu 2+ ), as reflected by the characterizations of ATR-FTIR, XPS, AFM, etc. Without the metal ion treatment, the optimal poly-TaDb membrane shows the permeance of 31, 17, 20.4 L m −2 h −1 MPa −1 respectively for organic solutions of tetrahydrofuran (THF), methanol (MeOH) and acetonitrile (ACN), with the rejection rates higher than 90% toward EB dye (879.8 g.mol −1 ). After interacting with these metal ions, the obtained Fe-binding poly-TaDb membranes possess higher permeance, with the THF, MeOH, and ACN permeance enhanced respectively to 113, 35, and 24.2 L m −2 h −1 MPa −1 , meanwhile maintaining high dye rejection rate. While the Cu- and Zn-binding membranes show much higher permeance but lower dye rejection rate. Density functional theory (DFT) calculations disclose that the void size of the toplayer network increases in the order of: the pristine (11.40 Å) < Fe-binding (11.54 Å) < Zn-binding (11.60 Å) < Cu-binding membrane (11.63 Å). It is the suitable void size and charge density distribution in the toplayer network of Fe-binding membrane. that results in the superior nanofiltration performance. This work provides a facile route to explore new polymeric network materials for highly effective separation performance in organic solvents.

Interfacial void ripening in Cu Cu joints

Cu-to-Cu direct bonding technology has been adopted to achieve ultra-high- density interconnects for 3D IC integration. Although the voids in the bonding interface was observed, there was no report on the size distribution and evolution of voids due to ripening. In this paper, we used focused ion beam to prepare specimens for transmission electron microscopy (TEM) study under a specific direction to obtain the plan view images of interface. The voids size distributions under 200 °C for 30, 60 and 120 min were obtained. The interfacial ripening effect was observed. The average void diameter is 49 nm, 70 nm, and 96 nm for the joint annealed for 30, 60, and 120 min, respectively. In addition, the void count within 10 μm2 is 183, 65 and 34 for the three conditions. A simple kinetic model of ripening is developed, and the relationship between radius and annealing time is in good agreement with experimental data.

A Statistical Hot Spot Reaction Rate Model for Shock Initiation of Polymer‐Bonded Explosives

AbstractA statistical hot spot reaction rate model for shock initiation and detonation of heterogeneous solid explosives is developed to devote a physically realistic description of the formation, growth, or coalescence of hot spots, combustion, and rapid transition to detonation. One of the most significant advantages of the statistical model is that it predicts well the influence of mesoscopic void size distribution and critical size of activated hot spots on the shock initiation and detonation growth of PBXs. The calculated pressure histories in both TATB‐based LX‐17 and HMX‐based PBX9501 are found to be in good agreement with the experimental data. It is found that the shock initiation behavior of the HMX‐based PBX is mainly controlled by the number density of hot spots and shows an accelerated reaction characteristic and that of the TATB‐based PBX is determined by the combustion reaction process, which featured by a stable reaction characteristic. It is also found that the void size inside the explosive should be restricted as small as possible to effectively reduce the initiation sensitivity. The results could advance our knowledge of shock initiation of PBXs and provide guidelines for the synthesis of insensitive high composite explosives.

Effect of fly ash on the air void size distribution entrained by selected anionic, cationic and nonionic surfactants in hardened cement mortars

In this study, three types of surfactants, as anionic (mainly SDS), cationic (DTAB) and nonionic (AEO-9), were selected as the air-entraining agent (AEA). Air void size distributions of air-entrained mortars with or without fly ash were measured. The surface tension, the zeta potential of solid particles, and rheological properties were examined. The zeta potential alteration is used to characterize the interaction between surfactants and solid particles, as larger zeta potential alteration represents stronger interaction. Results indicate that the overall air-entraining behavior is governed by the surface tension, as lower surface tension leads to finer air voids. In addition, fly ash influences the air void structure by changing the interaction between surfactants and solid particles. Stronger interaction leads to finer small air voids and coarser large air voids, especially for the anionic and cationic surfactants. For the nonionic surfactant, this interaction is relatively weak, thus large air voids are affected by viscosity, as higher viscosity results in coarser large air voids.

A mechanistic model for creep lifetime of ferritic steels: Application to Grade 91

In this work, a physics-based crystal plasticity model is developed to predict failure in Grade 91 steel. A microstructure-sensitive dislocation kinetics law defines local plastic slip, an Arrhenius creep law is used to model vacancy-mediated plasticity, and strain hardening evolves with local dislocation density. As voids nucleate, a reaction–diffusion framework is adopted to dynamically track the local void size distributions, which grow by coupled viscoplastic and diffusive processes. Upon accurately reproducing the temperature and stress dependencies in primary, secondary, and tertiary creep seen experimentally for Grade 91 steel, the model is exercised to generate a material response database across a wide range of operating conditions. A new reduced-order lifetime predictor is developed from numerical predictions, and a Bayesian framework is used to quantify prediction uncertainties. When compared to current empirically-derived lifetime relations, the proposed lifetime assessment tool predicts rupture times up to several orders more conservative.

Microstructural characterization of 3D printed concrete

3D concrete printing technique offers a novel approach to build new infrastructure. However, the microstructure of 3D printed concrete has rarely been studied so far. This study presents the microstructural characterization of 3D printed concrete by using MIP and X-ray CT. The void size distribution reported in this study covers a wide spectrum. The mechanical performance of the 3D printed concrete, along different directions, is also investigated. This study reveals that a higher amount of macropores and large voids are present in the 3D printed samples, attributed to the movement of the printhead, elimination of vibration, and rapid moisture loss. Moreover, the morphologies of the voids are more irregular and elongated. This is more pronounced at the interlayer than in the filament. As a result, the strength of 3D printed samples is lower than that of mould-cast specimens.

Nanocavitation mechanisms in deformed High Density PolyEthylene (HDPE) using synchrotron radiation NanoTomography

The nanocavitation distribution in deformed High Density PolyEthylene (HDPE) was investigated. A flat notched sample of HDPE, deformed under steady crosshead speed up to the end of the stress softening, was observed by classical synchrotron parallel beam microtomography and magnified holotomography with a true spatial resolution of 0.7 μm and 0.05 μm, respectively. Image processing, 3D image segmentation and 2D-Fast Fourier Transform, was shown to lead to quantification of void volume fraction and void size distributions. Nanocavitation morphology and distribution in HDPE reveal to be similar to cavitation mechanisms in other semi-crystalline polymers, but at a different scale. While a slight decrease of the void diameter was seen, from the end of the neck shoulder to the extended neck region, a significant increment of the height was evidenced. Furthermore, the results provide compelling evidence of morphology transition of cavities within the test sample from an oblate to a prolate geometry.

Mechanical enhancement for EMW-absorbing cementitious material using 3D concrete printing

Ordinary electromagnetic wave-absorbing concrete can substantially reduce electronic pollution but usually has limited EMW-reflecting capacity. Thus, EMW-absorbing macrostructure can be applied to further reduce EMWs. These concrete macrostructures could be manufactured using 3D cementitious printing technology as conventional casting methodology is not sufficient in standard manufacturing. However, many printed concretes may demonstrate inferior mechanical performance to that of the cast counterpart due to improper fabrication procedure. Therefore, the present study demonstrates the mechanical capacity improvement of EMW-absorbing concrete through optimizing chemical additive and printing parameters. EMW-absorbing experiment confirms that 3D printing technology using enhancement methodology enhances the microwave absorption. Mechanical results show that the hydration velocity as well as the hydration degree acceleration for the printed sample are improved. The compressive strength of the printed samples is improved by 9% and 40% at 28 days and 7 days, respectively. The mercury intrusion porosimetry test indicates that 3D printing provides a favorable squeezing effect to decrease concrete porosity, reducing the void fraction from 17.47% to 11.13% and narrowing the medium void size distribution from 48 nm to 22 nm. To this end, the proposed enhancement methods improve the mechanical and EM absorbing behavior of 3D-printed elements.

Characteristics of Entrained Air Voids in Hardened Concrete with the Method of Digital Image Analysis Coupled with Schwartz-Saltykov Conversion.

This paper presents the characteristics of air void systems in hardened concrete with the method of digital image analysis (DIA) coupled with Schwartz-Saltykov (SS) conversion. The results indicate that the DIA method coupled with SS conversion estimates the air content with more accuracy than it would without SS conversion; the correlation between air content obtained from the DIA method, and that from the thin section (TS) method is as good as the correlation observed between the pressure saturation (PS) method and the TS method. It was also found that the DIA method shows a better correlation with the TS method when the spacing factor without SS conversion is considered, while both methods show poor correlations when the corresponding specific surface is considered. In addition, it indicates that the peak of three-dimensional size distribution (3-DSD) of air voids after SS conversion falls in smaller voids, and 3-DSD of air voids shifts to a narrow size range, in comparison with the 2-DSD without SS conversion; the shape of the 3-DSD air voids remains constant irrespective of the class widths. Increasing the number of classes can minimise the standard deviation in the estimation, however, it also results in a leap in voids volume density, which will influence the estimation of air content.

Mesostructure of Foamed Cement Paste and Its Influence on Macromechanical Behavior

Foamed cement paste has been used increasingly as a backfill material in geotechnical engineering applications, such as embankments, tunnels, and retaining walls. However, limited studies have focused on the influence of void structures of the foamed cement paste on its macromechanical behavior. This study used X-ray computed tomography (X-CT) to investigate the void structures of foamed cement paste and their influence on the macromechanical properties (strength and modulus). The test results showed that the void volumes had a unimodal distribution. The reduction in the density of the specimen indicated more large voids and a wider distribution of void sizes; however, surrounded-type voids were dominant in terms of percentage of total volume. The sphericity of the voids ranged from 0.1 to 0.6, indicating their highly irregular shapes. Sensitivity analysis showed that the volume of the surrounded-type voids played a primary role in the compressive behavior of the foamed cement paste, the void shape was the secondary influence factor, and the void size was the tertiary influence factor.