Void Configuration Research Articles

Void configuration-induced change in microstructure and deformation mechanisms of nano-porous materials

Void configuration plays an essential role in the deformation behavior of ductile metals; its effects on intervoid interference has not been investigated systematically until now. In the present study, molecular dynamics simulation was employed to study the void configuration-induced change in mechanical properties and deformation mechanisms during tensile loading. The results show that void configuration has a significant influence on the yield stress and yield strain, while its effect on the elastic modulus is about 3.14 ± 0.23%. The deformation mechanisms of porous materials with various void configurations at micro- and nanoscale are proposed: (i) local plastic deformation and (ii) homogeneous plastic deformation. Further analysis indicates that the difference between the above two deformation mechanisms is mainly caused by the competition and synergy between the stacking faults and dislocation. Local plastic deformation is mainly controlled by stacking faults. Homogeneous plastic deformation is dominated by dislocation motion with only a small amount of stacking shear motion, which gives material a superior plastic elongation. The result is of great value for improving the plasticity limit of nanostructures.

Spallation of polycarbonate under plate impact loading

We investigate spallation of polycarbonate under plate impact loading. The Hugoniot equation of state up to ∼1.3GPa (corresponding to a peak particle velocity ∼380m/s) is obtained, and spall strength and corresponding strain rates are determined at peak shock stresses up to ∼2.4GPa (corresponding to a peak particle velocity ∼600m/s). With increasing shock strength, the transition from strain-hardening to softening at shock states occurs as a result of shock heating; spall strength remains approximately constant, followed by a rapid drop upon strain softening. Release/tensile melting occurs at higher impact velocities. Three-dimensional void configurations of the postmortem samples are obtained via X-ray computerized tomography. The small voids are flat and curved for low-speed shots but become ellipsoidal for high-speed shots, and their coalescence leads to different shapes likely due to different damage mechanisms.

A new method for assessing anisotropy in fused deposition modeled parts using computed tomography data

Voids in fused deposition modeled (FDM) parts are assumed to be a key driver for their anisotropic behavior. However, these assumptions are based on investigations of voids using only 2D data (microscopy images). This paper presents a new method to measure such voids by analyzing 3D-data of from X-ray computed tomography (CT), and application of this data for assessment of mechanical parameters. The article is divided into three parts, where the first part elaborates on a proposed method to assess and characterize the void geometry throughout uniaxial printed FDM parts using CT-data. The second part presents an investigation of the void configurations in samples manufactured using different process parameters, aiming to understand how variation in extrusion rate and compensation for non-linear dynamic extrusion behavior affects the void sizes. The third part displays how the information regarding void sizes could be further related to global mechanical properties, using a multiscale finite element approach. The present method of CT-data analysis gives a clear overview of the spatial variation of the void geometry, and findings from the investigated samples suggest that the size of voids have a large non-random spatial variation, highly dependent on the turning points of the toolpaths, and also significantly affected by accumulation of excess material. Printing at a low extrusion rate increases the void sizes considerably, while implementation of an extrusion dynamics compensation algorithm was found to have low impact on the void sizes. The multiscale finite element approach predicts anisotropic elastic behavior, significantly more compliant in the vertical and transversal direction, relative to the printing direction of the infill. It also predicts a non-isotropic strain energy density throughout the specimen, where the location and magnitude of the most energy dense locations vary significantly for different directions of loading, which implicates an anisotropic behavior in terms of failure, in accordance with literature.

Void-by-void versus multiple void interaction under mode I-mode II or mode I-mode III loading conditions

Ductile crack initiation, governed by void growth to coalescence, is controlled by two distinct mechanisms, namely: (i) void-by-void growth, where substantial growth of the void closest to the crack tip precedes growth of other voids, and (ii) multiple void interaction, where the voids grow simultaneously and interact in the vicinity of the crack tip prior to crack advance. Tvergaard and Hutchinson (2002) [Int. J. Solids and Struct, 39: 3581-3597] studied the two mechanisms in detail under far-field mode I loading conditions and demonstrated their interference by changing the size and spacing of discretely modeled voids. The present numerical work focuses on mixed mode loading conditions rather than limiting the study to mode I and takes up the question; how will a change in the far-field loading conditions affect the shift between the two mechanisms? A pre-existing straight crack with nearby discretely modeled voids in an elastic-plastic material is considered by a specialized 2D plane strain setting capable of out-of-plane displacements to allow for combinations of mode I, mode II, and mode III. Details of the void-by-void growth versus the multiple void interaction mechanisms are laid out for a range of load cases and geometrical void configurations, and increasing the mode mixity is found to favor void-by-void growth accompanied by a higher load intensity required on the far-field boundary to initiate the crack. The change between the two mechanisms is tied to a change in the deformation field that surrounds the voids, and the study reveals significant deformation that stretches above (and below) the row of discrete voids for increasing shear mode contributions yielding an increase in the void rotation.

Structural transformations during periodic deformation of low-porosity amorphous materials

Atomistic simulations are employed to study microstructural evolution of pore ensembles in binary glasses under periodic shear deformation with varied amplitude. The consideration is given to porous systems in the limit of low porosity. The initial ensembles of pores are comprised of multiple pores with small sizes, which are approximately normally distributed. As periodic loading proceeds, the ensembles evolve into configurations with a few large-scale voids and significantly reduced number of small pores. These structural changes are reflected in skewed shapes of the pore-size distribution functions and the appearance of a distinct peak at large length scales after hundreds of shear cycles. Moreover, periodic shear causes substantial densification of solid domains in the porous systems. The structural evolution of pore ensembles is found to stem from the formation of shear band like regions of enhanced particle mobility after a number of transient cycles. The spatial extent of regions with increased atomic mobility depends strongly on the strain amplitude. A simple theoretical model is developed to qualitatively describe the transformation of the initial configurations of small-size voids into larger-scale void agglomerates.

Void detection of cementitious grout composite using single-walled and multi-walled carbon nanotubes

Cement-based materials containing carbon nanotubes (CNT) has enhanced electrical and thermal characteristics. This study aimed to detect voids in prestressed concrete (PC) grout using CNT. CNT cement grout specimens were fabricated and tested inside ducts. Test parameters were type of CNTs, concentration of CNTs, and void ratio of grout. Thermal imaging analysis and electrical resistance analysis were performed to detect and identify the void configuration. The field emission scanning electron microscopy (FE-SEM) images were taken to obtain detailed information of microstructure of tested specimen. It was concluded that MWCNT grout was appropriate to detect the void in duct using thermal image analysis. SWCNT grout was more suitable to identify void in duct through electrical resistance analysis.

Modeling the modified drug release from curved shape drug delivery systems – Dome Matrix®

The controlled drug release from hydrogel-based drug delivery systems is a topic of large interest for research in pharmacology. The mathematical modeling of the behavior of these systems is a tool of emerging relevance, since the simulations can be of use in the design of novel systems, in particular for complex shaped tablets. In this work a model, previously developed, was applied to complex-shaped oral drug delivery systems based on hydrogels (Dome Matrix®). Furthermore, the model was successfully adopted in the description of drug release from partially accessible Dome Matrix® systems (systems with some surfaces coated). In these simulations, the erosion rate was used asa fitting parameter, and its dependence upon the surface area/volume ratio and upon the local fluid dynamics was discussed. The model parameters were determined by comparison with the drug release profile from a cylindrical tablet, then the model was successfully used for the prediction of the drug release from a Dome Matrix® system, for simple module configuration and for module assembled (void and piled) configurations. It was also demonstrated that, given the same initial S/V ratio, the drug release is independent upon the shape of the tablets but it is only influenced by the S/V evolution. The model reveals itself able to describe the observed phenomena, and thus it can be of use for the design of oral drug delivery systems, even if complex shaped.

Microstructural analysis of an epitaxial AlN thick film/trench-patterned template by three-dimensional reciprocal lattice space mapping technique

Three-dimensional (3D) lattice plane microstructures were investigated at local regions in an epitaxial AlN thick film grown on a trench-patterned AlN/sapphire template. A 3D reciprocal lattice space mapping technique combined with cross-sectional X-ray microdiffraction using an appropriate Bragg reflection quantitatively revealed the inhomogeneity of the lattice structures in the AlN film without loss of spatial resolution. The results showed a strong correlation of the lattice plane tilt/twist and variations with respect to the void configuration, the patterning structure of the template, and the dislocation morphologies confirmed by transmission electron microscopy.

Model Analysis of Ridge and Rib Types of Silicon Waveguides With Void Compositions

Detailed numerical study on mode characteristics and light confinement properties of ridge and rib waveguides with vertical, horizontal, and cross types of void configurations is presented. The study starts from the mode analysis for rectangular ridge and rib waveguides. Then, the analysis is extended to semicircular ridge and rib waveguides with and without slots. The percentages of the electric and magnetic field components and light confinement capabilities are compared. We observe that polarization type of the mode and strong field confinement in the low refractive index medium can be achieved by the incorporation of void structures into the regular optical waveguide.

The Effects of Void on Natural Ventilation Performance in Multi-Storey Housing

Enhancing natural ventilation performance in multi-storey housing is very important for the living environment in terms of health and thermal comfort purposes. One of the most important design strategies to enhance natural ventilation in multi-storey housing is through the provision of voids. A void is a passive architectural feature, which is located in the middle of deep plan buildings. It is very crucial to consider the configurations of voids in the buildings for enhancing natural ventilation, especially for multi-storey housing. In this study, Malaysian Medium Cost Multi-Storey Housing (MMCMSH), which is an example of multi-storey housing located in a suburban area, has been selected in this study. This study aims to investigate the potential of void for enhancing natural ventilation performance in multi-storey housing by the comparison of two different void configurations. Field measurement of MMCMSH has been conducted to validate Computational Fluid Dynamic (CFD) model and Atmospheric Boundary Layer (ABL) is an important parameter for setting up the CFD Model’s domain. Ventilation rate (Q), which is necessary for comfort and health reasons, is an important parameter for the comparison of the different void configurations. This study revealed that the provision of void can enhance natural ventilation performance in multi-storey housing with an increase in the value of Q, from 3.44% to 40.07%, by enlarging the void’s width by 50% compared to the existing void.

Role of intratumoral flow void signs in the differential diagnosis of intracranial solitary fibrous tumors and meningiomas

An absence of signal on magnetic resonance (MR) images caused by blood or cerebral spinal fluid flow is known as a flow void, and may be related to intracranial tumors such as intracranial solitary fibrous tumor (SFT) or meningioma. However, the differential diagnosis of these neoplasms based on flow void configuration is controversial. This study investigated common intratumoral flow void patterns for differentiating intracranial SFT from meningioma. From May 2008 to May 2015, 14 patients (10 men, 4 women; 14-63y) received a pathologic diagnosis of primary intracranial SFT, and in 85 patients (23 men, 62 women; 20-76y) a pathologic diagnosis of meningioma was made. Intratumoral flow void signs were retrospectively observed on MR images of all these patients and classified by radiologists blinded to the pathology findings. Statistical significance was established by chi-squared tests. In intracranial SFT patients, the sunburst flow void was detected in 1 case (7.1%) and the serpentine in 13 cases (92.9%). In meningioma patients, the sunburst flow void was detected in 82 cases (96.5%) and the serpentine in 3 cases (3.5%). The differences in the prevalence of the flow void types between the 2 groups were significant (χ(2)=64.348; P<0.01). Intracranial SFTs on MR images were more likely to show a serpentine than a sunburst flow void, while the sunburst pattern was far more typical of meningioma. Thus, the pattern of the intratumoral flow void sign may be a useful tool to assist in differentiating these tumors.

Finite element analysis of the effect of process-induced voids on the fatigue lifetime of a lead-free solder joint under thermal cycling

Process-induced voids remain one of the key concerns in thermo-mechanical reliability of solder alloys. Previous studies reported that the void effect on fatigue failure reliability of solder joints depends on the void configuration and some other specific characteristics of the electronic package. This paper investigates the void effect on the solder material layers used in power modules subjected to thermal passive cycles. The Anand's visco-plastic model of the solder alloy is identified based on experimental data obtained with a micro-tester. The constitutive model is then used in a finite element analysis to study the behaviour of Innolot Pb-free solder joint used in an electronic assembly. An algorithm called Monte Carlo Representative Volume Element Generator is used to generate, based on the statistical probability law for the diameters, the 2D disk distribution of the voids (thereafter extruded in the form of cylinders) within the solder layer. The dissipated plastic energy is considered as a damage variable indicator representing the void effect on the fatigue lifetime of the solder. Results suggest that the fatigue reliability of solder joints depends not only on the size, location and ratio of the voids but also on their statistical distribution. The critical sites for damage are located at the corners of the joint, as well as at the border of voids. Fatigue lifetime of the solder joint decreases as the volume fraction of voids increases. Moreover, voids near the critical sites facilitate initiation of damage significantly. On the contrary, the solder joint behaviour is almost not affected by voids located far from the critical sites.

Architected Cellular Materials

Additive manufacturing enables fabrication of materials with intricate cellular architecture, whereby progress in 3D printing techniques is increasing the possible configurations of voids and solids ad infinitum. Examples are microlattices with graded porosity and truss structures optimized for specific loading conditions. The cellular architecture determines the mechanical properties and density of these materials and can influence a wide range of other properties, e.g., acoustic, thermal, and biological properties. By combining optimized cellular architectures with high-performance metals and ceramics, several lightweight materials that exhibit strength and stiffness previously unachievable at low densities were recently demonstrated. This review introduces the field of architected materials; summarizes the most common fabrication methods, with an emphasis on additive manufacturing; and discusses recent progress in the development of architected materials. The review also discusses important applications, including lightweight structures, energy absorption, metamaterials, thermal management, and bioscaffolds.

Topography and Frontier: Gibellina's City of Art

Topography and Frontier: Gibellina's City of Art

Ductility of post-Northridge connections with Angelina beams

Sinusoidal voids of the same size at the beam web can be easily created during fabrication of a castellated beam, when a sinusoidal cut shape is used. Such castellated beams are known as Angelina beams and are available in the market place. This study is designed to investigate the efficiency of using single and multi-sinusoidal voids at the beam web area, to enhance the ductility of the post-Northridge connections. For this purpose, a parametric study is carried out to determine the best configuration of such voids using the finite-element method. The investigation is carried out on more than 440 specimens of different design parameters. The optimum size of sinusoidal voids is proposed to achieve both adequate strength of connections and ductility. Present data indicate that creating a single void at the beam web with optimum size is only effective when the beam overall depth is limited to 750 mm, and the ratio of the beam length to the beam overall depth is equal to or greater than 11·5. For deeper and shorter beams, adequate strength of connection and ductility can be achieved when a modified specimen is used, with either multiple same-size voids with web stiffeners or multiple different-sized voids without web stiffeners.

Union operation image processing of data cubes separately processed by different objective filters and its application to void analysis in an all-solid-state lithium-ion battery.

In this article, we propose a smart image-analysis method suitable for extracting target features with hierarchical dimension from original data. The method was applied to three-dimensional volume data of an all-solid lithium-ion battery obtained by the automated sequential sample milling and imaging process using a focused ion beam/scanning electron microscope to investigate the spatial configuration of voids inside the battery. To automatically fully extract the shape and location of the voids, three types of filters were consecutively applied: a median blur filter to extract relatively larger voids, a morphological opening operation filter for small dot-shaped voids and a morphological closing operation filter for small voids with concave contrasts. Three data cubes separately processed by the above-mentioned filters were integrated by a union operation to the final unified volume data, which confirmed the correct extraction of the voids over the entire dimension contained in the original data.

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

Abstract 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.

Predicting ductile fracture in pure metals and alloys using notched tensile specimens by an ellipsoidal void model

The ductile fracture of nonferrous pure metals and alloys during notch tensile testing was predicted using an ellipsoidal void model. Simulated and experimental tensile tests were performed using four types of nonferrous sheets and bars. Two magnitudes of prestrain were induced in the sheets by rolling and in the bars by drawing. Six notched sheet specimens and thirteen notched bar specimens with different notch-root radii were prepared. A void configuration and void shape for pure metals and those for alloys were assumed. The effects of the prestrain and notch-root radius on the reduction in area calculated agreed reasonably well with those obtained experimentally.

Bio-Inspired Design of Lightweight Metal Structure Based on Microstructure of Fully Ripe Loofah

It is well known that a naturally matured loofah has some distinct inherent properties including extremely low density and high absorbability, due to its unique fibrous microstructure and macro-cellular architecture including the spatial configuration of voids and solids. In this paper, we focus on patents and literature related to the development of a three-dimensional (3D) lightweight metallic structure by means of a design bio-inspired by the cellular architecture of a fully ripe loofah. To simplify the design, the cellular architecture of the loofah is assumed to be a two-layered open-cell foam structure, and each layer is a porous shell wall. The two layers are connected by three porous radial plates. With this simplification, a lightweight metallic structure with hexagonal pores is developed that achieves the desired balance of weight and mechanical properties. The resulting metallic structure has low density (190.39 kilograms per cubic meter) and the Young’s modulus is estimated by compressive testing using a finite element technique. Keywords: Bio-inspired design, finite element, foam, lightweight metal, loofah.

Plastic deformation in nanoporous aluminum subjected to high-rate uniaxial compression

The mechanical behavior of nanoporous monocrystal aluminum subjected to uniaxial compressive loading at a rate of 2109 s- 1 along [110] crystallographic orientation is studied using molecular dynamics simulations. Subjected to such a loading, nanovoids act as the effective sources of dislocation nucleation and emission, four of the twelve {111}110 slip systems may be activated. With the same strain of 3.8%, dislocation nucleation will occur in both the sample of multiple voids and that with a single void. The configuration of multiple voids decreases the required stress for the onset of dislocation nucleation and emission in comparison with the sample with an isolated void of the same size. Because of the emission of trial partials, the accumulation of dislocation density can be changed into a piecewise linear process by the dislocation density propagation rate dd/d: in the initial stage of plastic deformation we obtain dd/d1.071018 m-2, but this changes to dd/d5.361018 m-2 at higher deformation. The velocity of dislocation is calculated to be subsonic and is a variable value during the plastic deformation. Dislocation loop pairs emit from the same void, glide and approach to each other, leading to the reduction of dislocation velocity. Then one loop of each pair continues to glide to intersect mutually and finally interact with the loops emitted from other voids, causing a strain hardening to reach the peak flow stress of 4.3 GPa. There is a post-yield softening corresponding to the onset of rapid dislocation density proliferation at higher dislocation densities. With the temperature evolution of the sample with multiple voids during plastic deformation, the density of mobile dislocations is calculated to be one magnitude lower than the total dislocation density. There is a decrease of mobile dislocation densities at large strains, showing that the mobile dislocation are diminished by the formation of dislocation forest and junctions. At the onset of their nucleation, the dislocations are all Shockley partials, however, when dislocation intersection happens, the majority are still Shockley partials, while the rest consists of Frank partials, perfect fcc dislocations and other dislocation ingredients. Voids collapse at the strain of 11.8%. No twins are found in the present simulation due to the high stacking-fault energy of aluminum. Prismatic dislocation loop emission is observed in this simulation.