Void Configuration Research Articles

Effect of Shape and Distribution of Secondary Voids on Ductile Crack Path

Ductile fracture in structural metals and alloys comprises of three distinct stages: namely nucleation, growth, and coalescence of voids. In this failure mode voids, either pre-existing in the material or nucleated during deformation grow until they coalesce to form a continuous fracture path. The larger voids nucleating early in the deformation history are often referred to as the primary voids whereas the smaller ones (typically around 0.1 to 0.01 times the size of the primary voids) nucleate much later and are described as the secondary voids. As the imposed deformation increases, voids grow, and the ductile crack path is influenced by the interaction and coalescence of the two-scale voids. The present work numerically models the interaction between the two populations of pre-existing voids lying ahead of the crack tip. The influence of secondary voids attributes, in particular, the shape and distribution of secondary voids on ductile crack propagation is analysed. A plane-strain modified boundary layer (MBL) model, under small-scale yielding condition, subjected to remote mode-I loading is analysed. Both primary and secondary voids lying near the crack tip are modelled explicitly. In the absence of secondary voids, as expected, the crack path is controlled by the distribution of primary voids. The presence of small secondary voids, however, influences the crack growth and, hence, the crack driving force. Various initial configurations of secondary voids are modelled and some details of crack propagation and the numerically computed driving force versus crack extension (J-Δa) curves are presented.

Flexible nanomechanical bit based on few-layer graphene.

Mechanical computers have gained intense research interest at size scales ranging from nano to macro as they may complement electronic computers operating in extreme environments. While nanoscale mechanical computers may be easier to integrate with traditional electronic components, most current nanomechanical computers are based on volatile resonator systems that require continuous energy input. In this study, we propose a non-volatile nanomechanical bit based on the quasi-stable configurations of few-layer graphene with void defects, and demonstrate its multiple quasi-stable states by deriving an analytic relationship for the void configuration based on a competition between the bending energy and the cohesive energy. Using this nanomechanical bit, typical logic gates are constructed to perform Boolean calculations, including NOT, AND, OR, NAND and NOR gates, and demonstrate reprogrammability between these logic gates. We also study the accuracy and the stability of the nanomechanical bits based on the few-layer graphene. These findings provide a novel approach to realize the nanomechanical computing process.

Level-Set and Learn: Convolutional Neural Network for Classification of Elements to Identify an Arbitrary Number of Voids in a 2D Solid Using Elastic Waves

We present a new convolutional neural network (CNN)-based element-wise classification method to detect a random number of voids with arbitrary shapes in a two-dimensional (2D) plane-strain solid subjected to elastodynamics. We consider that an elastic wave source excites the solid including a random number of voids, and wave responses are measured by sensors placed around the solid. We present a CNN for resolving the inverse problem, which is formulated as an element-wise classification problem. The CNN is trained to classify each element into a regular or void element from measured wave signals. Element-wise binary classification enables the identification of targeted voids of any shapes and any number without prior knowledge or hint about their locations, shape types, and numbers, while existing methods rely on such prior information. To this end, we generate training data consisting of input-layer features (i.e., measured wave signals at sensors) and output-layer features (i.e., element types of all elements). When the training data are generated, we utilize the level-set method to avoid an expensive remeshing process, which is otherwise needed for each different configuration of voids. We also analyze how effectively the CNN performs on blind test data from a non-level-set wave solver that explicitly models the boundary of voids using an unstructured, fine mesh. Numerical results show that the suggested approach can detect the locations, shapes, and sizes of multiple elliptical and circular voids in the 2D solid domain in the test data set as well as a blind test data set.

Development and performance of pouring self-luminescent asphalt mixture

The application of long afterglow-based materials to the self-luminescent pavement could significantly reduce the power consumption of lighting at night and improve the traffic safety. This study designs a pouring self-luminescent asphalt mixture (PSAM) through pouring a grouting material including the long afterglow luminescent material (SrAl2O4:Eu, Dy) and epoxy resin into the porous pavement void configuration. First, the mechanical and afterglow properties of the epoxy resin-based long afterglow grouting material were systematically tested to determine the optimal composition ratio of raw materials. Then, the influence of time and viscosity on the grouting was studied by obtaining the luminous images of the cross and longitudinal sections of self-luminous asphalt mixture specimens. According to the volume characteristics of the grouting material and the mixture, the relationship between the theoretical grouting consumption and the actual grouting consumption was analyzed. Finally, the influence of porous mixture air void on afterglow characteristics and road performance of the PSAM was analyzed. From the acquired results, it was demonstrated that the optimum proportion of each component of the epoxy resin long afterglow grouting material was the following: epoxy resin: curing agent: toughener: long afterglow material = 100:18:15:14. Moreover, it was found that the temperature and pre-curing time have remarkable effects on the viscosity of grouting materials. In order to ensure a good grouting effect, the viscosity of the grouting material should be controlled within 1.7 Pa s. There was a good linear relationship between the actual grouting amount and the connected air void, although the actual grouting consumption was less than the theoretical grouting consumption. Furthermore, PSAM exhibited good afterglow performance and road performance. The afterglow brightness and water stability had no obvious relationship with the air void. In contrast, with the further increase of the air void, both the low-temperature crack resistance and the skid resistance were improved, and the high-temperature stability was reduced.

Behavior of hollow self-compacted concrete beams reinforced with aluminum sections

This paper presents a study on improving the flexural performance of self-compacting hollow concrete beams with embedded aluminum sections, both experimentally and numerically. The experimental program involved testing three specimens to illustrate the impact of the bare and aluminum-reinforced voids on the mechanical properties of the hollow beams relative to the solid reference beam. A three-dimensional (3D) non-linear finite element model (FEM) was initially developed using ABAQUS software and validated against experimental results. The validated model examined the effects of void area (16–49% of the beam cross-section), void configuration (square or circular), longitudinal tensile reinforcement ratio (0.41–1.19%), and shear reinforcement ratio (0–0.56%) on the failure patterns and load–displacement relationships of hollow reinforced concrete (RC) beams with or without reinforced internal aluminum sections. Results showed that the embedded aluminum reinforcement promoted a more uniform distribution of flexural cracks along the effective length of the hollow RC beams and delayed the formation of shear cracks compared to the bare specimens. Furthermore, the embedded aluminum reinforcement effectively restored the ultimate capacity of the hollow beams with a void area ranging from 16 to 36% of the beam cross-section. Despite having the same area, the ultimate load-carrying capacity of both bare and reinforced beams featuring circular voids was higher than those with square voids.

Analytical study on influence of subsurface voids on moment capacity of singly reinforced concrete beam – Inspired by a project case study

Ultrasonic echo tomography is a non-destructive testing technique that has been used to detect subsurface construction defects located within reinforced concrete members. In order to reasonably define the scope of the repair work for the detected subsurface construction defects, it is important to understand the relationship between the extent of the construction defects and the performance of structural members. This preliminary analytical study was conducted to evaluate the moment capacity of singly reinforced concrete beam sections containing subsurface voids. It was determined in this study that if the location of void remained the same, an increase in the depth of void resulted in a reduction in the moment capacity. As the void extended outside the compression zone and the depth of void continued to increase, the moment capacity did not decrease further. If the depth of void remained the same, a void located closer to the top of beam resulted in more reduction in the moment capacity than the same void located closer to the neutral axis. A parameter named the percentage of remaining moment capacity was proposed to be used to identify the unacceptable void configurations from the standpoint of moment capacity.

A percolation-based micromechanical model for elastic stiffness and conductivity of foam concrete

Void morphology effect on percolation and even physico-mechanical performance of foam concrete is of great interest in the evaluation of service-life of civil and hydraulic infrastructures. For experiments, it is a huge challenge to quantify the percolation threshold of voids affected by their morphologies, and the dependence of elastic modulus and conductivity of foam concrete on void configurations. In this work, we focus on the prolate spheroidal void morphologies with the aspect ratios of 2.5 and 2, following the microscopic measurements reported in the literature. A numerical framework is developed to capture the percolation threshold characterized by the critical porosity of voids with both morphological types. For the verification purpose, Measurement on the critical porosity of spherical voids using the present framework as a benchmark is compared against the percolation threshold of monodisperse overlapping spheres reported in literature. Furthermore, this work proposes a simple and powerful percolation-based micromechanical model for precisely predicting the effective elastic modulus and thermal conductivity of foam concrete. It can be convinced of a general micromechanical framework to elucidate the intrinsic relationship of void morphology and percolation to the physico-mechanical properties of concrete. The present framework is capable of tailoring physical and mechanical properties through void configuration and enable foam concrete design and multifunctional applications.

Cosmic web & caustic skeleton: non-linear constrained realizations — 2D case studies

The cosmic web consists of a complex configuration of voids, walls, filaments, and clusters, which formed under the gravitational collapse of Gaussian fluctuations. Understanding under what conditions these different structures emerge from simple initial conditions, and how different cosmological models influence their evolution, is central to the study of the large-scale structure. Here, we present a general formalism for setting up initial random density and velocity fields satisfying non-linear constraints for specialized N-body simulations. These allow us to link the non-linear conditions on the eigenvalue and eigenvector fields of the deformation tensor, as specified by caustic skeleton theory, to the current-day cosmic web. By extending constrained Gaussian random field theory, and the corresponding Hoffman-Ribak algorithm, to non-linear constraints, we probe the statistical properties of the progenitors of the walls, filaments, and clusters of the cosmic web. Applied to cosmological N-body simulations, the proposed techniques pave the way towards a systematic investigation of the evolution of the progenitors of the present-day walls, filaments, and clusters, and the embedded galaxies, putting flesh on the bones of the caustic skeleton. The developed non-linear constrained random field theory is valid for generic cosmological conditions. For ease of visualization, the case study presented here probes the two-dimensional caustic skeleton.

Formation processes and spatial patterning in a late prehistoric complex cave in northern Israel informed by SLAM-based LiDAR

A SLAM-based Zeb Horizon LiDAR system is used for high-resolution 3D mapping of Har Sifsof Cave (HSC) in the Upper Galilee, northern Israel. Located within a dense karst province, HSC is a subterranean system featuring a composite morphology, the result of prolonged aging processes of a large hypogenic chamber. With a total length of 518 m, it is one of the largest caves in the Galilee district. The cave contains diverse archeological remains from the early fifth millennium BCE associated with ritual activities. In the current project the scanned point cloud contains over 884 million measured points, captured in two days along 10 scanning routes. One scan was conducted by a drone recording surface topography above the cave, and nine scans were conducted manually, carrying the scanner through the cave, with an average of 1850 points per m2, and measurement accuracy of 10 cm. We address the cave 3D documentation strategy and methods in the context of site formation process and spatial distribution of archeological deposits. The study shows that by the fifth millennium BCE the subterranean system already assumed its present configuration, indicating that ancient human activity in the cave involved overcoming multiple environmental obstacles and hazards. In tandem, changes in the configuration of voids and shafts in the upper part of the subterranean system led to secondary deposition of external flint-bearing sediments alongside the primary deposited remains in the cave, and to blockage of other conduits. Since the cave is located in a rapidly-developing rural area, high-resolution documentation of the subterranean system and its surroundings is a valuable tool not only for research purposes, but also for promoting statutory protection of the cave and its natural and archeological legacies.

Interaction between an edge dislocation and faceted voids in body-centered cubic Fe

Irradiation defects cause mechanical property degradation in reactor materials. The relationship between dislocations and defects in these materials is of particular importance to mechanical strength. In the current study, we investigate the interaction between an edge dislocation and different faceted void geometrical combinations using molecular dynamics to clarify the cutting mechanism and void effects on irradiation hardening in pure iron. We elucidate the difference in obstacle strength and the cutting process between spherical voids and faceted voids, especially for different faceted void configuration types. The highest critical shear stress in the type-a faceted void configuration is observed from the lower to upper void region, whereas that in the type-b faceted void configuration is observed at the void center, with the critical shear stress decreasing with increasing and decreasing normalized distance from the void center (dnorm). As for the type-c faceted void configuration, the highest critical shear stress is observed at the lower region of the void. The difference in the cutting process is due to the faceted plane; the highest critical shear stress is observed at the region where the dislocation cuts the {110} plane of the faceted void. The {110} plane is the close packed plane of the body center cubic structure, and therefore the largest amount of energy is required to cut the atomic binding and slip the next equivalent {110} plane compared to other planes.

Deeply subwavelength giant monopole elastodynamic metacluster resonators.

The giant monopole resonance is a well-known phenomenon, employed to tune the dynamic response of composite materials comprising voids in an elastic matrix which has a bulk modulus much greater than its shear modulus, e.g. elastomers. This low frequency resonance (e.g. for standard elastomers, where and are the compressional wavelength and void radius, respectively) has motivated acoustic material design over many decades, exploiting the subwavelength regime. Despite this widespread use, the manner by which the resonance arising from voids in close proximity is affected by their interaction is not understood. Here, we illustrate that for planar elastodynamics (circular cylindrical voids), coupling due to near-field shear significantly modifies the monopole (compressional) resonant response. We show that by modifying the number and configuration of voids in a metacluster, the directionality, scattering amplitude and resonant frequency can be tailored and tuned. Perhaps most notably, metaclusters deliver a lower frequency resonance than a single void. For example, two touching voids deliver a reduction in resonant frequency of almost 16% compared with a single void of the same volume. Combined with other resonators, such metaclusters can be used as meta-atoms in the design of elastic materials with exotic dynamic material properties.

A heteroencoder architecture for prediction of failure locations in porous metals using variational inference

In this work we employ an encoder–decoder convolutional neural network to predict the failure locations of porous metal tension specimens based only on their initial porosities. The process we model is complex, with a progression from initial void nucleation, to saturation, and ultimately failure. The objective of predicting failure locations presents an extreme case of class imbalance since most of the material in the specimens does not fail. In response to this challenge, we develop and demonstrate the effectiveness of data- and loss-based regularization methods. Since there is considerable sensitivity of the failure location to the particular configuration of voids, we also use variational inference to provide uncertainties for the neural network predictions. We connect the deterministic and Bayesian convolutional neural network formulations to explain how variational inference regularizes the training and predictions. We demonstrate that the resulting predicted variances are effective in ranking the locations that are most likely to fail in any given specimen.

Parametric study on vertical void configurations for improving ventilation performance in the mid-rise apartment building

Double-loaded affordable apartments, which are commonly seen in tropical developing countries, suffer from poor cross-ventilation, particularly on the leeward side of the buildings. This study aims to parametrically evaluate closed-vertical void configurations to improve the ventilation performance of the double-loaded apartment building using the validated CFD models. The configurations include the aspect ratio of the void, pilotis size and fin size with various wind directions. The results showed that the most influential parameter for enabling natural ventilation by augmenting pressure coefficient differences on the windward and leeward sides of the building was the fin size, followed by wind direction and aspect ratio. Wind direction influenced the windward side more than the leeward side, whereas aspect ratio influenced the leeward side over the windward side. It was concluded that the mass flow rates on both sides of the building could be maximised by optimally combining the void's fin size and aspect ratio under the specific prevailing wind directions. These findings would help create sustainable design guidelines for improving natural ventilation by incorporating a closed-vertical void in the mid-rise apartment buildings in the tropics.

Secondary void interactions under shear in thin porous sheets

A compressive stress state decreases the mechanism of void growth in metals. However, in reality, void sheets are evidenced on the fracture surface of metallic shear-compression specimens. The prevailing stress state in the gauge is in contrast to the features observed on the fracture surface. This work examines the mechanical fields in the surroundings of a void under shear as a realization of the mechanical behavior of a ligament between coalescing voids. It is shown that the presence of secondary voids in the surroundings of one primary void reduces the average triaxiality in the cell and homogenizes its distribution; thus, it can be surmised to be mechanically favored. This paper examines this point from a micromechanical perspective utilizing the periodic unit cell method with a single void in its center as a reference case to be compared with other configurations of voids distributed in the cell. It is hypothesized that nucleated voids grow and promote nucleation and growth of voids around them – a mechanism to describe void sheet formation as a positive feedback loop process. The effect of extra secondary voids on the mechanical fields is examined and correlated to their size and location with respect to the existing primary void.

Simulation of the influence of Lode parameter on ductile fracture using an ellipsoidal void model

The influence of the Lode parameter μ on ductile fracture is simulated using an ellipsoidal void model proposed previously by the author, and is analyzed using an elementary theory proposed in this study. The equivalent strain at fracture calculated using the ellipsoidal void model in the case of μ=+1 is almost identical to that in the case of μ=-1. The equivalent strain at fracture calculated using the ellipsoidal void model in the case of μ=±1 is larger than that in the case of μ=0. The void shape and the void configuration at fracture calculated using the ellipsoidal void model significantly depend on the stress triaxiality but scarcely depend on the Lode parameter. The proposed elementary theory indicates that the equivalent strain at fracture in the case of μ=±1 is 2/3 times larger than that in the case of μ=0. The equivalent strain at fracture analyzed using the elementary theory almost agrees with that calculated using the ellipsoidal void model.

A Brief Review on Additive Manufacturing of Polymeric Composites and Nanocomposites.

In this research article, a mini-review study is performed on the additive manufacturing (AM) of the polymeric matrix composites (PMCs) and nanocomposites. In this regard, some methods for manufacturing and important and applied results are briefly introduced and presented. AM of polymeric matrix composites and nanocomposites has attracted great attention and is emerging as it can make extensively customized parts with appreciably modified and improved mechanical properties compared to the unreinforced polymer materials. However, some matters must be addressed containing reduced bonding of reinforcement and matrix, the slip between reinforcement and matrix, lower creep strength, void configurations, high-speed crack propagation, obstruction because of filler inclusion, enhanced curing time, simulation and modeling, and the cost of manufacturing. In this review, some selected and significant results regarding AM or three-dimensional (3D) printing of polymeric matrix composites and nanocomposites are summarized and discuss. In addition, this article discusses the difficulties in preparing composite feedstock filaments and printing issues with nanocomposites and short and continuous fiber composites. It is discussed how to print various thermoplastic composites ranging from amorphous to crystalline polymers. In addition, the analytical and numerical models used for simulating AM, including the Fused deposition modeling (FDM) printing process and estimating the mechanical properties of printed parts, are explained in detail. Particle, fiber, and nanomaterial-reinforced polymer composites are highlighted for their performance. Finally, key limitations are identified in order to stimulate further 3D printing research in the future.

Influence of Void Configurations on Hot Spot Temperature in PBXs under Impact Loading

AbstractIn this study, similarity analysis and mesoscale simulations were performed to investigate hot spot temperatures in triangular voids inside polymer‐bonded explosives (PBXs). We found that there are two different void collapse modes, namely single and double hydrodynamic jet modes. The hot spot temperatures achieved in the double hydrodynamic jet mode are much higher than previous predictions, such as predictions based on circular voids, which agrees with recent experimental observations. Additionally, we verified that void configuration (position and shape) plays a significant role in increasing hot spot temperatures, implying that when establishing a mesoscale reaction rate model, it is insufficient to use void volume alone to characterize the impact sensitivity of PBXs. Finally, two semi‐empirical analytical expressions are proposed to represent the dependence of hot spot temperature on both the void configuration and shock intensity. The resulting theoretical predictions are in good agreement with the numerically simulated results. Such mesoscale analytical expressions can be used for establishing theoretical mesoscale hot spot ignition rate models in the future.

An Estimate of the Sensitivity of Muon Radiography Detectors to Voids in the Ground

Model simulations of test experiments on muon radiography for several configurations of voids in the ground and different muon detector positions are presented. The sensitivity of the method for the considered cases is estimated. This study lays the basis for quantitative analysis of data in muon radiography and can be used for planning future experiments.

Static and dynamic shear-compression response of additively manufactured Ti6Al4V specimens with embedded voids

The shear-compression response of additively manufactured Ti6Al4V specimens containing discrete artificial voids is investigated under quasi-static and dynamic loadings. Specimens containing spherical voids are designed with one or three voids with variable spacing between them. Specimens containing spheroidal prolate voids are designed with different void orientations with respect to the shear direction, while the total volume fraction of all void configurations is kept constant in the whole study. It is found that the presence of the void(s) reduces the displacement to failure, compared to the dense specimens, in both quasi-static and dynamic regimes. The shape and the number of voids have a noticeable effect on results only in the quasi-static regime. However, changing the distance between the spherical voids or changing the orientation of the prolate voids does not affect significantly the load-displacement curves in both strain rate regimes. Fractographic observations show that shear-compression specimens fail dominantly by shear, whereas previously investigated shear-tension specimens fail dominantly by tension. These observations are supported by preliminary numerical simulations.

Investigation on effects of solder paste voids on thermal and optical performance of white high-power surface-mounted device LEDs

Purpose The presence of voids in the solder layer has been considered as one of the main issues causing reliability problems in optoelectronic devices. Voids can be created due to trapped gas, clean-up agent residues (fluxes), poor wettability at interface or shortcoming of the reflow process. The voids hinder the heat conduction path and subsequently, the thermal resistance will increase. The purpose of this paper is to investigate the influence of lead-free water-washable Sn96.5Ag3.0Cu0.5 (SAC305) solder paste (SP) voids on the thermal and optical performance of white high-power (HP) surface-mounted device (SMD) light-emitting diode (LED). Design/methodology/approach Five LEDs are mounted on five SinkPAD substrates by using the SP. The SMT stencil printing is used to control the thickness of the SP and reflow oven for the soldering process. The fraction of voids in the SP layer is calculated using the X-ray machine software. The thermal parameters of the LEDs with different voids fraction and configuration are measured using a thermal transient tester (T3Ster) system. In addition, the optical characterizations of the LEDs are determined by the thermal and radiometric characterization of power LEDs (TeraLED) and the electroluminescence by using the spectrometer. Findings The results showed that the thermal performance and temperature distribution are improved for the LED with lower voids fraction and good filling state of soldering. In addition, luminous flux, efficacy and color shift of the LEDs with different fraction and configurations of voids on the SP layer are compared and discussed. It is found that the color shift of LED1 of low voids fraction and higher thickness are less than other LEDs. Originality/value The paper provides valuable information about the effect of water-washable SAC305 SP voids fraction and filling state of solder on the thermal and optical performance of ThinGaN HP SMD LED. A comprehensive overview of the outcomes is not available in the literature. It was shown experimentally that the voids fraction, height and configuration of the SP layer could strongly influence the heat dissipation efficiency and thermal resistance. This study can help in heat diffusion investigation and failure analysis of HP SMD LEDs.