Effect Of Void Size Research Articles

Interaction between nano-voids and migrating grain boundary by molecular dynamics simulation

Understanding the interaction between void and grain boundary (GB) is important to the design of radiation resistant materials by GB engineering and to achieve high quality metallurgical diffusion joining. In this study, the interaction between nano-voids and GBs has been systematically investigated by molecular dynamics simulations. The bicrystal Cu sample was used throughout the work, and the dynamic GB-void interaction was achieved by GB migration under shear deformation. Both high-angle GBs (Σ5 (310) GB, Σ5 (210) GB) and low-angle GBs (Σ37 (750) GB, Σ61 (650) GB) were investigated, and the effect of void size and temperature on the simulation result was examined. The transition of the deformation mechanism from GB migration to dislocation propagation was observed during the interaction between voids and high-angle GBs at low temperature (T = 10 K). At higher temperature (T = 300 and 600 K), the migrating GB can be pinned to voids, freely traversed voids, or dissolved voids in the process of their interaction. The void-drag effect on GB motion was analyzed based on the Zener-like equation, which indicates that the retarding pressure applied to the migrating GB by a void is closely related to the surface area of the void, the degree of contact between GB and void, and GB energy. By investigating the thermal stability of a void at the stationary GB, it was found that the dissolution of voids at a moving GB cannot be attributed solely to the thermal diffusion mechanism. The dynamic migration of high-angle GBs can significantly accelerate the dissolution time of the void. Atomistic analysis indicated that the migrating GB rearranged the atoms on the void surface by the collective motion of structural units, and the GB structural phase transformation provided an efficient diffusion channel for transporting the vacancies. The low-angle GBs show a reduced ability to dissolve the voids than the high-angle GBs, which can be ascribed to their low GB energy and diffusion coefficient, the fast GB migration velocity, and the discrete GB structure.

A homogenized model for size-effects in porous metals

A simple method for enriching conventional homogenized porous plasticity models with size-dependence for micron scale voids is proposed. A recent strain gradient plasticity theory with dissipative gradient effects is generalized to finite strains in order to quantify size-scale effects on void growth under different loading conditions. Based on numerical cell model studies for regularly distributed voids, the size-dependence of homogenized yield surfaces is quantified in terms of effective stress and mean stress. It is proposed to account for void size effects by two simple extensions to conventional models. One is the establishment of a new size-dependent effective void volume fraction smaller than the physical one. The other is the introduction of a size-dependent reduction of the influence of the mean stress. The results obtained are based on numerical unit cell calculations, but it is conjectured that they may be used for any isotropic yield surface for porous metals that includes explicit dependence on porosity and mean stress.

Void Content Measurement in Fiber Reinforced Plastic Composites by X-Ray Computed Tomography

The void formation in FRP composites is unavoidable and the void content measurement is very important to study its deleterious effects on the mechanical properties of the material. Generally destructive methods are used to calculate the void volume fraction. But the recent advances in X-ray computed tomography can be used to detect and quantify the void content in the composites in a non-destructive manner. In this study average area method is proposed and validated for the void volume measurement from the X-ray CT image slices through digital image processing. The effects of void size, shape and position in the accuracy of the measurement is studied and presented here. The void volume fraction of a CFRP laminate manufactured with compression molding technique is calculated by this method and found to be around 1%.

Effect of surface void percentage (SVP) on the unconfined compressive strength (UCS) of porous rocks

In this paper, the effect of areal void size on unconfined compressive strength (UCS) depending on the core diameter is investigated and discussed in terms of obtaining more accurate and reliable strength values relating to the effect of the local increase in stress and corresponding decrease in area on the loading surface. According to the results obtained from UCS tests, the overall trend was a significant loss in the UCS with increasing surface void per cent. It can be concluded that the areal size of voids on loading surfaces is an important indicator of porosity as well as rock strengths when considering rock engineering in vuggy anisotropic rock masses.

Limit analysis and homogenization of nanoporous materials with a general isotropic plastic matrix

In this paper, a closed-form expression of a macroscopic strength criterion for ductile nanoporous materials is established, by considering the local plastic behavior as dependent on all the three isotropic stress invariants and by referring to the case of axisymmetric strain-rate boundary conditions. The proposed criterion also predicts void-size effects on macroscopic strength properties. A homogenization procedure based on a kinematic limit-analysis is performed by addressing a hollow-sphere model comprising a rigid-ideal-plastic solid matrix. Void-size effects are accounted for by introducing an imperfect-coherent interface at the cavity boundary. Both the interface and the solid matrix are assumed to obey to a general isotropic yield function, whose parametric form allows for a significant flexibility in describing effects induced by both stress triaxiality and stress Lode angle. Taking advantage of analytical expressions recently provided by Brach et al. [Int J Plasticity 2017; 89: 1–28] for the corresponding support function and for the exact velocity field under isotropic loadings, a parametric closed-form relationship for the macroscopic strength criterion is obtained as the solution of an inequality-constrained minimization problem, the latter being faced via the Lagrangian method combined with Karush-Kuhn-Tucker conditions. Finally, several comparative illustrations are provided, showing the influence of local-yield-function parameters on the established criterion, as well as the model capability to describe the macroscopic strengthening, typical of nanoporous materials, induced by a void-size reduction for a fixed porosity level.

Influence of dust void on neon DC discharge

The diffusion/drift model of the positive column of glow discharge in neon with fine dust particles was used to study the role of a dust cloud with a void in the interaction between plasma and dust particles in the range of neon pressure and discharge current where dust particles may form structures with cavities. The results represent the nonlocal effect of void size on plasma composition, configuration of electric field and on distributions of plasma components in discharge with voids in dust structures. Simulations show that the electric field strength and the metastable atom concentration inside the void are higher than in the discharge without dust particles, while electron concentration may be either higher or lower.

Nanoporous materials with a general isotropic plastic matrix: Exact limit state under isotropic loadings

In this paper, hydrostatic strength properties of nanoporous materials are investigated by addressing the limit state of a hollow sphere undergoing isotropic loading conditions. Void-size effects are modelled by treating the cavity boundary as a coherent-imperfect homogeneous interface. The hollow sphere is assumed to be comprised of a rigid-ideal-plastic material obeying to a general isotropic yield criterion. The latter is defined by considering a simplified form of the yield function proposed by Bigoni and Piccolroaz in [Int J Solids Struct; 41: 2855–2878], resulting able to account for a broad class of pressure-sensitive materials whose plastic response is also affected by the stress Lode angle. The corresponding support function is consistently derived and discussed. The exact solution of the limit-state problem is fully determined, providing a closed-form description of stress, strain-rate and velocity fields, as well as the macroscopic hydrostatic strength of nanoporous media. Proposed approach allows to consistently generalise available analytical solutions for porous and nanoporous materials, by accounting for a general plastic response of the solid matrix and for void-size effects. Finally, present exact solution, as well as the identification of the support function for the adopted general strength criterion, open towards novel kinematic limit-analysis approaches for describing macroscale strength properties of nanoporous materials under arbitrary triaxial loadings.

A computational insight into void-size effects on strength properties of nanoporous materials

In this paper, strength properties of nanoporous materials are addressed aiming to establish novel insights into the influence of void-size effects. To this end, a virtual spherically-nanovoided sample of an aluminium single crystal is investigated by adopting a Molecular-Dynamics computational approach. Elasto-plastic mechanical response, under triaxial strain-based conditions and including axisymmetric and shear states, are numerically experienced, identifying the corresponding limit stresses. Computed strength measures are used to furnish estimates of strength domains, described in terms of meridian and deviatoric profiles. The influence of void-size effects on the computed strength properties is clearly quantified for different porosity levels, numerical results confirming a strengthening of the sample when the void radius reduces. Moreover, it is shown that the occurrence and the amount of void-size effects are strongly dependent on the Lode angle, resulting in a shape transition of both meridian and deviatoric strength profiles when the void radius is varied. Finally, present results suggest porosity-dependent threshold values for the void radius above which void-size effects tend to disappear. With respect to the actual state-of-the-art, useful benchmarks for assessing the effectiveness of available theoretical models are provided, resulting in a novel incremental contribution towards the definition of advanced modelling strategies for describing strength properties of nanoporous materials.

Impact of liquid-filled voids within the active layer on transport through thin-film composite membranes

Transport through composite membranes is strongly influenced by the morphologies of both the porous support membrane and overlying selective thin-film (i.e., active layer). Recently, the occurrence of water-filled voids within the active layer has been suggested in the literature; however, their effects on transport are uncertain. Here, we theoretically consider, through numerical modeling, the effect that liquid filled-voids have on the transport of water and solutes through supported thin-film morphologies. Specifically, we evaluated the effect of volume void fraction, void size, and relative location of the voids within the active layer with respect to both surface roughness features and the pores of the support. Transmission electron microscopy image analysis was used to obtain evidence supporting the existence of voids in two commercial brackish and seawater reverse osmosis membranes; the volume fraction of the active layer the voids occupied was determined to be about 30% for both membranes. Our calculations show that, for films with a constant polymer volume, a rough film containing voids is more permeable than an equivalent, homogeneous flat film, due to the creation of shorter paths for diffusion, not due to increased surface area, though the latter is shown to correlate positively with permeability when no base-film exists under the void. Results further illustrate the importance of void position within the thin-film, indicating that even with a significant void fraction, the presence of an underlying polymer base-film negatively impacts the permeability. Voids created closer to the bottom of the active layer will increase membrane permeability. Conversely, results show that even at significant void fractions, voids located closer to the top of the active layer negatively impact the permeability. Variations in the proportion of the active layer overlaying the voids will impact the flux distribution along the membrane, and may be used to reduce flux ‘hotspots’, which may enhance localized concentration polarization and fouling propensity. A strategy for creating high permeability membranes with relatively even flux distributions may include a combination of a rough film with a reduced base thickness, and with thicker regions of the film aligned with the support pore locations. Understanding the role of the voids in determining the transport properties of the membranes provides motivation for controlling their formation.

Molecular Dynamics Study of Uniaxial Deformation in Perfect and Defective Aluminum

In materials, strength is usually controlled by the occurrence of grain boundaries, dislocations, voids, and other microstructural defects. molecular dynamics (MD) simulations with the embedded atom method potential have been employed to investigate the uniaxial tension and compression of single crystal aluminum with and without defects. In this paper, defect refers to a void or grain boundary. For perfect crystal aluminum, the ideal strengths and stress-strain curves obtained from the MD simulations agree well with those obtained by first principles calculations. The strength and ductility of Al decrease with the occurrence of void and grain boundaries. The effect of void size on the uniaxial stress-strain relations is investigated. Also, dislocation nucleation and emission from the void free surface are observed. In order to study the effect of a grain boundary, we generate a symmetric tilt grain boundary Σ5(310) and strain is imposed perpendicular to the boundary interface. We can clearly see that fracture starts in the grain boundary. The observed defect pattern is expected to provide some helpful insights into the damage mechanism of ductile materials at the microscale. The effects of temperature, strain rate, and crystalline direction on the ideal strength are also discussed.

Effect of air volume fraction and void size on the performance of acoustically thin underwater noise isolation panels

We present experimental measurements of the acoustic behavior of underwater sound absorbing and isolating panels composed of three deeply sub-wavelength layers. The panels were constructed with an inner layer of Delrin plastic containing through-thickness circular air-filled holes sandwiched between two rubber layers. Panel isolation efficacy was quanitifed by measuring insertion loss for non-dimensional panel thicknesses, kd, ranging from 7.2 x 10-3 to 8.5 x 10-2. Finite element numerical simulations will be discussed which indicate that the panel structure mimics a planar encapsulated bubble screen exactly one bubble thick but displays measured performance that is significantly more broadband. Experimental and numerical results indicate that panels with kd ranging from 2.8 x 10-2 to 7.2 x 10-2 can generate a frequency-averaged insertion loss ranging from 5 to 12 dB, dependent on air volume fraction in the panel. The effect of individual air pocket resonance frequency on insertion loss will also be discussed. [Work supported by ONR.]

Numerical assessment, implementation and application of an extended Gurson model accounting for void size effects

Abstract Using recent results obtained by Dormieux and Kondo (2010) on strength properties of ductile nanoporous media, we aim at investigating the mechanical behaviour of this class of materials. To this end, we first assess and validate the yield criterion proposed by these authors by performing numerical limit-analyses, based on the standard finite element method including elasticity, of an elementary porous cell. This requires special care in order to account for surface stresses at the void's boundary. Then, taking advantage of the normality property for the flow rule to derive the porosity and void size evolution equations, we formulate a complete model for ductile nanoporous materials. Owing to the parametric form of the yield locus, a specific procedure is developed for the numerical implementation of the model. The problem of plastic correction of the elastic stress predictor is reduced to the resolution of two nonlinear equations. Finally, the model is applied to proportional loadings, for different stress triaxialities. The void size appears to have a strong influence on the mechanical behaviour and on the porosity evolution.

Solder void position and size effects on electro thermal behaviour of MOSFET transistors in forward bias conditions

This research aims to enhance the understanding on position and size effects on the electro thermal behaviour of low voltage power MOSFET transistors in forward bias condition. The numerical simulations are based on a fractional design of experiments (DoE). The performance of a finite elements model is discussed by comparing thermal and electrical measurements to results of finite elements simulation on a module of free void and voided solder. The void in the model is afterwards parameterized on position and size, according to the fractional DoE of the study. The combined functions issued from the parametric simulations and the DoE show the main impact of void size on temperature of the device and on the surface temperature of the bonding wires. From the numerical viewpoint, the most impacting position of void depends highly on the void size. The redistribution of current density and temperature on MOSFET chip and bonding wires due to solder void is also observed. A future experimental study in respect to the same DoE is expected in prospect, in order to fulfil the complementarity for this approach.

Microstructure effects on shock response of Cu nanofoams

Microstructure effects on shock response of Cu nanofoams are investigated with molecular dynamics simulations, including elastic-plastic deformation, Hugoniot states, void collapse, nanojetting, and vaporization. The microstructure features examined include pore shape, arrangement and size, as well as grain boundaries. The elastic-plastic transition, void collapse, and jetting including vaporization, are dependent on the microstructure, although to different extents. The void arrangement and aspect ratio play an important role. The effects of grain boundaries and void size are less pronounced. Considering the measurement scatter inherent for porous materials, the high pressure Hugoniot states are not sensitive to microstructure. Jetting during void collapse is due to tensorial velocity gradients (direction and amplitude), and a combined result of forward, divergent, and convergent flows with varying contributions; this mechanism and related processes are common for different microstructures. Free surface jetting involves necking and cavitation. Elliptical voids with large aspect ratios, and with their centers aligned linearly with the shock direction, are particularly efficient in inducing high speed jetting and vaporization.

A Gurson-type model accounting for void size effects

In this paper we present an extension of the Gurson model of cavity growth which includes the void size effect. To this end, we perform the limit analysis of a hollow sphere made up of a Fleck and Hutchinson’s strain gradient plasticity material. Based on the trial velocity field of Gurson, we derive an approximate closed form expression of the macroscopic criterion. The latter incorporates the void size dependency through a non dimensional parameter defined as the ratio of the cavity radius and the intrinsic length of the plastic solid. The accuracy of this approximate criterion is demonstrated by its comparison with numerical data. In the last part of the paper we present a complete plasticity model involving the damage rate and a power-law strain hardening of the matrix. It is shown that the cavity size effect has a strong dependency on damage growth as well as on the stress strain response.

Analysis of void shape and size in the collector plate and polycaprolactone molecular weight on electrospun scaffold pore size

AbstractMany variations in the electrospinning process have emerged to control pore size needed in tissue scaffolds including introducing voids in the collector plate. However, the effect of different shapes and sizes of voids on fiber and pore sizes is not well understood. In this study, we evaluated the effect of void size and shapes in the collector plate on polycaprolactone (PCL) fiber size and pore size. First, we performed experiments using three different sizes of circular voids (0.9, 1.4, and 1.9 cm). Also, we evaluated the effect of triangular and rectangular voids. Further, changes in the electrical field within the voids were evaluated using computational fluid dynamic software COMSOL. Fibers using mixtures of PCL (high, medium, and low molecular weight) formed using void collector plate and performed degradation characteristics for 30 days in physiological conditions (37°C and 7.4 pH). These results showed that the size and the morphology of fibers are not affected by the geometrical patterns of the voids. However, fiber alignment is affected by the void shape due to favorable rearrangement in the electrical field. Fibers can be formed by mixing PCL of different molecular weight, and degradation of PCL fiber was accelerated by the mixture with low molecular weight. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Effect of non-uniform void size and shape distributions on deformation failure in cast aluminum alloy

Tensile tests were conducted on several cast aluminum specimens with different degrees of porosity. The effects of non-uniform void size and shape distributions, including spherical and non-spherical types, on stress-strain behavior resulting from different initiation mechanisms were investigated. A micro-mechanics-based statistical approach was employed, and the heterogeneous microstructures could therefore be modeled during the deformation process. The predicted changes of the distributions of void size and void shape gener- ally agreed with experimental results. Void spatial variation was also quantified, and its effects on the level of failure were analyzed. The void spatial variation facilitated development of inhomogeneous deformation, which results in failure.

Coupling effects of void shape and void size on the growth of an elliptic void in a fiber-reinforced hyper-elastic thin plate

The growth of a prolate or oblate elliptic micro-void in a fiber reinforced anisotropic incompressible hyper-elastic rectangular thin plate subjected to uniaxial extensions is studied within the framework of finite elasticity. Coupling effects of void shape and void size on the growth of the void are paid special attention to. The deformation function of the plate with an isolated elliptic void is given, which is expressed by two parameters to solve the differential equation. The solution is approximately obtained from the minimum potential energy principle. Deformation curves for the void with a wide range of void aspect ratios and the stress distributions on the surface of the void have been obtained by numerical computation. The growth behavior of the void and the characteristics of stress distributions on the surface of the void are captured. The combined effects of void size and void shape on the growth of the void in the thin plate are discussed. The maximum stresses for the void with different sizes and different void aspect ratios are compared.

A renormalization approach to model interaction in microstructured solids: Application to porous elastomer

The overall physical behavior of microstructured (heterogeneous or periodic) materials depends strongly on the size, shape, and spatial distribution of the separate constituents. To predict the macroscopic constitutive response for these materials, we apply a coarse-graining transformation – termed renormalization – developed in statistical physics that bridges between the macroscopic quantities and those at finer scales. Based on this coarse-graining transformation, a two-scale generalized model for microstructured solids is built by utilizing the mathematical framework of generalized continuum mechanics. The model is labeled as generalized because it contains extra degrees of freedom representing microscopic deformation. The present study applies the renormalization technique to develop a two-scale continuum material model for porous elastomer and implements it into commercial finite element software. Direct numerical simulation of porous elastomer computational cells is used to calibrate the model parameters. The complex effect microstructure (voids) has on constitutive behavior is examined in detail with particular attention paid to void size and distribution effects in the context of multi-axial loading, macroscopic instability (buckling), bending, and fracture.

Semi-analytical investigation of stress interfacial effects in ductile media with nanosized spheroidal cavities

In this paper we investigate the interfacial stress effects on the macroscopic yield strength of plastic porous media containing nanosized spheroidal cavities. The solid matrix is assumed to obey the von Mises criterion with associated flow rule. Analysis of a rigid-ideal plastic spheroidal unit cell, containing a confocal spheroidal cavity, and subjected to arbitrary mechanical loadings is made. Void size effects are captured by considering at the interface between the matrix and the cavity a surface stress model, which relates the jump of the traction vector to the interfacial residual stress and to interfacial plastic strain. The resulting macroscopic criterion for the nanoporous material exhibits unusual features such as (i) an increase of the yield stress when the void size is decreased, (ii) asymmetry between the yield stress in uniaxial tension and compression.