Random Distribution Of Voids Research Articles

Thermomechanical analysis of carbon fiber-reinforced polymer representative volume elements with voids

This work presents results of numerical simulations to investigate the effect of different void percentages on composite materials’ Coefficient of Thermal Expansion (CTE) and local stress fields. A random distribution of voids is considered within the Representative Volume Element (RVE) matrix, and different types of microstructures are considered, including square-packed and randomly distributed fibers. The use of a higher-order beam model within the framework of Carrera Unified Formulation (CUF) leads to a Component-Wise (CW) approach, resulting in an accurate, 3D description of the cross-section although using a 1D formulation. Numerical results for different fiber volume fractions and void concentration percentages demonstrate the agreement of the computed effective coefficients of thermal expansion of the present micromechanical thermoelastic model with references from the literature. The local stress fields are affected by voids, with higher effects over the matrix. Furthermore, higher void fractions lead to higher variability of stress peaks.

An analysis of Lode effects in ductile failure

An isotropic multi-surface model of porous material plasticity is derived and employed to investigate the effects of the third stress invariant in ductile failure. The constitutive relation accounts for both homogeneous and inhomogeneous yielding of a material containing a random distribution of voids. Individual voids are modeled as spheroidal but the aggregate has no net texture. Ensemble averaging is invoked to operate a scale transition from the inherently anisotropic meso-scale process of single-void growth and coalescence to some macroscopic volume that contains many voids. Correspondingly, expressions for effective yield and associated evolution equations are derived from first principles, under the constraint of persistent isotropy. It is found that the well-known vertex on the hydrostatic axis either disappears for sufficiently flat voids or develops into a lower-order singularity for elongated ones. When failure is viewed as the onset of an instability, it invariably occurs after the transition to inhomogeneous yielding with the delay between the two depending strongly upon the Lode parameter. The strain to failure is found to be weakly dependent on the Lode parameter for shear-dominated loadings, but strongly dependent on it near states of so-called generalized tension or compression. Experimentally determined fracture loci for near plane stress states are discussed in light of the new findings.

Micromechanical modelling of pervious concrete reinforced with treated fibres

Pervious concrete has been widely used in parking lots and airport fields. However, excessive cracking and spalling issues of pervious concrete remain a challenge for its adoption in wider applications, since its binding material proportion is low and the use of fine aggregates is nearly zero. When loaded in compression, failure appears first in the weak concrete zone, induced by the random distribution of voids. The process continues until failure of the whole specimen. Fibre-reinforcement delays crack generation and enhances the strength of the host matrix. However, the bonding mechanism between the fibre and concrete matrix is a controversy in the literature. In this study, the influence of polypropylene fibre inclusion on the strength and stiffness of pervious concrete microstructure was studied through delicate interface modelling. Two-dimensional microstructures of pervious concrete were generated using Matlab and Ansys parametric design languages. A cohesive-zone technique was then used to model the interface between fibres and the concrete matrix. Load–displacement plots were thus obtained in Ansys for specimens under compression for various void–fibre percentages and different treated interface properties.

Characterization of pervious concrete through image based micromechanical modeling

Pervious concrete has been widely used in parking lots and airport fields. However, past experience has shown durability and strength of the pervious concrete remains a challenge for adopting them in wider applications, as the binding material proportion is low and the use of fine aggregates is nearly zero. When loading in compression, the failure appears first in the weak concrete zone induced by the random distribution of voids. The process continues until the failure of the whole specimen occurs. Thus, the position, size and shape distribution of voids in the pervious concrete microstructure control the overall behavior of the material. In this work, the influence of the distribution of voids on the strength, stiffness, and permeability of pervious concrete microstructure is studied by 2D image analysis and finite element modeling. The position and size distribution of the microstructure voids are first summarized through Fast Fourier Transformation (FFT) analysis. A finite element analysis is then conducted on the microstructure models regenerated through the derived microstructure characteristics using ANSYS Parametric Design Language (APDL). The effect of the sample size of 2D microstructures is studied by varying the section size and a possible representative volume element (RVE) is therefore found. Predicted stress-strain plots are generated for the 2D specimen under compressive load and the obtained results, including stiffness, strength and permeability are compared with the results from the experiments conducted following ASTM standards. The comparison between models and physical experiments showed around or above 90% accuracy, which validates the suggested micromechanical analysis method of the previous concrete composite.

The role of disorder in the domain wall dynamics of magnetic nanostrips

We study the role of the disorder in the dynamics of the domain walls (DW) in nanostrips with in-plane magnetization. In contrast with previous works where the disorder is due to edge roughness, we consider the role of a random distribution of voids, thus simulating local changes of the magnetization saturation value. By making use of the high-speed computational capability of GPUs, and an ad hoc micromagnetic code, we compute the speed of DWs under both applied fields (up to 15 mT), and spin-polarized currents (up to 30 A/μm2), for four different void densities. Field and currents are applied for 20 ns. We also consider both adiabatic and non-adiabatic spin-torque effects (ξ parameter equal 0 and 0.04, respectively). For all the cases, we repeat the simulation for 50 realizations of the void distributions. No thermal effects are considered. While some results can be understood in the line of the models reported in the literature, some others are much more peculiar. For instance, we expect a lower value of the maximum DW speed. This actually occurs in the field driven case, but with a less dramatic drop at the Walker breakdown, due to the difficulty to nucleate an antivortex DW. When nucleated, it gets easily pinned, thus preventing its retrograde motion typical for disorder-free strips. In the case of current drive with non-adiabatic spin-transfer torque, the Walker breakdown current increases strongly with the void density. This results in an increased value of the maximum speed available. Another important consequence of the disorder is that at low fields/currents the depinning transition regions appear to be more rounded, resembling creep behavior. This can have important consequences in the interpretation of experimental data.

Mesoscopic picture of fracture in porous brittle material under shock wave compression

Void is one of the most common type of structure flaws existing in brittle materials, which dramatically affects the shock loading response of brittle materials. A quantitative discrete element method is employed in this work to study the fracture characteristics of porous isotropic brittle material under shock wave compression. Scenarios of isolated void, three types of simple distribution and random distribution of voids are computed, from which we find that shear fracture and local tensile fracture are two type of basic fracture modes for brittle material under shock wave compression. Coalescence of damage bands between voids can induce the collapse of voids at relatively low pressure, while stress relaxation caused by damage can shield fracture evolution in a certain zone. The combination of amplification and shielding effects of damage results in a unique pattern of alternate distribution of severe and mild damage zones. These simulation results present a basic physics picture for the understanding of evolution process and mechanism of fracture in porous brittle material under shock wave compression.

Modeling the dislocation-void interaction in a dislocation dynamics simulation

ABSTRACTIn this paper, we propose a model for dislocation-void interaction in Iron that is amenable to dislocation dynamics simulations. Voids are treated as shearable particles whose shear resistance and thermal activation parameters are obtained from atomistic calculations. The modeling is first validated by direct comparison with molecular dynamics calculations. A good agreement is found especially at 0K and high temperature. The interaction with a random distribution of voids is then investigated.

Compressive Deformation in Aluminum Foam Investigated Using a 2D Object Oriented Finite Element Modeling Approach

The stochastic nature of aluminum foam structure, having a random distribution of voids, makes it difficult to model its compressive deformation behavior accurately. In this paper, a 2-dimensional simplified modeling approach is introduced to analyze the compressive deformation behavior that occurs in Alporas aluminum foam (Al foam). This has been achieved using image analysis on real undeformed aluminum foam images obtained by VHX-100 digital microscope. Finite element mesh for the cross sectional model is generated with Object Oriented Finite element (OOF) method combined with ABAQUS structural analysis. It is expected that OOF modeling enable prediction of the origin of failure in terms of localized deformation with respect to the microstructural details. Furthermore, strain concentration sites leading to the evolution of the deformation band can be visualized. Thus, this investigation addresses the local inhomogeneity in the Al foam structure. This study implies that the OOF modeling approach combined with experimental observations can provide better insight into the understanding of aluminum foam compressive deformation behavior.

Kinetic Monte Carlo simulations of void lattice formation during irradiation

Over the last decade, molecular dynamics simulations of displacement cascades have revealed that glissile clusters of self-interstitial crowdions are formed directly in cascades and that they migrate one-dimensionally along close-packed directions with extremely low activation energies. Occasionally, under various conditions, a crowdion cluster can change its Burgers vector and glide along a different close-packed direction. The recently developed production bias model (PBM) of microstructure evolution under irradiation has been structured specifically to take into account the unique properties of the vacancy and interstitial clusters produced in the cascades. Atomic-scale kinetic Monte Carlo (KMC) simulations have played a useful role in understanding the defect reaction kinetics of one-dimensionally migrating crowdion clusters as a function of the frequency of direction changes. This has made it possible to incorporate the migration properties of crowdion clusters and changes in reaction kinetics into the PBM. In the present paper we utilize similar KMC simulations to investigate the significant role that crowdion clusters can play in the formation and stability of void lattices. The creation of stable void lattices, starting from a random distribution of voids, is simulated by a KMC model in which vacancies migrate three-dimensionally and self-interstitial atom (SIA) clusters migrate one-dimensionally, interrupted by directional changes. The necessity of both one-dimensional migration and Burgers vectors changes of SIA clusters for the production of stable void lattices is demonstrated, and the effects of the frequency of Burgers vector changes are described.

Frequency‐dependent elastic properties of rubberlike materials with a random distribution of voids

A self‐consistent multiple scattering approach using the T‐matrix method with suitable pair‐correlation function has been employed to study the frequency‐dependent properties of rubber materials containing a random distribution of voids. Numerical results indicate that for a certain range of concentration of voids, the resonance of the voids controls the frequency‐dependent effective properties such that the imaginary part of the effective wavenumber becomes greater than the real part. The so‐called “superviscous” propagation in which the propagating wave may be damped out over a distance smaller than a wavelength, seems to occur over a bandwidth controlled by the concentration of voids (c). The bandwidth increasing with concentrations in the range 0 0.10, this kind of behavior does not exist for the material system considered.

Frequency‐dependent dynamic properties of rubber materials with a random distribution of voids

A self‐consistent multiple scattering approach using the T‐matrix method with suitable pair‐correlation function has been employed to study the frequency‐dependent properties of rubber materials containing a random distribution of voids. Numerical results indicate that for a certain range of concentration of voids, the resonance of the voids controls the frequency‐dependent effective properties such that the imaginary part of the effective wavenumber becomes greater than the real part. The so‐called “superviscous” propagation in which the propagating wave may be damped out over a distance smaller than a wavelength, seems to occur over a bandwidth controlled by the concentration of voids (c). The bandwidth increasing with concentrations in the range 0 0.10, this kind of behavior does not exist for the material system considered.

Computer simulation of ductile fracture in a random distribution of voids

A theory is developed to calculate the fracture strain in a material with a random distribution of voids. The voids are of equal size and are cylindrical in shape with a square cross section. Random arrays of voids are generated using a computer. The material is assumed to be rigid and perfectly plastic and is exposed to plane strain tension. The theory is based on the assumption that the material deforms homogeneously until localized deformation along a row of voids intervenes. Upper-bound solutions are used to derive criteria for the onset of inhomogeneous deformation. The fracture is assumed to occur so rapidly after the beginning of localized deformation that the fracture strain is equal to the strain to inhomogeneous deformation. Several fracture paths have been selected from the computer-generated arrays of voids and the fracture strain has been computed as a function of the tensile hydrostatic pressure. the fracture propagates along the path in the material which has the smallest fracture strain. Distribution functions are presented for intervoid spacings along the fracture paths. The study includes two volume fractions of voids: 4% and 0.25%.

THE USE OF VACUUM-EVAPORATED GOLD FILMS TO INVESTIGATE THE OXIDATION MECHANISM OF METALS

Vacuum-evaporated gold films were used as inert markers to study the oxidation mechanism of cobalt at 850°C. Gold-marked and unmarked specimens were simultaneously oxidized for 138 hr. With the exception of the gold marker appearing as a dispersed row of spheriods within the CoO layer closest to the metal-oxide interface, and the random distribution of voids in the specimens, the microstructures of the gold-marked and unmarked specimens were identical, and are in agreement with microstructures reported in the literature.