Showing papers on "Micromechanics published in 2014"

Characterization of electrical and thermal properties of carbon nanotube/epoxy composites

Abstract: The electrical and thermal conductivity of pristine, oxidized, and fluorinated single-wall/multi-wall carbon nanotube (CNT) mixtures, dispersed in epoxy, were investigated as a function of CNT concentration. The effect of fabrication parameters, such as stirring rate and degree of epoxy pre-curing, on CNT dispersion was analyzed. The electrical conductivity increased by 10 and 6 orders of magnitude for pristine and oxidized CNT composites, respectively, relative to neat epoxy, while fluorinated CNT composites showed no increase in electrical conductivity. An increase of up to 5.5% was observed in thermal conductivity for pristine CNT composites while oxidized and fluorinated CNTs provide less enhancement in thermal conductivity. A micromechanics model, based on the composite cylinders method, was implemented to study the electrical and thermal conductivity of these composites. Effects in electrical and thermal conduction, such as electron hopping and thermal interface resistance, respectively, were incorporated into the model to accurately simulate the acquired experimental results.

Behavior of fresh and fouled railway ballast subjected to direct shear testing: discrete element simulation

Abstract: This paper presents the three-dimensional discrete element method (DEM) that was used to study the shear behavior of fresh and coal fouled ballast in direct shear testing. The volumetric changes and stress-strain behavior of fresh and fouled ballast were simulated and compared with the experimental results. Clump logic in particle flow code in three dimensions (PFC3D) incorporated in a subroutine was used to simulate irregular-shaped particles in which groups of 10–20 spherical balls were clumped together in appropriate sizes to simulate ballast particles. Fouled ballast with a various void contaminant index (VCI) ranging from 20 to 70% VCI was modeled by injecting a specified number of miniature spherical particles into the voids of fresh ballast. The DEM simulation captures the behavior of fresh and fouled ballast as observed in the laboratory, showing that the peak shear stress of the ballast assembly decreases and the dilation of fouled ballast increases with an increasing VCI. Furthermore, th...

Fabrication and characterization of fully biodegradable natural fiber-reinforced poly(lactic acid) composites

Abstract: Polymer composites were fabricated with poly(lactic acid) (PLA) and cellulosic natural fibers combining the wet-laid fiber sheet forming method with the film stacking composite-making process. The natural fibers studied included hardwood high yield pulp, softwood high yield pulp, and bleached kraft softwood pulp fibers. Composite mechanical and thermal properties were characterized. The incorporation of pulp fibers significantly increased the composite storage moduli and elasticity, promoted the cold crystallization and recrystallization of PLA, and dramatically improved composite tensile moduli and strengths. The highest composite tensile strength achieved was 121 MPa, nearly one fold higher than that of the neat PLA. The overall fiber efficiency factors for composite tensile strengths derived from the micromechanics models were found to be much higher than that of conventional random short fiber-reinforced composites, suggesting the fiber–fiber bond also positively contributed to the composites’ strengths.

Mechanical properties of hybrid composites using finite element method based micromechanics

Abstract: A micromechanical analysis of the representative volume element of a unidirectional hybrid composite is performed using finite element method. The fibers are assumed to be circular and packed in a hexagonal array. The effects of volume fractions of the two different fibers used and also their relative locations within the unit cell are studied. Analytical results are obtained for all the elastic constants. Modified Halpin–Tsai equations are proposed for predicting the transverse and shear moduli of hybrid composites. Variability in mechanical properties due to different locations of the two fibers for the same volume fractions was studied. It is found that the variability in elastic constants and longitudinal strength properties was negligible. However, there was significant variability in the transverse strength properties. The results for hybrid composites are compared with single fiber composites.

The role of flaw size and fiber distribution on tensile ductility of PVA-ECC

Abstract: Polyvinyl Alcohol-Engineered Cementitious Composites (PVA-ECC) designed based on micromechanics exhibit high tensile ductility (above 1%) and limited crack widths (below 100 μm). The tensile performance of ECC is dependent on the fiber and flaw size distributions. These parameters are known to be influenced by the matrix flowability and mix processing; however, a comprehensive quantitative analysis framework linking fiber and flaw size distributions to the tensile performance of PVA-ECC is needed to supplement theoretical understanding of the relationship between micromechanical parameters and composite macro-properties. In the present work, fiber distribution (dispersion and orientation) of two different ECCs in terms of matrix flowability was investigated using fluorescence microscopy and advanced digital image analysis. The maximum flaw size distribution along the specimens was also analyzed by cross-sectional image analysis. The influences of fiber and flaw size distributions on the composite behavior of PVA-ECCs were experimentally established.

Effect of carbon nanotube waviness on the effective thermoelastic properties of a novel continuous fuzzy fiber reinforced composite

Abstract: This paper deals with the investigation of the effect of carbon nanotube (CNT) waviness on the effective coefficient of thermal expansion (CTE) of a novel continuous fuzzy fiber reinforced composite (FFRC). This novel FFRC is composed of carbon fibers, sinusoidally wavy CNTs and epoxy matrix. The sinusoidally wavy CNTs are radially grown on the circumferential surfaces of the carbon fibers. Analytical micromechanics model based on the method of cells (MOC) approach is derived to investigate the influence of the waviness of CNTs on the effective CTEs of the FFRC. The present study reveals that if the amplitudes of the radially grown sinusoidally wavy CNTs are parallel to the axis of the carbon fiber then the thermoelastic properties of the FFRC are significantly improved over those of the FFRC being composed of straight CNTs.

Three-phase carbon fiber amine functionalized carbon nanotubes epoxy composite: processing, characterisation, and multiscale modeling

Abstract: The present paper discusses the key issues of carbon nanotube (CNT) dispersion and effect of functionalisation on the mechanical properties of multiscale carbon epoxy composites. In this study, CNTs were added in epoxy matrix and further reinforced with carbon fibres. Predetermined amounts of optimally amine functionalised CNTs were dispersed in epoxymatrix, and unidirectional carbon fiber laminateswere produced. The effect of the presence of CNTs (1.0 wt%) in the resin was reflected by pronounced increase in Young's modulus, inter-laminar shear strength, and flexural modulus by 51.46%, 39.62%, and 38.04%, respectively. However, 1.5wt% CNT loading in epoxy resin decreased the overall properties of the three-phase composites. A combination of Halpin-Tsai equations and micromechanics modeling approach was also used to evaluate the mechanical properties of multiscale composites and the differences between the predicted and experimental values are reported. These multiscale composites are likely to be used for potential missile and aerospace structural applications.

Micromechanics of ITZ–Aggregate Interaction in Concrete Part I: Stress Concentration

Abstract: At the macroscopic scale, concrete appears as a composite made of a cement paste matrix with embedded aggregates. The latter are covered by interfacial transition zones (ITZs) of reduced stiffness and strength. Cracking in the ITZs is probably the key to the nonlinear stress–strain behavior in the prepeak regime. For a deeper understanding of this effect triggered by tensile microstress peaks, we here employ and extend the framework of continuum micromechanics, as to develop analytical solutions relating the macroscopic stresses acting on a piece of concrete, to microtractions at the aggregates' surfaces and to three-dimensional stress states within the ITZs. In the latter context, a new aggregate-to-ITZ stress concentration tensor is derived based on the separation-of-scale principle, which implies that ITZs may be modeled as two-dimensional interfaces at the concrete scale, but as three-dimensional bulk phases at the scale of a few micrometers. Microtensile peaks occur both under uniaxial macroscopic tension and compression. To describe the respective microtraction and microstress fields, it is suitable to define aggregate's “poles” and “equator” by an “axis” through the aggregate center, directed in the uniaxial macroscopic loading direction. Accordingly, tensile microtraction peaks, induced by macro-tension and macro-compression, respectively, occur at the “poles” and at the “equator”, respectively. The largest tensile ITZ-microstresses occur at an offset of about π/8 from the “poles” and the “equator”, respectively. These fields of microtractions and ITZ microstresses are prerequisites for upscaling ITZ-related strength to the macroscopic concrete level, as presented in the companion paper (Part II).

MODELING AND STRESS ANALYSIS OF SMART CNTs/FIBER/POLYMER MULTISCALE COMPOSITE PLATES

Abstract: Modeling and nonlinear stress analysis of piezolaminated CNTs/fiber/polymer composite (CNTFPC) plates under a combined mechanical and electrical loading are investigated in this study. The governing equations of the piezoelectric CNTFPC plates are derived based on first-order shear deformation plate theory (FSDT) and von Karman geometric nonlinearity. Halpin–Tsai equations and fiber micromechanics are used in hierarchy to predict the bulk material properties of the multiscale composite. The CNTs are assumed to be uniformly distributed and randomly oriented through the epoxy resin matrix. An analytical solution is employed to determine the large deflection response and stress analysis of the nanocomposite plates. Finally, by solving some numerical examples for simply supported plates, the effects of the applied constant voltage, plate geometry, volume fraction of fibers and weight percentage of SWCNTs and MWCNTs on the deflection and stress analyses of the piezoelectric CNTs/fiber/polymer multiscale composite plate are studied. It is shown that the deflections significantly decrease with a small percentage of CNTs. Also, it is found that the SWCNTs reinforcement produces more pronounced effect on the bending and stress of the nanocomposite plates in comparison with MWCNTs.

A micromechanics based multiscale model for nonlinear composites

Abstract: A micromechanics model for fiber-reinforced composites that can be used at the subscale in a multiscale computational framework is established to predict the effective nonlinear composite response. Using a fiber–matrix concentric cylinder model as the basic repeat unit to represent the composite, micromechanics is used to relate the applied composite strains to the fiber and matrix strains by a six by six transformation matrix. The resolved spatial variations of the matrix fields are found to be in good agreement with corresponding finite element analysis results. The evolution of the composite nonlinear response is assumed to be governed by two scalar, strain-based variables that are related to the extreme value of an appropriately defined matrix equivalent strain, and the matrix secant moduli are used to compute the composite secant moduli for nonlinear analysis. The results from the micromechanics model are compared well with a full finite element analysis. The predictive capability of the proposed model is illustrated by two distinct fiber-reinforced material systems, carbon and glass, for the fiber volume fraction varying from 50 to 70 %. Since fully analytical solutions are utilized for the micromechanical analysis, the proposed method offers a distinct computational advantage in a multiscale analysis and is therefore suitable for large-scale progressive damage and failure analyses of composite material structures.

Stress analysis of functionally graded open cylindrical shell reinforced by agglomerated carbon nanotubes

Abstract: In this paper, stresses due to bending behavior of functionally graded carbon nanotube-reinforced (FGCNTR) open cylindrical shells subjected to mechanical loads is studied. The material properties of FGCNTR shells are assumed to be graded in the thickness direction, and are estimated using a two-parameter micromechanics model in which Eshelby–Mori–Tanaka approach is employed. The primary bending formulation is based on the linear, small-strain, three-dimensional elasticity theory. In addition, the cylindrical shells are analyzed using the third-order shear deformation theory (TSDT). In order to discretize the governing equations, the two-dimensional generalized differential quadrature method (2-D GDQM) in the thickness and longitudinal directions and the trigonometric functions in tangential direction are used. The effects of agglomeration parameters, CNTs volume fraction, and CNTs distribution through the thickness on the bending behavior of FGCNTR open cylindrical shells are studied. In addition, the mechanical stresses obtained from 3-D elasticity are compared with those obtained using TSDT for a different range of geometric and agglomeration parameters.

Micromechanics of ITZ‐Aggregate Interaction in Concrete Part II: Strength Upscaling

Abstract: Cracking in concrete typically starts in the immediate vicinity around the aggregates, i.e., in the region of the so-called interfacial transition zones (ITZs), but the process is still not fully understood. Notably, crushing of concrete in compression results in fragments with interesting aggregate surface textures. Part of the aggregate surfaces is cleanly separated from the ITZ, while another part of the aggregate surfaces remains covered with a thin layer of cement paste. This suggests two different types of failure: ITZ-aggregate separation and ITZ failure; which we here study based on the continuum micromechanics approach of the companion paper (part I). It provides access to both traction vectors acting on aggregate surfaces and three-dimensional stress states within representative ITZ volumes for loading states below the elastic limit of concrete. When inserting these microtractions and microstresses into Rankine-type strength criteria for the aggregate-ITZ interface and for the ITZ, respectively, the micromechanics model allows for upscaling this microscopic failure behavior to concrete-level criteria for crack onset. Comparing the latter to corresponding experimental results, reveals that under tension-dominated loading both ITZ failure and ITZ-aggregate separation appear to be realistic, while under compression-dominated loading ITZ failure appears as the more likely mechanism. Also, comparing model and experiments shows that the ITZ-aggregate separation strength amounts to at least half of the internal ITZ cohesion strength, but may be much larger than the latter.

Multiscale modeling of the effects of nanoscale load transfer on the effective elastic properties of unfunctionalized carbon nanotube-polyethylene nanocomposites

Abstract: A multiscale model is proposed to study the macroscale bulk elastic material properties under the influence of interfacial load transfer at the nanoscale in carbon nanotube?polyethylene (CNT?PE) nanocomposites. Molecular dynamic (MD) simulations are performed to characterize the nanoscale load transfer through the identification of representative nanoscale interface elements which are studied parametrically in terms of the length of the polymer chains, the number of the polymer chains and the ?grip? position. Once appropriate scales of these parameters are deemed to yield sufficiently converged results, the representative interface elements are subjected to normal and sliding mode simulations in order to obtain the force?separation responses at 100 and 300?K for unfunctionalized CNT?PE interfaces. Cohesive zone traction?displacement laws are developed based on the force?separation responses obtained from the MD simulations and are used in continuum level models to determine the influence of the interface on the effective elastic material properties of the nanocomposites using analytic and computational micromechanics approaches. It is found that the inclusion of the nanoscale interface in place of the perfectly bonded interface results in effective elastic properties which are dependent on the applied strain and temperature in accordance with the interface sensitivity to those effects, and which are significantly diminished from those obtained under the perfect interface assumption.

Determination of material parameters for discrete damage mechanics analysis of carbon-epoxy laminates

Abstract: Discrete damage mechanics (DDM) refers to micromechanics of damage constitutive models that, when incorporated into commercial finite element software via user material subroutines, are able to predict intralaminar transverse and shear damage initiation and evolution in terms of the fracture toughness of the composite. A methodology for determination of the fracture toughness is presented, based on fitting DDM model results to available experimental data. The applicability of the DDM model is studied by comparison to available experimental data for Carbon Epoxy laminates. Sensitivity of the DDM model to h- and p-refinement is studied. Also, prediction of modulus vs. applied strain is contrasted with ply discount results and the effect of in situ correction of strength is highlighted.

Investigation of uniaxial stretching effects on the electrical conductivity of CNT-polymer nanocomposites

Abstract: A theoretical study is carried out to investigate the effects of uniaxial stretching on the electrical conductivity of carbon nanotube (CNT)–polymer composites using a mixed micromechanics model, which incorporates two conductivity mechanisms: electron hopping and conductive networks. The uniaxial stretching induces volume expansion of the composites, re-orientation of CNTs and a change in conductive networks, which are characterized by the variation of the CNT concentration, the CNT orientation distribution function and the percolation threshold, respectively. Modelling results demonstrate that stretching decreases the electrical conductivity of the composite in both the longitudinal and transverse directions. It is also observed that stretching has more significant effects on the electrical conductivity of the composites with a lower CNT volume fraction. Furthermore, the effects of Poisson's ratio on the electrical conductivity are also investigated. Possible reasons for the observed phenomena are interpreted. This work can be claimed to provide a theoretical prediction on the trend of the stretching effects on the electrical properties of CNT–polymer composites.

Computational micromechanics analysis of electron-hopping-induced conductive paths and associated macroscale piezoresistive response in carbon nanotube–polymer nanocomposites:

Abstract: In this study, a computational model is developed using finite-element techniques within a continuum micromechanics framework to capture the effect of electron-hopping-induced conductive paths at t...

Viscous interfaces as source for material creep: A continuum micromechanics approach

Abstract: It is generally agreed upon that fluids may play a major role in the creep behavior of materials comprising heterogeneous microstructures and fluid-filled porosities at small length scales. In more detail, nanoconfined fluid-filled interfaces are typically considered to act as a lubricants, once electrically charged solid surfaces start to glide along fluid sheets, with the fluid being typically in a liquid crystal state, which refers to an “adsorbed”, “ice-like”, or “glassy” structure of fluid molecules. Here, we aim at translating this interface behavior into apparent creep laws at the continuum scale of materials consisting of one non-creeping solid matrix with embedded fluid-filled interfaces. To this end, we consider a linear relationship between (i) average interface dislocations and (ii) corresponding interface tractions, with an interface viscosity as the proportionality constant. Homogenization schemes for eigenstressed heterogeneous materials are used to upscale this interface behavior to the much larger observation scale of a matrix-inclusion composite comprising an isotropic and linear elastic solid matrix, as well as interacting parallel interfaces of circular shape, which are embedded in the aforementioned matrix. This results in exponentially decaying macroscopic viscoelastic phenomena, with both creep and relaxation times increasing with increasing interface size and viscosity, as well as with decreasing elastic stiffness of the solid matrix; while only the relaxation time decreases with increasing interface density. Accordingly, non-asymptotic creep of hydrated (quasi-) crystalline materials at higher load intensities may be readily explained through non-stationarity, i.e. spreading, of liquid crystal interfaces throughout solid elastic matrices.