孔隙率对复合材料单向板横向力学性能的影响

在考虑纤维和孔隙随机分布的情况下,通过随机算法生成包含孔隙的代表性体积单元Representative Volume Element(RVE)。对生成的RVE建立有限元模型,引入基体的塑性本构模型和界面的双线性本构模型,采用有限元方法研究了孔隙率对碳纤维/环氧树脂复合材料单向板横向力学性能的影响。研究显示,孔隙随机分布对横向力学性能的影响不是很大;当孔隙率不超过临界值时,孔隙对横向力学性能的影响相对较小;当孔隙率超过临界值后,材料横向弹性模量、横向拉伸强度和横向压缩强度都会有较大的下降。     

A multiscale modeling for predicting the thermal expansion behaviors of 3D C/SiC composites considering porosity and fiber volume fraction

Coefficients of thermal expansion (CTEs) are an essential design criterion of the three-dimensional carbon fiber reinforced SiC matrix composites (3D C/SiC). Representative volume element (RVE) models of microscale, void/matrix, and mesoscale developed in this work were used to investigate the CTEs of these composites. A coupled temp-displacement steady-state analysis step was created for assessing the thermal expansions behaviors of the composites by applying periodic displacement and temperature boundary conditions. Three RVE models of cuboid, hexagonal and fiber random distribution were respectively established to comparatively study the influence of fiber package pattern on the CTEs at microscale. Similarly, the effects of different void size, locations, and shapes on the CTEs of the matrix are comparatively analyzed by the void/matrix models. The prediction results at mesoscale corresponded closely to the experimental results. The effect of the porosities on the CTEs was studied by the void/matrix RVE models. The voids were effective in lowering the CTE of the 3D C/SiC composites. Furthermore, the effect of fiber volume fractions on the CTE were also taken into consideration. Equal in-plane and out-of-plane CTEs were realized by selecting appropriate fiber volume fractions for the different directions. The multiscale models developed in this work can be used to predict the thermal expansion behaviors of other complex structure composites.     

On the Effect of Fiber Shape and Packing Array on Elastic Properties of Fiber-Polymer-Matrix Composites

Abstract

In this study, three-dimensional finite element simulations on the base of the cell model and micromechanics are made to predict effective elastic properties of fibrous composites. The effects of fiber shape, packing array and volume fraction on the overall elastic behavior of an epoxy resin containing unidirectional glass fibers are examined. The geometrical structure includes three types of periodic fiber arrangements in cubic, hexagonal and rectangular cells. The fibers are assumed to be of four shapes; square, circular, elliptic and rectangular. The numerical results indicate that the overall transverse elastic properties are rather sensitive to both fiber shape and packing array while fiber geometry has no effect on the apparent overall Young's modulus in the longitudinal direction of the fibrous composite.     

Numerical simulation on the permeability variations of a fiber assembly

Finite element simulations on permeability variations of an aligned fiber assembly have been conducted. Two dimensional viscous flow in longitudinal and transverse directions has been studied respectively, using the steady state Navier-Stokes equation but with small Reynolds numbers. Two groups of packing structures are investigated, both with introduced disturbances in the fiber assembly. One is the idealized packings, including hexagonal and square packings, but with selected fibers removed randomly, and with fiber center positions disturbed randomly. This gives random effect within the modeled cells, but on the macro-scale the structure is still periodic. Numerical simulations are performed using available finite element packages. The result shows clearly that different physical mechanisms doninate the flow process in longitudinal and transverse permeabilities in the longitudinal flow case, the disturbance introduces openings within the bundle, and these openings become between fibers. Thus flow resistance increases substantially in the idealized packing cases. Numerical data are then compared with available data from other studies, particularly that from using self-consistent methods.     

Preform permeability predictions by self-consistent method and finite element simulation

The self-consistent method and finite element simulations have been used to estimate the permeability of an aligned fiber bundle. The self-consistent method gives out formulas for both longitudinal and transverse permeabilities as functions of the fiber volume fraction. The finite element simulation presents the solutions of various periodic fiber packings. The results for square and hexagonal fiber packings are found coincident with the literature results. It is shown that the permeability is not only related to the fiber volume fraction of porosity, but is also greatly influenced by the packing structure or micro-level disturbance. A unified empirical model is proposed, which uses as variables ultimate fiber volume fraction in addition to fiber volume fraction. The model predications agree with numerical simulation results in different cases.     

An improved self-consistent method for estimating the permeability of a fiber assembly

This paper presents an improved self-consistent method for estimating the permeability of an aligned fiber assembly, in both longitudinal and transverse directions. In this method an insertion is assumed to include open space surrounded by densely packed fibers. This improvement allows us to describe effectively the permeability of dense structures containing distributed voids. As used in self-consistent methods, the insertion is placed into a homogeneous medium with an unknown permeability. Stokes flow and Darcy flow are then considered, respectively, at different regions. Boundary and interface conditions as well as two consistency conditions, including the total amount of the flow and the dissipation energy, are applied accordingly. The permeability is solved from these considerations. This improved permeability model captures the flow characteristics of a fiber bundle. In the longitudinal flow case, the openings within a bundle due to disturbance dominate the flow path. In the transverse flow case, the gaps between neighboring fibers govern the flow resistance. The derived expression for the transverse permeability contains two variables, the averaged fiber volume fraction and the maximum packing efficiency, which adequately describe the status of a fiber bundle. These two variables can also be measured experimentally. The predictions agree with available data reported. The result for the longitudinal flow shows not only the influence of these two parameters, but also the very strong effect of the openings within the bundle on the permeability. This explains the significant differences between the data of idealized packings, such as square and hexagonal packing, and those measured from real fiber bundles. The comparison also provides an estimation of the average opening sizes within a fiber bundle as a function of fiber volume fraction. Numerical simulation results of previous studies are also used to verify this approach.     

Transverse compression failure of unidirectional composites

The mechanical response of unidirectional composites subject to uniaxial transverse compressive loads was measured and analyzed by finite element simulation. Consistency in failure plane orientation was observed when comparing simulated matrix shear band angle to measured crack angle. A model based on hexagonal packing of fibers was proposed and the shear band angle was shown to depend on the fiber volume fraction. The effects of strong and weak fiber–matrix interfaces were considered using models with randomly distributed fibers for a valid statistical analysis. The results of these models showed that the composite compressive strength increased with the fiber loading for the strong interface case, while the strength was independent of the fiber loading for the weak interface case because of interface debonding. POLYM. COMPOS., 36:756–766, 2015. © 2014 Society of Plastics Engineers     

Strength of short-fiber reinforced composites

The structural utility of short, glass fiber-reinforced epoxy composities is experimentally investigated for fiber volume fractions from 0.15 to 0.5. The strength and stiffness of systems with randomly oriented fibers are compared with those of similar composites with aligned fibers. The ultimate strength of both types of material increses in a reasonably linear fashion with volume fraction up to 0.5. For all volume fractions in this range, strength of the random composites is slightly higher than the longitudinal and much higher than the transverse strength of equivalent compsites with aligned fibers. The modulus of the random system is approximately two-thirds the longitudinal and twice the transverse modulus of the unidirectional material. The structural utility of the flow molded material is greatest in uniaxial, stiffness critical situations. The greater strength and planar isotropy of the random composites make them preferable in all strength limited or multiaxial applications.     

Micromechanical modeling of large deformation in sepiolite reinforced rubber sealing composites under transverse tension

This article presents an experimental and numerical study on mechanical behavior of sepiolite reinforced rubber sealing composites (SRRC), which are subjected to the transverse tensile loads. A finite element model of composites with fibers in square and random distribution is adopted for the numerical study. The representative volume elements with different fiber volume fraction are established and analyzed. A successive remeshing strategy is employed to achieve the large deformation of SRRC. The results show that the tensile strength and breaking elongation of SRRC can be improved by addition of sepiolite fiber, and they reach a maximum at 42% fiber volume fraction. The stress‐strain curve of SRRC with fibers in the random distribution agrees well with that measured in the experiments. However, there exists a significant difference between experimental and numerical results as fiber volume fraction increases. POLYM. COMPOS., 38:381–388, 2017. © 2015 Society of Plastics Engineers     

异形纤维阵列过滤特性的数值模拟

为研究异形纤维排布方式对其过滤性能的影响,以矩形及交错排布为基础,在保持纤维体积分数不变的条件下,通过改变纤维的列间距调整阵列结构,运用CFD-DEM耦合方法对含尘空气通过具有不同异形纤维阵列结构的简化过滤器模型过程进行数值模拟。结果表明：当纤维的体积分数保持不变,交错纤维阵列较矩形阵列过滤效率高出35%,且对于颗粒的吸附力更强;而在交错阵列的基础上,调整列间距得到的前密阵列和后密阵列均可保持80%左右的过滤效率,且不影响颗粒吸附力的大小,但前密阵列产生的压降更低,即具有更高的品质因数;在整个过滤过程中,纤维-颗粒间的黏附作用远远高于颗粒与颗粒间的相互作用,说明颗粒过滤主要来源于纤维-颗粒作用的贡献。     

Numerical Study of an Electret Filter Composed of an Array of Staggered Parallel Rectangular Split-Type Fibers

The flow field through a staggered array of parallel, rectangular split-type electret fibers was numerically modeled. The particle trajectory and the collection efficiency were simulated by solving the equation of particle motion, taking into account the effects of diffusion, interception, inertial impaction, and electrostatic forces. The model was validated against results calculated from semiempirical expressions. The model was applied to investigate the role of the inertial impaction and the interception mechanisms in the particle collection by an electret fiber, the particle trajectories under various filtration conditions, the effect of the aspect ratio of the rectangular fiber on the filter penetration, and the distribution of the deposited particles on the surface of the fiber. The simulated results indicate that the inertial impaction and interception mechanisms account for a major portion of neutral particles collected by an electret fiber when the Stokes number is higher than 0.5. For neutral particles, fibers with an aspect ratio of 38/10 have almost the same penetration as fibers with an aspect ratio of 10/38; while for singly charged particles, fibers with an aspect ratio of 38/10 achieve a much lower penetration when the electrophoretic collection mechanism dominates. In addition, it is predicted that a filter composed of fibers with an aspect ratio of 38/10 will result in a lower flow resistance and thus a slower clogging process when the dielectrophoretic collection mechanism dominates.     

Flow along and across glass‐fiber wicks: Testing of permeability models through experiments and simulations

A rigorous test of important theoretical models for permeability of glass‐fiber wicks, backed by numerical simulations, is conducted using a novel small‐scale experiment. The models include those for flow along and across aligned fibers and for flow through random fibers. The domains for numerical simulations were created by randomly distributed parallel fibers in a cube‐like unit‐cell using Geodict. Two separate simulations were considered: (1) Stokes‐flow solution using GeoDict, (2) Whitaker's closure‐formulation solution using COMSOL. The falling‐head parameter was adapted to measure the permeability along and across the fibers. Multiple measurements were conducted for each of the wicks to establish repeatability and estimate scatter. The permeabilities obtained through experiments matched with those from the theoretical and numerical methods. But numerical permeabilities for the longitudinal flow were exceptionally accurate. Also, the specialized models for longitudinal and transverse flows were more accurate than the random‐fiber models. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3491–3501, 2018     

Investigation on scratching force and material removal mechanism of 3D SiCf/C–SiC composites during single grain scratching

As one of the novel fiber-reinforced ceramic matrix composites, SiC_f/C-SiC composites are difficult to machine materials and the material removal mechanism is not completely clear yet. Single grain scratching tests with gradual depth have been carried out to analyze the influence of fiber orientation and woven structure on the material removal mechanism and scratching force in three typical scratched surfaces defined by the fiber orientation. Results indicate that the scratching force waveform is affected by the weaving method, fibers orientation, and porosity distribution of the scratched area, resulting in periodic fluctuations. Diamond indenter causes greater damage to the surface of SiC_f/C-SiC composites when scratching along transverse fibers, while leaving narrow but deep grooves on the surface when scratching along perpendicular fibers. The dominant mode of material removal of SiC_f/C-SiC composites is brittle fracture, and the damage features mainly include direct fiber breakage, matrix fragmentation, fiber pullout, fiber outcropping.     

纺丝温度对中间相沥青炭纤维结构性能的影响

以萘催化合成中间相沥青(MP-1)和热缩聚法制备中间相沥青(MP-2)两种中间相沥青为原料,对其进行簇组成分析、偏光显微镜观察及红外光谱分析,研究其组成及结构.采用实验室气压式单孔纺丝装置在不同温度下对两种中间相沥青进行熔融纺丝,探讨纺丝温度对炭纤维结构及性能的影响.研究表明:MP-1低温获得无规结构,高温出现中心放射状边缘洋葱皮混合结构;MP-2随纺丝温度升高依次出现无规结构、准洋葱皮结构和洋葱皮结构.中间相沥青原料的性质影响着纤维截面结构随纺丝温度变化的规律.     

Distributions for residual autocovariances in parsimonious periodic vector autoregressive models with applications

The asymptotic distribution of the residual autocovariance matrices in the class of periodic vector autoregressive time series models with structured parameterization is derived. Diagnostic checking with portmanteau test statistics represents a useful application of the result. Under the assumption that the periodic white noise process of the periodic vector autoregressive time series model is composed of independent random variables, we demonstrate that the finite sample distributions of the Hosking‐Li‐McLeod portmanteau test statistics can be approximated by those of weighted sums of independent chi‐square random variables. The quantiles of the asymptotic distribution can be computed using the Imhof algorithm or other exact methods. Thus, using the (single) chi‐square distribution for these test statistics appears inadequate in general, although it is often recommended in practice for diagnostic methods of that kind. A simulation study provides empirical evidence.     

Gas transport properties of electrospun polymer nanofibers

Although the basic principles of gas flow through unidirectional fibers have been widely studied and well understood since the 1950s, questions arise when these principles are applied to electrospun polymer nanofibers. Classic theories based on orderly packed coarse fibers are inadequate in accounting for the influences of random fiber distribution and slip flow. In this work, a mechanistic model in terms of fiber volume fraction and fiber radius is presented to determine the through-plane permeability of electrospun nanofiber layers. The fibrous system is subdivided into a series of cells of orthogonal fibers with random volumes. A single factor is proposed to quantify the effect of randomness of fiber distribution on flow behaviors. When the fiber radius is comparable with the mean free path of air molecules, the slip flows in the nanoscale fibrous media are particularly explored. The solutions obtained are successfully validated through comparison with experimental and numerical results. It is demonstrated that the through-plane permeability of electrospun nanofibers is enhanced by the slip effect and randomly distributed fibers are more permeable than ordered structures.     

Study of interfaces of high-performance glass fibers and DGEBA-based epoxy resins using single-fiber-composite test

The single-fiber-composite (SFC) technique was used to study the interfacial behavior between two flexible blends of diglycidylether of bisphenol A (DGEBA)-based epoxy and polyglycol epoxide and three glass fibers. Dog-bone-shaped SFC specimens were made and strained to obtain a distribution of fragment lengths. The fibers were tension-tested at two different gauge lengths. The fragment length distributions, the fiber strength data, and a Monte Carlo simulation of a Poisson/Weibull model for fiber strength and flaws were used to obtain the effective interfacial shear strength values. The results show that the interface does not fail. Instead, penny-shaped transverse cracks appear at every fiber break and grow as the specimen is strained. The interfacial shear strength values are many times higher than the yield shear strength values of bulk epoxy obtained from the tension test.     

Parametric numerical simulation of impact response of carbon nanotube/polymer nanocomposites

A numerical model has been developed using the explicit FE code LS-DYNA in order to study the effect of geometrical and material parameters on the low-velocity impact response of carbon nanotube (CNT)/polymer nanocomposites. The model is based on a Representative Volume Element (RVE). The RVE is prismatic with a rectangular cross-section while the impactor is spherical. The simulations show that the presence of CNT significantly enhances the impact stiffness and the energy absorption capacity of the material. The enhancement increases with the CNT's volume fraction and it is larger at larger impact velocities. The effect of CNT's aspect ratio is found to be minor. The orthotropic behaviour of CNT assigns the RVE a higher energy absorption capacity than the isotropic behaviour at small impact velocities. The prediction of impact damage at large impact velocities indicates that the CNT makes the polymer more susceptible to fracture.     

The electronic and structural characteristics of carbon fibers from mesophase pitch

The electronic properties (resistivity, magnetoresistance, and electron spin resonance) of mesophase pitch-based carbon fibers have been studied in relation to the fiber structure and processing conditions. Reproducible correlations between the electronic properties and the structure demonstrate that the electronic properties are a sensitive indicator of the degree of graphite order in the fibers. Fibers having radial transverse structure are more easily graphitized than fibers with random transverse structure. The effect of differences in the thermosetting procedure is relatively unimportant. The ultimate degree of graphitization attainable by these fibers is comparable to that of similarly heat-treated pyrolytic carbons, whereas PAN-based carbon fibers are ultimately capable only of properties comparable to pitch-based fibers treated in the 1700–2300°C range.     

The effects of preparation temperature on the SiCf/SiC 3D4d woven composite

Continuous silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites have promising applications in aero-engine due to their unique advantages, such as low density, high modulus and strength, outstanding high temperature resistance and oxidation resistance. As SiC fibers are main reinforcements in SiC_f/SiC composites, the crystallization rate and initial damage degree of SiC fibers are seriously influenced by preparation temperatures of SiC_f/SiC composites, namely mechanical properties of SiC fibers and SiC_f/SiC composites are influenced by preparation temperatures. In this paper, KD-II SiC fibers were woven into 3D4d preforms and SiC matrix was fabricated by PIP process at 1100 °C, 1200 °C, 1400 °C and 1600 °C. Digital image correlation (DIC) method was adopted to measure the uniaxial tensile properties of these SiC_f/SiC composites. In addition, finite element method (FEM) based on representative volume element (RVE) was adopted to predict the mechanical properties of SiC_f/SiC composites. The good agreements between numerical results and experimental results of uniaxial tensile tests verified the validity of the RVE. In last, the transverse tensile, transverse shear, uniaxial shear properties were predicted by this method. The predicted results illustrated that axial tensile, transverse tensile and axial shear properties were greatly influenced by the preparation temperatures of SiC_f/SiC composites while transverse shear properties were not significantly various. And the mechanical properties of SiC_f/SiC composites peaked at 1200 °C among these four temperatures while their values reached their lowest points at 1600 °C because of thermal damage and brittle failure of SiC_f/SiC composites.