Range Of Fiber Volume Fractions Research Articles

Micromechanical Analysis of GFRP Composite with Micro-Level Defects

Fiber-reinforced plastic (FRP) composites are subjected to micro-level defects such as fiber-matrix debond and/or matrix cracks after a period of their service due to the increasing brittleness of matrix material. Prediction of the degraded elastic properties of a lamina through micromechanical studies by incorporating micro-level defects gives an idea of the health condition of such structures. Due to the limitations of classical mathematical approaches in solving complex structures, numerical mathematical methods like the finite element method (FEM) can be employed. The present investigation deals with the micromechanical analysis of Glass fiber-reinforced plastic (GFRP) composite with micro-level defects to predict some of the elastic properties. The composite is idealized as an array of square unit cells, and the micromechanical behavior of one such unit cell is simulated in ANSYS software using the three-dimensional finite element method to predict Young’s moduli and Poisson’s ratios in principal material directions. The converged finite element solution for longitudinal modulus is validated by the rule of mixtures and the other properties using the Maxwell–Betti reciprocal theorem. Variations of Young’s moduli and Poisson’s ratios due to an incremental internal failure of composite such as low-level, medium-level, and high-level defects at an expected range of fiber volume fractions (50% - 60%) are evaluated and estimated the percentage degradation with respect to a corresponding defect-free composite.

Transverse isotropic compression behavior of aligned steel fiber UHPC produced by electromagnetic field

Aligned steel fiber ultra-high performance concrete (ASF-UHPC) demonstrates transverse isotropic mechanical properties. Specifically, when subjected to compressive stress perpendicular to the fiber orientation, surface cracks emerge proximate to the fiber tips, resulting in a significant enhancement of the material's compressive strength. Conversely, under compressive stress applied parallel to the fiber direction, fissures manifest along the fiber orientation, leading to a notable reduction in compressive strength. Consequently, the primary contributory factor underlying augmented compressive strength is the lateral restraint effectuated by the fibers. The optimization of fiber distribution for the aim of altering compressive strength is subject to the influence of the qualities of the UHPC matrix. Moreover, a comprehensive analysis extends to the examination of pertinent parameters, encompassing fiber volume fractions, fiber lengths, and fiber shapes. Investigation of these facets reveals an optimal range of fiber volume fraction, approximately 1.0% to 1.5%, which imparts maximal compressive strength in ASF-UHPC subjected to compressive stress perpendicular to the fiber orientation. When forces are applied parallel to the fiber direction, the incorporation of longer fibers or hooked-end fibers mitigates the reduction in compressive strength perpendicular to the applied stress. The consideration of the transverse mechanical attributes exhibited by aligned steel fibers, coupled with the optimization of fiber distribution characteristics, not only attenuates steel fiber consumption but also underscores a sustainable approach by curtailing CO2 emissions.

The unique flexibility feature of moso bamboo: Arising implications for biomimetic material design

Bamboo, renowned for its sustainability, renewable nature and extraordinary flexibility, has long been widely used as engineered materials. This coveted attribute can be attributed to the unique gradient microstructure of the bamboo culm, characterized by the volume fraction of reinforcing fibers diminishes exponentially from the epidermis to the endodermis. This inherent structural heterogeneity engenders disparate fracture behaviors and mechanical properties between the compression and tension sides during bending, i.e. the asymmetric bending behavior. Through a qualitative analysis with the shear-lag theory, this study has discerned that a critical range of fiber volume fraction, spanning from 25% to 30%, serves as the critical condition for whether fiber break occurs before or after pull-out. Moreover, it has been elucidated that the significant degree of radially discrepancy in fiber volume fraction manifests as a key determinant of the degree of asymmetry. By parameterizing the variation trend of the fiber volume fraction along the loading direction, it becomes feasible to predict the mechanical properties when subjected to loading from different sides. Such endeavors furnish a pivotal scientific foundation for characterizing the performance of gradient materials with similar densities, while concurrently offering novel insights for the structural design of high-performance biomimetic material.

Optimizing the thermal properties of fiber reinforced phthalonitrile composites

AbstractThe development of lightweight composites with desirable thermo‐mechanical properties is progressively increasing. Phthalonitrile (PN) based composites have shown great potential in this regard. However, the basic thermal properties of PN composites required for engineering design are not yet fully understood. In this work, we investigated the thermal stability, thermal expansion behavior and thermal conductivity of PN composites reinforced with carbon fiber (CF) and high silicon fiberglass (HSF) via combined experimental studies and numerical simulations. The results indicated that CF/PN performs better in thermostability than HSF/PN at temperatures below 500°C. Moreover, the incorporation of CF and HSF lowered the coefficient of thermal expansion (CTE) and thermal insulation of PN. At room temperature, the in‐plane CTE of CF/PN and HSF/PN were 1.97E‐6 and 9.24E‐6°C−1, respectively, while the out‐plane thermal conductivities of CF/PN and HSF/PN were 0.65 and 0.34 W/(m K). It is worth noting that an excessively high fiber volume fraction would lead to poor thermal insulation and lightweight properties of the PN composites, while a low fiber volume fraction would result in poor stiffness and thermal dimensional stability. Determined via TOPSIS model, a fiber volume fraction range of 50%–60% was ideal for both composites with comprehensive optimal properties, respectively.

Permeability simulation of kidney-bean shaped carbon fibers

Cost saving efforts in carbon fiber manufacturing may result in irregularly shaped cross-sections, often as a kidney-bean shape. In this research, flow simulations were performed to determine transverse permeability of these kidney-bean shaped fibers. Three different unit-cell packing arrangements for the kidney-bean shaped fibers and unit-cell orientations of 0°, + 90°, and − 90° were considered over the fiber volume fraction range 0.30 – 0.75. The kidney-bean shaped fibers had two distinct hexagonally packed unit-cell geometries because of the fiber’s asymmetry with different packing efficiency. The maximum fiber packing fraction for kidney-bean shaped fibers was 0.923, which was larger than the 0.907 for circular fibers. When compared to circular fibers with hexagonal packing, the kidney-bean shaped fibers had up to a 74% reduction in permeability. The ±90° orientations for KB-fibers with hexagonal packing had an infiltration time up to 3.86 longer than the hexagonal packed circular fibers for the same volume fraction.

About the self-sensing behavior of smart concrete and its interaction with the carbon fiber percolation status, sand connectivity status and grain size distribution

The addition of conductive fibers to ordinary cementitious matrix permits to reduce its electrical resistivity, and consequently create its self-sensing behavior. This guarantees to the smart concrete a multifunctional behavior, where it satisfies the structural and self-sensing expectations simultaneously.This study intends to provide a better understanding of the self-sensing behavior of smart concrete function of the connectivity of carbon fibers, sand particles structure and its grain size distribution. The results showed that for cement paste, over carbon fiber volume fraction range, sensibility presented two local peaks. For mortar, in presence of diluted sand and partially connected sand, local peaks persisted. In presence of 50 % of sand volume fraction, sensibility to external compression forces was significantly reduced.

Prediction of permeability of five-harness satin fabric by a modified Kozeny constant determined from experiments

Permeability is a critical parameter not only in flow simulation analysis but also in liquid composite molding process. When a liquid resin is infused into a dry preform, the impregnation is mainly characterized by the permeability. The permeability of a dry preform can be obtained through theoretical and experimental methods. In the theoretical estimation of permeability, the effects of fiber arrangement as well as fabric type and form for various types of preforms are not sufficiently reflected in the calculation. Thus, there is a gap between the theoretical and experimental permeability. Recently, experimental determination has been gaining considerable attention as a mean to obtain accurate permeability values; however, it requires a number of trials. In this study, the permeability of the Hexforce G0926 5HS (five-harness satin) carbon fabric preform is estimated using representative theoretical prediction models, the Gebart and Kozeny–Carman equations. In addition to the Kozeny–Carman permeability (using the Kozeny constant values from literature), the Kozeny constant obtained through experiments was used to obtain a modified Kozeny–Carman permeability. All three calculated permeabilities were compared and verified with the fabric manufacturer’s reference value. The results showed that the modified Kozeny–Carman permeability using the experimentally determined Kozeny constant was closest to the reference value at 57% fiber volume fraction. Further, the predicted permeability was compared with other experimental permeability values from literature over the 40%–65% range of fiber volume fraction. We found that the modified Kozeny–Carman permeability once again came closest to the literature values. Finally, an optimized fitting equation was proposed to replace the Kozeny–Carman equation for predicting the permeability of Hexforce G0926 5HS carbon fabric over the 40%–65% fiber volume fraction range.

Elastic modulus prediction based on thermal conductivity for silica aerogels and fiber reinforced composites

The speed of sound is a critical parameter in the test of mechanical and thermal properties. In this work, we proposed a testing method to obtain the elastic modulus of silica aerogel from the sound speed formulas. The solid thermal conductivity of the silica aerogel is experimentally measured for predicting the sound speeds, and then the elastic modulus is calculated based on the elasticity sound speed model. The experimental data of the solid thermal conductivity of silica aerogels with different densities are employed and the obtained elastic modulus is fitted as a power-law exponential function of the density. Two existing sound speed models and three groups of available experimental data are also employed to validate the present fitting relation, and good agreement is obtained for the silica aerogel in the density range of 150–350 kg/m3. The fitting formula can also be extended to estimate the elastic modulus of the glass fiber-reinforced silica aerogel composite. The results show that the elastic modulus of the aerogel composite is sensitive to the glass fiber volume fraction, while the thermal conductivity is weakly dependent on the glass fiber volume fraction at room temperature in the studied range of fiber volume fraction.

Elastic Properties of Polymer Modified Steel Fibre Reinforced High Strength Concrete

The paper deals with the experimental and theoretical results of elastic constants of polymer modified high strength concrete with high fibre volume fraction. To evaluate elastic properties of this composite system, compressive and flexural strengths were obtained experimentally. Elastic properties such as modulus of elasticity and Poisson’s ratio are first obtained based on experimental results. Simplified equations based on micromechanics, and solid mechanics theories are utilized for the evaluation of elastic properties of polymer modified randomly oriented short steel fibre reinforced high strength concrete. The micromechanics equations for the modulus of elasticity and the Poisson’s ratio are based on the fibre volume fraction and the elastic moduli of the fibre and polymer modified concrete matrix. The existing empirical equations are also used to obtain elastic constants. These equations are applied in the full range of fiber volume fraction (1% to 10%). The comparison of experimental results with theoretical values shows the good agreement with each other. The novelty of the present paper is that the modulus of elasticity of this composite system is obtained experimentally using four point bending test and the Poisson’s ratio is obtained as a function of flexural and compressive strengths with excellent accuracy for the first time.

An analytical approach to evaluate the effective coefficients of piezoelectric composites

In this paper, the effective coefficients of piezoelectric fiber-reinforced composites are determined through two micromechanics methods. The exact solutions are derived to predict the effective electro-elastic coefficients with modifications in the conventional strength of materials approach and strain energy approach to account for the effects of fiber packing arrangements and orientations on these properties. A representative volume element subjected to a mechanical loading in the longitudinal direction to the fiber and an electrical loading in the transverse direction to the fiber is used to model the problem mathematically and derive the results. The effective elastic, piezoelectric and dielectric coefficients are estimated by these two methods, and the results are compared with those predicted by the strength of materials method of the literature. From the results, it is observed that results obtained for the elastic, piezoelectric and dielectric coefficients by the strength of materials method and modified strength of materials method are in good agreement with each other in a useful range of fiber volume fractions, while the results obtained through the energy approach shows some deviations when compared with previously stated two methods. The strength of actuation capabilities in the direction longitudinal to the fiber shows significant improvements in the results obtained with the strain energy approach compared with those of the other two micromechanics methods.

Deep learning in heterogeneous materials: Targeting the thermo-mechanical response of unidirectional composites

In this communication, a multi-task deep learning-driven homogenization scheme is proposed for predicting the effective thermomechanical response of unidirectional composites consisting of a random array of inhomogeneity. Toward this end, 40 000 repeating unit cells (RUCs) comprising an arbitrary number of locally irregular inclusions are generated over a wide range of fiber volume fractions. The finite-volume direct averaging micromechanics is then employed to evaluate the homogenized thermo-mechanical moduli of each RUC. Subsequently, a two-dimensional deep convolution neural network (CNN) is constructed as a surrogate model to extract the statistical correlations between the RUC geometrical information and the corresponding homogenized response. The RUC images together with their homogenized moduli are divided into two datasets in a ratio of 9:1 with the former part used for training the CNN model and the latter part used for verification. The results presented in this contribution demonstrate that the deep CNN predictions exhibit remarkable correlations with the theoretical values generated by the finite-volume micromechanics, with a maximum relative prediction error of less than 8%, providing good support for the data-based homogenization approach.

Numerical Evaluation of the Thermal Properties of UD-Fibers Reinforced Composites for Different Morphologies

In this paper, the numerical homogenization technique is used to evaluate the representative volume element and to compute the effective transverse thermal properties of unidirectional fibers reinforced composites. Three unit cells are constructed including square, hexagonal and random arrangement. The results obtained by finite element analysis showed the effect of unit cells configurations on the prediction of effective transverse thermal properties of unidirectional fibers composites for a wide range of fiber volume fractions and contrasts of properties. The numerical results are compared to available analytical models in the thermal conductivity of composites and experimental test results from the literature.

The morphological effect of carbon fibers on the thermal conductive composites

The locally-exact homogenization theory (LEHT) with thermal conductive capability is developed to investigate the morphological effect of carbon/graphite fibers on the effective and localized thermal responses of periodic composites. Based on the orientations of basal planes, the material properties of carbon fibers can be transversely isotropic, radially orthotropic or circumferentially orthotropic, possibly influencing the microscopic behavior of thermal conductive composites. By taking fiber-fiber interactions into account, repeating unit cells (RUCs) of hexagonal and rectangular geometries are considered with large fiber volume fractions. The efficiency and stability of the LEHT are guaranteed by solving the complete internal eigenvalue functions, imposing point-wise continuity conditions, as well as implementing the generalized variational principle. It is demonstrated that the present theory exhibits good agreement with the independently developed Eshelby solutions and Hashin's formula. The morphological effects are also tested by generating effective coefficients over a wide range of fiber volume fractions and recovering the local thermal field concentrations within the composite microstructures. The results clearly indicate that even if the homogenized properties are not significantly affected by the morphologies of carbon fibers by satisfying the replacement scheme, the large heat-flux gradients within the orthotropic fibers could still lead to the split of carbon reinforcement.

Transverse compaction of 2D glass woven fabrics based on material twins – Part II: Tow and fabric deformations

In Liquid Composite Molding (LCM), compaction of the reinforcement occurs during several stages of the entire process, including before and during resin injection, which leads to significant deformations of fibrous architecture. This article aims to study by X-ray microtomography the mesoscopic deformations of 2D glass woven fabrics under transverse compaction for a range of fiber volume fractions encountered in high performance composite applications. In Part I, material twin geometric models of dry fibrous reinforcements were created at different levels of compaction to study the evolution of the morphological features and displacements of fiber tows when compressed during processing. Part II proposes a new approach to track the motion of contour points on cross-sections of fiber tows during compaction. This will allow investigating more precisely the behavior of fiber tows by calculating their mesoscopic deformations. In particular, it will also be possible to study in more details the effect of contacts between tows and of nesting between fabric plies on the mesoscopic deformations of textile preforms during compression.

Transverse compaction of 2D glass woven fabrics based on material twins – Part I: Geometric analysis

In Liquid Composite Molding (LCM), compaction of the reinforcement occurs during several stages of the entire process, including before and during resin injection, which leads to significant deformation of the fibrous architecture. This affects not only the manufacturing process, but also the mechanical properties of final parts. This article aims to study by X-ray microtomography the mesoscopic deformations of 2D glass woven fabrics under transverse compaction for a range of fiber volume fractions encountered in high performance composite applications. The analysis is based on the recently proposed Micro-CT Aided Geometric Modeling (AGM) technique Huang et al. (2019) [1], which is used to create, from the three-dimensional images of dry textile preforms obtained by microtomography, “material twin” geometric models representative of fiber architecture. Three “material twin” mesostructural geometric models of a stack of multiple layer 2D woven fabrics are generated from microtomographic images at different compaction levels. Since all the fiber tows are reconstructed from real fabrics and are labeled separately, the deformation and displacement of fiber tows during compaction can be subsequently studied, which is not necessarily possible with other reconstruction methods. The results show that contacts between fiber tows have an effect on the evolution of their morphological features, which is related to the material variability of fiber tows in real fabrics. Tracking of the fiber tows positions also reveals that the vertical displacement of fiber tows are significant, while their horizontal movements are negligible.

Homogenization and localization of imperfectly bonded periodic fiber-reinforced composites

An elasticity-based micromechanics model is derived analytically to investigate the homogenized and localized responses of unidirectional composites with imperfect interfaces. To mimic the perturbation of the inclusion positioning caused by the consolidation process, an arbitrarily parallelogram-shaped repeating unit cell (RUC) is developed with periodic boundary conditions. Different from the finite-element/volume approaches which solve a boundary-value problem in the discretized domain, the present work avoids the complicated mesh discretization and instead adopts the Trefftz concept that employs the complete elastic solutions with unknown coefficients to represent the internal trial displacement/stress fields. The solutions of the unknown coefficients are obtained by applying the flexible separation relations along the fiber/matrix interface that govern the interfacial traction reciprocities and displacement discontinuities, as well as the periodic balanced variational principle at the circumference of microstructures. The homogenization equations are then applied to generate the effective properties of composite materials with different architectures over a wide range of fiber volume fractions. In addition to validating the effective properties and underpinning stress fields against other micromechanical techniques in the literature, new results are generated extensively aiming at demonstrating the effects of the interfacial stiffness, radius of fiber and shape of the unit cell on the homogenized and localized composite responses. The analytical framework developed in this communication also facilitates the identification of the interfacial stiffness that minimizes the difference between the effective moduli and targeted material properties through the incorporation of Particle Swarm Optimization algorithm.

Fabrication and Mechanical Characterization of Short Fiber-Glass Epoxy Composites

Abstract In this article, the epoxy-based composites were developed by combining the short E-glass fibers into epoxy matrix. The fiber content in the composite was varied from 0.05 to 0.40 by weight fraction, and the variation of mechanical properties, such as tensile, flexural, impact, and hardness properties, in each case, were investigated. Chemical analyses of the constituents by X-ray spectroscopy and a microstructural analysis of the fracture area complete the mechanical tests. The obtained results show that although ultimate tensile strength and flexural strength of the composites decreased with increasing fiber content, Young’s and flexural moduli increased. The reduction in the failure strain is caused by an embrittlement effect as the stiffness of the composites is improved when the fiber weight fraction is increased. In the investigated glass fiber volume fraction range, the notched Charpy impact energy of composites increases with an increase of the glass fiber volume fraction. The morphological analysis of the tensile fracture surfaces of the composites observed using scanning electron microscopy brings an important clarification that explains the obtained behavior. The micromechanics analysis of behavior of epoxy composites reinforced with short glass fibers was performed using Digimat 6.0 software, and the obtained results of Young’s modulus revealed a good convergence of the calculated and measured values.

Semianalytical compressive strength criteria for unidirectional composites

The compressive strength of unidirectional composites is strongly influenced by the elastic and strength properties of the fiber and matrix phases, as well as by the local geometrical properties, such as fiber volume fraction, misalignment, and waviness. In the present investigation, two microbuckling criteria are proposed and examined against a large volume of measured data of unidirectional composites taken from the literature. The first criterion is based on the compressive strength formulation using the buckling of Timoshenko’s beam. It contains a single parameter that can be determined according to the best fit to experimental data for various types of polymeric matrix composites. The second criterion is based on buckling-wave propagation analogy using the solution of an eigenvalue problem. Both criteria provide closed-form expressions for the compressive strength of unidirectional composites. We propose modifications of the two criteria by a fitting approach, for a wide range of fiber volume fractions, applied to four classes of unidirectional composite systems. Furthermore, a normalized form of the two models is presented after calibration in order to compare their prediction against experimental data for each of the material systems. The new modified criteria are shown to give a good match to a wide range of unidirectional composite systems. They can be employed as practical compression failure criteria in the analysis and design of laminated structures.

Tailoring the moduli of composites using hollow reinforcement

Effects of hollow reinforcement on homogenized moduli and stress fields in unidirectional composites are investigated using an elasticity-based homogenization theory for periodic materials with hexagonal symmetries. The theory employs Fourier series representations for hollow fiber and matrix displacement fields in the cylindrical coordinate system which satisfy exactly the equilibrium equations and continuity conditions in the interior of the unit cell. The inseparable exterior problem requires satisfaction of periodicity conditions efficiently accomplished using previously introduced balanced variational principle. This principle ensures rapid solution convergence in the presence of thick or very thin-walled hollow fibers with relatively few harmonic terms. The solution’s analytical framework and stability facilitate parametric and optimization studies aimed at rapid identification of fiber wall-thickness impact on homogenized moduli and local fields in wide range of fiber volume fractions. This is illustrated for two material systems containing hollow glass fibers and alumina nanotubes as reinforcement, revealing new results of interest in the design of multifunctional porous materials. The results of parametric studies are made more precise upon incorporation of the locally-exact homogenization theory into the Particle Swarm Optimization algorithm, with the new computational capability employed to identify tube thickness as a function of the porosity volume fraction which minimizes the differences between target homogenized moduli and the corresponding matrix moduli. Analysis of a generalized Kirsch problem with an underlying microstructure based on targeted homogenized moduli around the cylindrical cavity demonstrates the homogenization theory’s utility in tailoring the local stress field with the aim of enhancing material toughness.

低体積分率におけるキュプラ・ポリエステル短繊維集合体の有効熱伝導率の測定

In this study, the effective thermal conductivity of Cupra, Polyester and Polytrimethyleneterephthalate (PTT) fiber assemblies in low fiber volume fraction is measured using KES-F7 Thermo Labo II apparatus. In order to eliminate thermal perturbation by external heat, the radiation shielding board is set up between samples and operator. Heat flux to calculate thermal conductivity is measured including heat leakage from the side wall of sample and is calibrated in the analysis. The results are analyzed using non-linear regression method. The results are obtained as follows. Thermal conductivity curve is convex downward within the range of fiber volume fraction measured. The effective thermal conductivity, λ is expressed as following equation, λ = Aφ + B/φ + C where, φ: fiber volume fraction, A, B: coefficients and C: constant determined by non-linear regression analysis. Based on this equation, the effective thermal conductivity is divided into three parts, i.e. Aφ: heat conduction within fiber, B/φ: radiative heat transfer and C: heat conduction within air. Component of conduction in air, C plays a most important role in thermal conductivity of fiber assembly, and component of heat conduction in fiber, Aφ follows in higher fiber volume fraction.