Actual Fiber Research Articles

Macroscopic characterization of fiber micro-buckling and its influence on composites tensile performance

As one of the most common defects induced by Automated Fiber Placement, in-plane fiber micro-buckling is characterized by a macroscopic value and its influence on composites tensile properties are studied in this article. The mathematical relationship between fiber microscopic distribution and geodesic curvature of non-geodesic fiber path is derived. Influences of in-plane fiber micro-buckling on tensile properties are analyzed by off-axis tensile theory and finite element analysis, and verified by experiments. Actual fiber microscopic distribution of in-plane fiber micro-buckling shows that it is reasonable to evaluate the scale of fiber micro-buckling by the geodesic curvature radius of curved fiber trajectory. Longitudinal tensile properties of lamina are dramatically influenced by in-plane fiber micro-buckling, while the influences of in-plane fiber micro-buckling on transversal tensile properties can be ignored. When A/ L reaches 0.023, the longitudinal tensile modulus and strength of lamina decreased by 25.5% and 57.7%, respectively, compared with the lamina without any defects. Based on the conclusions made in this article, the scale of in-plane fiber micro-buckling can be predicted without metallographic observation. And the increase of structural efficiency and the loss of mechanical performance in consequence of non-geodesic fiber path could be evaluated for the optimization of fiber trajectory.

Flow‐induced anisotropic viscosity in short fiber reinforced polymers

AbstractThe commonly used flow models for fiber reinforced polymers often neglect the flow induced mechanical anisotropy of the suspension. With an increasing fiber volume fraction, this plays, however, an important role. There are some models which count on this effect, they are, however, phenomenological and require a fitted model parameter. In this paper, a micromechanically based constitutive law is proposed which considers the flow induced anisotropic viscosity of the fiber suspension. The introduced viscosity tensor can handle arbitrary anisotropy of the fluid‐fiber mixture depending on the actual fiber orientation distribution. A homogenization method for unidirectional structures in contribution with orientation averaging is used to determine the effective viscosity tensor. The motion of rigid ellipsoidal fibers induced by the flow of the matrix material is described by Jeffery's equation. A numerical implementation of the introduced model is applied to representative flow modes. The calculated stress values are analyzed in transient and stationary flow cases. They show a less pronounced anisotropic viscous behaviour in every investigated case compared to the results obtained by the use of the Dinh‐Armstrong constitutive law. The reason for the qualitative difference is that the presented model depends on the complete orientation information, while the other one is linear in the fourth‐order fiber orientation tensor. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Crosstalk in rectangular cross-section heterogeneous multicore fiber

Using neighboring cores with different mode propagation constants (indexes) is a well-known way to reduce crosstalk in multicore fiber (MCF). However, in actual field-deployed fiber, random bends can cause a reduction in the difference between the mode indexes of neighboring cores, which consequently increases crosstalk. The level of crosstalk induced by bending in both rectangular cross-section and circular cross-section heterogeneous MCF with cores arranged in a line was investigated. The experimental results obtained indicate that in contrast to circular cross-section MCF, no bending-induced crosstalk occurs in rectangular cross-section MCF wound on the mandrel without special control of cross-section orientation. Thus, to eliminate undesirable bending-induced crosstalk in heterogeneous MCF a rectangular cross-section should be employed.

Mapping 3D fiber orientation in tissue using dual-angle optical polarization tractography.

Optical polarization tractography (OPT) has recently been applied to map fiber organization in the heart, skeletal muscle, and arterial vessel wall with high resolution. The fiber orientation measured in OPT represents the 2D projected fiber angle in a plane that is perpendicular to the incident light. We report here a dual-angle extension of the OPT technology to measure the actual 3D fiber orientation in tissue. This method was first verified by imaging the murine extensor digitorum muscle placed at various known orientations in space. The accuracy of the method was further studied by analyzing the 3D fiber orientation of the mouse tibialis anterior muscle. Finally we showed that dual-angle OPT successfully revealed the unique 3D "arcade" fiber structure in the bovine articular cartilage.

Fiber orientation distribution and tensile mechanical response in UHPFRC

In this study, the crucial effect of the fiber orientation distribution on the tensile mechanical response of ultra high performance fiber reinforced concretes (UHPFRC) is discussed. A direct tension test method was used to characterize the tensile response of a UHPFRC material as well as to assess the actual tensile response along the principal directions in a real-scale UHPFRC structural element. Moreover, the actual fiber orientation distribution was evaluated in representative sections through an image analysis technique. The experimental results validated the anisotropy in the fiber orientation distribution and, consequently, in the tensile mechanical properties as a consequence of the casting process and the flow pattern. The concept of the fiber orientation factor was discussed as well as the approaches currently adopted to implement robust and reliable safety factors accounting for the fiber orientation distribution impact on the design methodologies for UHPFRC. Finally, the need of a comprehensive design framework for UHPFRC structures was highlighted in order to allow for fully exploitation of the material properties.

Coupled influence of strain rate and heterogeneous fibre orientation on the mechanical behaviour of short-glass-fibre reinforced polypropylene

Short-fibre-reinforced thermoplastic polymers (SFRT) are appealing materials for use in technical applications, due to high rigidity/density ratio, in particular. However, SFRT have complex anisotropic mechanical behaviour, because of specificities of thermoplastic matrix behaviour and heterogeneous reinforcement characteristics, in particular in terms of fibre orientation. For instance, matrix behaviour can be viscoelastic and/or viscoplastic (i.e. sensitive to strain rate) pressure sensitive, damageable, etc. Moreover, mechanisms of load transmission from matrix to fibres, at fibre/matrix interface, are still not very well known, although they play a crucial role in SFRT mechanical behaviour. For instance, there is no information about their eventual strain-rate sensitivity. In this study, the 3D microstructure of an injection-moulded short-fibre-reinforced polypropylene is reconstructed using micro-computed tomography. The tensile macroscopic behaviour of the composite is then studied under different angles of loading with respect to injection flow direction and different displacement rates, in quasi-static and dynamic range (from 1mm/min to 1m/s). The anisotropy of the composite behaviour can therefore be analysed not only in terms of dependence on macroscopic loading direction, but also on actual fibre distribution of orientation relatively to tensile direction, at the microscopic scale. In an original way, coupled influence of fibre orientation and strain rate is then studied. One of the most significant results is that fibre/matrix interfacial behaviour, which governs load transmission from matrix to fibres, does not show strain rate sensitivity for the considered composite and strain-rate range (5.10−4 to 50 s−1).

The Tensile Behavior of High-Strength Carbon Fibers.

Carbon fibers exhibit exceptional properties such as high stiffness and specific strength, making them excellent reinforcements for composite materials. However, it is difficult to directly measure their tensile properties and estimates are often obtained by tensioning fiber bundles or composites. While these macro scale tests are informative for composite design, their results differ from that of direct testing of individual fibers. Furthermore, carbon filament strength also depends on other variables, including the test length, actual fiber diameter, and material flaw distribution. Single fiber tensile testing was performed on high-strength carbon fibers to determine the load and strain at failure. Scanning electron microscopy was also conducted to evaluate the fiber surface morphology and precisely measure each fiber's diameter. Fiber strength was found to depend on the test gage length and in an effort to better understand the overall expected performance of these fibers at various lengths, statistical weak link scaling was performed. In addition, the true Young's modulus was also determined by taking the system compliance into account. It was found that all properties (tensile strength, strain to failure, and Young's modulus) matched very well with the manufacturers' reported values at 20 mm gage lengths, but deviated significantly at other lengths.

Influence of reinforcing bar layout on fibre orientation and distribution in slabs cast from fibre‐reinforced self‐compacting concrete (FRSCC)

AbstractFibre orientation and volume distribution affect the post‐cracking tensile strength, which is one of the main design parameters of fibre‐reinforced concrete (FRC). This paper discusses the influence of unidirectional and grid reinforcement on fibre orientation and distribution in FRC slabs. Slabs without conventional reinforcing bars were used as a reference. The slab size was 1200 × 1200 × 150 mm. Numerical simulations were used to predict the fibre orientation and X‐ray computed tomography (CT) to determine the actual fibre orientation and distribution. Beams were sawn from each slab, CT‐scanned and tested in three‐point bending tests in accordance with EN 14651. Both the numerical simulations and the CT results show that the rebars caused a more isotropic fibre orientation in the lower halves of the slabs. This was confirmed in the bending tests, where the lowest variation and highest residual tensile strengths were documented for beams sawn from slabs with grid reinforcement. Fibre migration from the upper layer to middle and lower layers of the slabs due to gravity was observed in all slabs, and in the reinforced slabs, migration also depended on the distance from the casting point. The reinforcement led to an accumulation of fibres above the rebars in the middle layer of each reinforced slab. A set of mechanisms is proposed to explain the experimental results.

Advanced Ultrasonic Non-destructive Evaluation for Metrological Analysis and Quality Assessment of Impact Damaged Non-crimp Fabric Composites

Composite materials are nowadays massively utilized in a very large number of industrial applications. Thus, it has become essential to characterize the service behaviour they can provide depending on their working conditions. In this paper, the study of the influence of impact conditions on damage generation in high performance composite materials, consisting of non-crimp fabric composite laminates, is carried out through the application of an advanced ultrasonic non-destructive evaluation technique, known as full volume ultrasonic scanning. This technique is based on the pulse-echo immersion testing method and allows for the quantitative analysis of the internal material structure in the entire composite volume. The aim of the ultrasonic non-destructive evaluation analysis is the metrological characterization of the non-crimp fabric composite laminates in terms of actual thickness estimation and stacking sequence fiber orientation verification as well as their quality assessment in terms of impact damage development within the whole composite material volume.

Modeling melt blowing fiber with different polymer constitutive equations

The characteristics of molten polymer plays a major role in fiber formation in the melt blowing (MB) process. In this paper, the Maxwell model and two kinds of the standard linear solid (SLS) models in the bead-viscoelastic element model are proposed for melt blown fiber formation simulation. The fiber diameter, velocity and stress are studied with these different constitutive equations of polymer. The trajectory path of fiber whipping is obtained using numerical simulation and compares with the actual fiber motion which is captured with a high-speed camera. The results present that the Standard Linear Solid Model (SLS) is better than Maxwell model to predict the melt blown fiber’s characteristics under the same air drawing conditions, including fiber diameter, velocity and stress. The whipping motion of the fiber also can be well expressed by SLS constitutive model. The mathematical model with SLS model provides a clear understanding on the mechanism of the formation of microfibers during melt blowing.

An approach to characterize dielectric properties of fiber-reinforced composites with high volume fraction

Fiber-reinforced composite materials are widely used in aeronautics and automotive industries due to their excellent mechanical properties. Composites with high conductive fibers embedded have good performance of shielding effectiveness and become possible candidates to replace metals. One approach for analyzing the electromagnetic (EM) interaction of fiber-reinforced composites is to use full numerical methods, which allow precise modeling and give accurate results. However, numerical methods may lead to prohibitive computational time and memory capacity due to the strong dependence on the shielding properties from heterogeneous microstructures.In composite materials, two important parameters, effective permittivity eff and effective permeability eff, determine the interaction between the electromagnetic field and the materials. For estimating the effective parameters, homogenization techniques have been developed to describe a composite mixture in terms of a spatially homogeneous electromagnetic response, mostly under static conditions. The well-known rules are the Maxwell-Garnett (MG) formula and the Bruggeman formula.These rules are usually applied to the dilute composite materials and provide satisfactory results as long as the wavelength remains large compared to the size of the heterogeneities. Recently, revised homogenization models have been developed to extend the frequency range. Some of them are presented with the help of numerical method but still require substantial computational time and resources to be performed. One recently proposed homogenization model, called dynamic homogenization model (DHM), is an extension of quasi-static homogenization methods for microwave frequencies. It is obtained by introducing a microstructure-dependent characteristic length for the composites made of a square array of circular cylinders buried in the matrix, based on the basic inclusion problems. The DHM overcomes the limitations of standard static homogenization tools, but only applicable to low fiber volume fraction (less than 20%).In this paper, we focus on the microstructure in the case of a square array of circular 2D conductive long fibers embedded in a dielectric matrix. A revised DHM is proposed to describe the effective permittivity of the composite materials with different inclusion concentrations, including higher fiber volume fraction. Firstly, an iterative procedure is employed to estimate an effective permittivity, which is then used to modify the wavelength in the DHM. Secondly, an empirical formula-based characteristic size of the microstructure is presented by considering the current distribution of the fibers under the EM wave illumination in the case of high fiber volume fractions. Therefore, the final modified homogenization model is given for the effective permittivity of composites with arbitrary inclusion concentrations. It can be used to efficiently calculate the reflection and transmission coefficients, as well as the shielding effectiveness by classical transmission-line methods. Three infinite sheets with different physical parameters are utilized for validation. We compare the results of the shielding effectiveness obtained from this homogenization model with those obtained from a full numerical solution of the actual fiber composites. Reasonable agreements obtained demonstrate that the proposed model could define the effective permittivity of the composites with high fiber concentration over a wide frequency range including microwave frequencies. Analogous formulas also hold for the magnetic permeability with permittivity replaced by permeability wherever it appears in the proposed model.

Prebiotics, Fermentable Dietary Fiber, and Health Claims.

Since the 1970s, the positive effects of dietary fiber on health have increasingly been recognized. The collective term "dietary fiber" groups structures that have different physiologic effects. Since 1995, some dietary fibers have been denoted as prebiotics, implying a beneficial physiologic effect related to increasing numbers or activity of the gastrointestinal microbiota. Given the complex composition of the microbiota, the demonstration of such beneficial effects is difficult. In contrast, an exploration of the metabolites of dietary fiber formed as a result of its fermentation in the colon offers better perspectives for providing mechanistic links between fiber intake and health benefits. Positive outcomes of such studies hold the promise that claims describing specific health benefits can be granted. This would help bridge the "fiber gap"-that is, the considerable difference between recommended and actual fiber intakes by the average consumer.

Sifat-sifat Tarik dan Flexural Komposit Serat Sabut Kelapa Unidireksional/Poliester

The purpose of this study is to investigate the tensile and flexural properties of unidirectional coconut fiber/polyester composite materials, and to describe their failure modes. Specimens were cut from fiber/polyester composite plates containing various fiber contents. Materials being used in this study are coconut fiber that was previously alkali-treated and polyester resin matrix. Whilst tensile testing was carried out in accordance with the ASTM D3039 standard, flexural testing was based on the ASTM D790 standard. Failure surfaces of the representative specimens were then observed under an optical microscope, and their digital photo macrographs were captured for image analysis in order to describe their respective fiber distribution pattern and to determine their respective actual fiber volume fraction, Vf, by means of an open source software called ImageJ. It was found out that the actual Vf of the four composite plates being produced were 10.7%, 17.6%, 27.4% and 40.5%. It was revealed that while tensile strength increases with the increase of Vf, while failure strain, modulus elasticity and flexural strength decreases. The average highest tensile strength, tensile failure strain, and tensile modulus of elasticity were found being 30.01 MPa at Vf = 40.5%, 0.027 mm/mm at = 0%, and 1.47 GPa at Vf = 0%, respectively. The average highest flexural strength, failure strain and modulus of elasticity were observed being 153.92 MPa at Vf = 10.7%, 0.0358 mm/mm at Vf = 0%, and 3.242 GPa at Vf =10.7%, respectively. It was observed that specimens were failed by fiber pull out and debonding.

Distribution of fibers in SFRC segments for tunnel linings

This paper presents research results regarding the distribution of steel fibers in concrete used to build precast tunnel segments for Line 9 of the Barcelona Metro. The fiber distribution was studied using the actual fiber contents obtained by means of crushed cores drilled from different points of three full-scale tunnel lining segments. A statistical analysis determined that the fiber content in the ends of segments tends to be greater than in the central zone. The way of transporting, pouring and compacting concrete influences the fiber content and the fiber distribution across the thickness of the segment. In addition, cores with a diameter of 150mm were found to have a lower scatter in the fiber content than smaller diameter specimens. Finally, based on probabilistic approaches, a minimum of 11 cores is proposed to control the fiber content in FRC segments.

The Influence of Different Retting Processes on the Strength of Fibres obtained from Poliostigma raticulatum, Grewia mollis, Cissus populnea and Hibiscus sabdariffa

<p class="1Body">The extraction of vegetable fibres from different parts of plants has been a major focal point in the search for natural fibres that would substitute synthetic fibres. Fibres from <em>Poliostigma reticulatum</em>, <em>Grewia mollis,</em> <em>Cissus populnea</em> and<em> Hibiscus sabdariffa</em> were extracted by water and chemical retting. In chemical retting different concentrations of NaOH and NH<sub>4</sub>OH were used. The extracted fibres were further purified and their tensile strength measured. The pH of water was measured as retting progresses and observed to increase as the retting time increases. This was ascribed to the secretion of enzymes by microorganisms as they acted on the mucilaginous matter of the bast with the resultant loosening of the fibres. The tensile strength of the fibres was determined using the Shirley Testometric 220D and was observed to gradual decline as the retting time increases. This was attributed to the continual removal of non-fibrous matter and the freeing of the fibres in the composite. However, beyond the fifth week of retting, no appreciable change in tensile strength was observed. This suggested that most of the non-fibrous matter have been solublized and utilized as source of nutrients and energy by the microorganisms. Acidic metabolites were produced due to microbial activities may have changed the pH of the medium and subsequently hindered their growth. In chemical retting, as the concentration of the medium increases the strength of the fibres decreases to a minimum then remain constant. This was accredited to the breakdown of inter- molecular bonds between non-fibrous substances and the fibres. And subsequently, the non-fibrous components separate from the fibres and dissolved in the medium revealing the actual fibres strength. It was therefore, opined that retted fibres in water, 5% NH<sub>4</sub>OH and 15% NaOH are clearer and lustrous for all sample except those from Kargo<em>.</em></p>

Thermoforming and Structural Analysis of Combat Helmets

To reduce combat casualties, military helmets are designed to provide protection against projectiles. Modern combat helmets are constructed of relatively lightweight composite materials that provide ballistic protection to the soldier. The manufacture of most composite helmets is labor intensive and involves the manual application and smoothing of individual layers of reinforcement to a concave mold surface. The recently developed double diaphragm deep drawing thermoforming process turns as-purchased, flat-form composite materials into structurally efficient three-dimensional shapes. Using this process, prototype shells have been produced and subsequently tested structurally. The success of the outcome has been greatly assisted through the use of specialized virtual prototyping techniques to provide insight into the thermoforming process of the shells and subsequently their structural performance by accounting for the actual fiber orientations of those finished shells.

Rapid Quantification of 3D Collagen Fiber Alignment and Fiber Intersection Correlations with High Sensitivity.

Metastatic cancers aggressively reorganize collagen in their microenvironment. For example, radially orientated collagen fibers have been observed surrounding tumor cell clusters in vivo. The degree of fiber alignment, as a consequence of this remodeling, has often been difficult to quantify. In this paper, we present an easy to implement algorithm for accurate detection of collagen fiber orientation in a rapid pixel-wise manner. This algorithm quantifies the alignment of both computer generated and actual collagen fiber networks of varying degrees of alignment within 5°°. We also present an alternative easy method to calculate the alignment index directly from the standard deviation of fiber orientation. Using this quantitative method for determining collagen alignment, we demonstrate that the number of collagen fiber intersections has a negative correlation with the degree of fiber alignment. This decrease in intersections of aligned fibers could explain why cells move more rapidly along aligned fibers than unaligned fibers, as previously reported. Overall, our paper provides an easier, more quantitative and quicker way to quantify fiber orientation and alignment, and presents a platform in studying effects of matrix and cellular properties on fiber alignment in complex 3D environments.

The theoretical yarn unevenness of cotton considering the joint influence of fiber length distribution and fiber fineness

The theoretical yarn unevenness has been discussed by considering the joint influence of fiber length distribution and fiber fineness. For fiber length distribution, the fiber length density function was derived from the specific Weibull parameters obtained from actual fiber length measurements using an USTER AFIS instrument. Meanwhile, fiber fineness was added to Suh’s model by expressing spun yarn irregularity in terms of fiber mass variation. Comparisons of the calculated value of theoretical yarn unevenness in this paper with the tested and previously researched ones for different cotton yarns have been made. It is shown that the theoretical yarn unevenness calculated in this paper could better reflect the effect of fiber length and fiber fineness on yarn unevenness. This might be the theoretical foundation for the prediction of theoretical cotton yarn unevenness on the basis of actual fiber length distribution and fiber fineness.

On the effects of leaflet microstructure and constitutive model on the closing behavior of the mitral valve.

Recent long-term studies showed an unsatisfactory recurrence rate of severe mitral regurgitation 3-5years after surgical repair, suggesting that excessive tissue stresses and the resulting strain-induced tissue failure are potential etiological factors controlling the success of surgical repair for treating mitral valve (MV) diseases. We hypothesized that restoring normal MV tissue stresses in MV repair techniques would ultimately lead to improved repair durability through the restoration of MV normal homeostatic state. Therefore, we developed a micro- and macro- anatomically accurate MV finite element model by incorporating actual fiber microstructural architecture and a realistic structure-based constitutive model. We investigated MV closing behaviors, with extensive in vitro data used for validating the proposed model. Comparative and parametric studies were conducted to identify essential model fidelity and information for achieving desirable accuracy. More importantly, for the first time, the interrelationship between the local fiber ensemble behavior and the organ-level MV closing behavior was investigated using a computational simulation. These novel results indicated not only the appropriate parameter ranges, but also the importance of the microstructural tuning (i.e., straightening and re-orientation) of the collagen/elastin fiber networks at the macroscopic tissue level for facilitating the proper coaptation and natural functioning of the MV apparatus under physiological loading at the organ level. The proposed computational model would serve as a logical first step toward our long-term modeling goal-facilitating simulation-guided design of optimal surgical repair strategies for treating diseased MVs with significantly enhanced durability.

Transverse cracking of cross-ply laminates: A computational micromechanics perspective

Abstract Transverse cracking in cross-ply carbon/epoxy and glass/epoxy laminates in tension is analyzed by means of computational micromechanics. Longitudinal plies were modeled as homogenized, anisotropic elastic solids while the actual fiber distribution was included in the transverse plies. The mechanical response was obtained by the finite element analysis of a long representative volume element of the laminate. Damage in the transverse plies was triggered by interface decohesion and matrix cracking. The simulation strategy was applied to study the influence of ply thickness on the critical stress for the cracking of the transverse plies and on the evolution of crack density in 0 2 / 90 n / 2 s laminates, with n = 1, 2, 4 and 8. It was found that the transverse ply strength corresponding to the initiation and propagation of a through-thickness crack was independent of the ply thickness and that the transverse strength of carbon/epoxy laminates was 35% higher than that of the glass fiber counterparts. In addition, the mechanisms of crack initiation and propagation through the thickness as well as of multiple matrix cracking were ascertained and the stiffness reduction in the 90° ply as a function of crack density was computed as a function of the ply thickness.