Aligned Short Fiber Composites Research Articles

Microstructural effects on failure modes in highly aligned short carbon fiber composites

AbstractAligned short carbon fiber (3‐5 mm) thermoplastic composites have potential for versatile manufacturability with little loss of mechanical properties in comparison to continuous reinforcement. Property prediction, failure behavior, and the effects of microstructural variation for these materials are not yet well explored. The finite element method was used to evaluate large domains encompassing characteristic discontinuous microstructures of thermoplastic matrix (PMMA) composites reinforced with short 3 to 5 mm carbon fibers with a high degree of alignment and conventional volume fractions. Virtual tests were performed on highly detailed microstructural simulations which included fiber‐matrix interface representation along with uniform or misaligned fiber morphologies. Stiffness, strength, and failure mechanisms were analyzed. It was shown that for ideal uniformly aligned short fiber reinforcements, the material maintains a high modulus within 95% of an equivalent continuous fiber‐reinforced polymer, along with relatively high strength at 80% of a carbon fiber reinforced polymer (CFRP). Failure modes depended on the toughness of the fiber‐matrix interface, transitioning from fiber rupture to a weaker fiber pullout failure for low interface properties. Simulations indicated that significant fiber misalignments on the order of 15% of fibers at ±15° off‐axis could sharply reduce local strength in the misaligned region to roughly 20% of the continuous fiber strength. Although partially affected by property dropoff owing to fiber off‐axis misalignment, this weakening was largely due to local matrix pockets in regions of high misalignment, which acted as failure initiation points.

Fabrication, Modeling and Characterization of Magnetostrictive Short Fiber Composites.

Magnetostrictive materials have a wide variety of applications due to their great capability as sensors and energy-harvesting devices. However, their brittleness inhibits their applications as magnetostrictive devices. Recently, we developed a continuous magnetostrictive Fe-Co-fiber-embedded epoxy matrix composite to increase the flexibility of the material. In this study, we fabricated random magnetostrictive Fe-Co short fiber/epoxy composite sheets. It was found that the discontinuous Fe-Co fiber composite sheet has the magnetostrictive properties along the orientation parallel to the length of the sheet. Finite element computations were also carried out using a coupled magneto-mechanical model, for the representative volume element (RVE) of unidirectional aligned magnetostrictive short fiber composites. A simple model of two-dimensional, randomly oriented, magnetostrictive short fiber composites was then proposed and the effective piezomagnetic coefficient was determined. It was shown that the present model is very accurate yet relatively simple to predict the piezomagnetic coefficient of magnetostrictive short fiber composites. This magnetostrictive composite sheet is expected to be used as a flexible smart material.

A multiscale approach for virtual testing of highly aligned short carbon fiber composites

This paper presents an efficient multiscale approach for high fidelity virtual testing of highly aligned short fiber reinforced composites (SFRCs), based solely on constituent material and interface properties with no need of calibration data. The method is based on a hierarchical, bottom-up characterization of SFRCs and the link of mechanical behaviors of materials and microstructures from a lower scale to a higher scale. It starts with a microscopic unit cell model to estimate the transverse and shear properties of the aligned short fiber composites. Next, two types of mesoscale models with explicit consideration of fiber discontinuity and possible local fiber misalignments are employed to obtain effective properties of material domains with such morphologies. Such obtained meso-scale mechanical properties and damage behaviors are then integrated into the macroscale virtual laminar via the stochastically integrating of possible material defects. Results indicate that for perfectly aligned SFRCs the strength reduction is about 25% as compared to that of the continuously reinforced composites. However, misalignment of short fibers can cause a further strength reduction of 20–35% with the volume fraction of the misalign regions varying from 1% to 10%.

Numerical analysis on viscoelastic creep responses of aligned short fiber reinforced composites

Micro-mechanics analysis using the Representative Volume Element (RVE) approach implemented with the Finite Element Method has been widely used for computing material properties of unidirectional fibrous polymer matrix composites. However, little attention has been given to viscoelastic RVEs of discontinuous fiber reinforced composites. This paper develops a RVE-based Finite Element algorithm for evaluating the effective viscoelastic creep behaviors of aligned short fiber composites. A parametric study including considerations of fiber volume fraction, fiber aspect ratio and fiber packing geometry is performed through the proposed algorithm. Computed results indicate that increasing the fiber volume fraction decreases the mechanical compliance of the overall compound, and the effect of fiber reinforcements is particularly significant in the direction of fiber alignment. Additionally, increasing the fiber aspect ratio reduces the creep compliance coefficient along the direction of fiber alignment more than coefficients along other directions. The fiber packing geometry affects the values of axial compliance properties at low fiber volume fraction and its impacts become less as the fiber volume fraction increases. We also provide an application to simulate the equivalent viscoelastic creep response out of the RVE approach through ABAQUS user defined material subroutine, and the maximum absolute error between the two sets of data is only 1%.

An optimized representative volume element to predict the stiffness of aligned short fiber composites

The micromechanical prediction of mechanical properties of composites, using a finite element method, is highly affected by selecting a suitable representative volume element. In the present research, the effects of the representative volume element aspect ratio on the stiffness of aligned short fiber composites have been studied. First, an analytical method is suggested to predict the longitudinal and transverse moduli of composites considering the representative volume element aspect ratio. Then, the optimum aspect ratio of the representative volume element is estimated by comparing the results obtained by the present method with those of a micromechanical method. Comparing the stiffness of the optimized representative volume element with some experimental results available in the literature shows that the result of the present method is also in a good agreement with experiments.

Multiple closed loop recycling of carbon fibre composites with the HiPerDiF (High Performance Discontinuous Fibre) method

The aim of this article is to apply the concept of Circular Economy, where end-of-life products and production wastes are recycled into reusable materials, to carbon fibre reinforced plastics. This not only reduces the amount of material disposed into landfills, but also enables manufacturers to achieve significant savings. While current research focuses on the performance of recycled carbon fibre reinforced composites after one recycling process, this paper aims to investigate the performance of composites remanufactured from short carbon fibres that have undergone multiple recycling loops with the High Performance Discontinuous Fibre (HiPerDiF) method. The HiPerDiF method enables the production of aligned short fibre composites with exceptional mechanical properties. In addition, using short fibres makes the composite material intrinsically easy to recycle. Short virgin carbon fibres underwent two loops of fibre reclamation and remanufacturing. A correlation between the composites’ mechanical properties and the nature of the fibres, i.e. reduction in fibre lengths, as well as the residual matrix accumulation from the reclaiming process over a number of recycling loops, was established.

Numerical and analytical modeling of aligned short fiber composites including imperfect interfaces

Finite element calculations were used to bound the modulus of aligned, short-fiber composites with randomly arranged fibers, including high fiber to matrix modulus ratios and high fiber aspect ratios. The bounds were narrow for low modulus ratio, but far apart for high ratio. These numerical experiments were used to evaluate prior numerical and analytical methods for modeling short-fiber composites. Prior numerical methods based on periodic boundary conditions were revealed as acceptable for low modulus ratio, but degenerate to lower bound modulus at high ratio. Numerical experiments were also compared to an Eshelby analysis and to an new, enhanced shear lag model. Both models could predict modulus for low modulus ratio, but also degenerated to lower bound modulus at high ratio. The new shear lag model accounts for stress transfer on fiber ends and includes imperfect interface effects; it was confirmed as accurate by comparison to finite element calculations.

A novel manufacturing method for aligned discontinuous fibre composites (High Performance-Discontinuous Fibre method)

The High Performance-Discontinuous Fibre (HiPerDiF) method is a new high speed process to produce discontinuous fibre architectures with high volume fraction. It allows the manufacture of tow or tape type prepregs with highly aligned reinforcements directly from short fibres rather than from pre-existing tows.This paper introduces the principle of this unique short fibre alignment method and describes the improved orientation head design for obtaining tape type preforms with high productivity. Using this HiPerDiF method, tensile specimens with 67% of the fibres aligned within the range of ±3° were successfully produced from tape type preforms with 3mm long carbon fibres. Tensile modulus and strength in the fibre direction of specimens with a fibre volume fraction of 55% were 115GPa and 1509MPa, respectively, significantly higher than those of aligned short fibre composites made by conventional methods.

Aligned Discontinuous Fibre Composites: A Short History

The most popular methodologies employed to manufacture composite components still lack sufficient rate to meet demand. Furthermore, the state of the art in automated processes imposes restrictions on component geometrical complexity. Many of these restrictions arise directly as a result of the use of continuous fibre reinforcements. Processing and use of highly-aligned discontinuous fibres may avoid many of the manufacturing defects induced in continuous fibre architectures, which are a result of design decisions and manipulations during processing operations, while theoretically allowing for similar mechanical properties if alignment and orientation are tightly controlled. An overview of the history of aligned short fibre composites has been presented, with the focus upon process and application of highly aligned advanced composites with properties near to that of continuous fibre composites. When continuous carbon fibres were first developed, there remained an interest to create aligned discontinuous fibre composites in order to form more complex components. This interest has reappeared in recent years with methods divided into those which align short fibres and those which introduce discontinuities into aligned fibres. It is clear that discontinuous material systems have their uses and applications, but greater development is still required in order to better quantify and categorise the materials and their associated properties and processes. doi:10.12783/issn.2168-4286/2/3/4/Such

Modeling and characterization of dielectrophoretically structured piezoelectric composites using piezoceramic particle inclusions with high aspect ratios

In this work, the piezoelectric properties at high electric fields of dielectrophoretically aligned PZT—polymer composites containing high aspect ratio particles (such as short fibers) are presented. Polarization and strain as a function of electric field are evaluated. The properties of the composites are compared to those of PZT-polymer composites with equiaxed particles, continuous PZT fiber-polymer composites, and bulk PZT ceramics. From high-field polarization and strain measurements, the effective field dependent permittivity and piezoelectric charge constant in the poling direction are determined for dielectrophoresis structured PZT-polymer composites, continuous PZT fiber-polymer composites, and bulk PZT ceramics. The changes in dielectric properties of the inclusions and the matrix at high fields influence the dielectric and piezoelectric properties of the composites. It is found that the permittivity and piezoelectric charge constants increase towards a maximum at an applied field of around 2.5-5 kV/mm. The electric field at which the maximum occurs depends on the aspect ratio and degree of alignment of the inclusions. Experimental values of d33 at low and high applied fields are compared to a model describing the composites as a continuous polymer matrix containing PZT particles of various aspect ratios arranged into chains. Thickness mode coupling factors were determined from measured impedance data using fitted equivalent circuit model simulations. The relatively high piezoelectric strain constants, voltage constants, and thickness coupling factors indicate that such aligned short fiber composites could be useful as flexible large area transducers.

Second order shear lag theory

The standard shear lag theory for elastic aligned short-fibre composites is extended to allow for a gradient of overall strain. The result is a one-dimensional strain gradient theory of the Toupin–Mindlin type. All parameters are the same as in the standard theory, and in the limits of weak strain gradients, large fibre aspect ratios or low elastic modulus ratios, the standard theory is recovered. The gradient effect is illustrated by a simple one-dimensional boundary value problem: a vertical composite rod fixed at both ends and loaded by gravity. The fibre length significantly affects the solution when the fibres are rigid and their length is near the rod length; but otherwise the effect is weak.

Generalized Self-Consistent Model for Predicting Thermal Conductivity of Composites with Aligned Short Fibers

Thermal conductivities of aligned short fiber composites in the longitudinal and transverse directions are derived by extending the generalized self-consistent model proposed by Benveniste and Miloh. Their final forms are semi-explicitly expressed as functions of constituents’ thermal conductivities, fiber aspect ratio, and the aspect ratio of the matrix surrounding the fiber. The results are proved to be consistent with those of composites with extreme shapes of fillers such as sphere and continuous fiber by simply changing a confocality variable. The present results for aligned short fiber composites are compared with Wu’s upper and lower bounds to be validated. Finally, the predictions by the present model and modified Eshelby model are compared in terms of the thermal conductivity ratio of constituents, fiber aspect ratio, and fiber volume fraction and their accuracy for the predictions are evaluated by the experimental results from the literature. [doi:10.2320/matertrans.M2010249]

A study on validity of using average fiber aspect ratio for mechanical properties of aligned short fiber composites with different fiber aspect ratios

A closed-form solution using the actual distribution of the fiber aspect ratio is proposed for predicting the stiffness of aligned short fiber composite. The present model is the simplified form of Takao and Taya’s model and the extended version of Taya and Chou’s model, where Eshelby’s equivalent inclusion method modified for finite fiber volume fraction is employed. The validity of using average fiber aspect ratio for predicting the composite stiffness is justified in terms of the scatter of fiber aspect ratio, fiber volume fraction, and constituents‘ Young’s modulus ratio, comparing with the results by the present model. The guideline for selection of either the actual distribution or the average fiber aspect ratio is presented for the better prediction of the composite stiffness.

Prediction of Thermal Conductivity of Aligned Short Fibre Composites with Different Fibre Aspect Ratios

A closed-form solution using the distribution of fibre aspect ratios is proposed for predicting the thermal conductivity of aligned short fibre composites. The present model is the extended version of Hatta and Taya's model, where Eshelby's equivalent inclusion method modified for finite volume fraction of fibres is employed. The validity of using the average fibre aspect ratio for predicting the thermal conductivity is examined in terms of the distribution of the aspect ratios such as its average and standard deviation, fibre volume fraction, and constituents' thermal conductivity ratio. The guideline for the selection of either the actual distribution or the average fibre aspect ratio is presented in terms of the thermal conductivity ratio and the coefficient of the variation of the aspect ratio for the better prediction of the thermal conductivity of the composites.

Simplified Model for the Influence of Inclusion Aspect Ratio on the Stiffness of Aligned Reinforced Composites

A simplified model for the Young’s Modulus of aligned composites possessing spheroidal inclusions, of any aspect ratio α, is developed herein based on a triply periodic array of parallelpiped inclusions. The Young’s Modulus of continuous unidirectional fiber composites (α→∞), particulate composites (α = 1) and periodically bilaminated composites (α = 0) are obtained as limiting cases based on the Parallel-Series and Series-Parallel schemes of analyses. A reinforcing parameter, R, of the inclusion is then identified from the limiting cases such that R ∊ [0,1]. By curve-fitting for a relationship between the reinforcement parameter and the inclusion aspect ratio for α∊ [0,∞], the Young’s Modulus for aligned short fiber composites (α > 1) and aligned flake composites (α < 1) are obtained. For verification, the present model at limiting cases (α =0, 1, ∞) were reduced to other simple models and/or compared with experimental results. For nonlimiting cases (α <1, α > 1), comparison with other theoretical models show reasonable agreement.

The applicability of the generalized method of cells for analyzing discontinuously reinforced composites

The paper begins with a short overview of the recent work done in the field of discontinuous reinforced composites, focusing on the different parameters which influence the material behavior of discontinuous reinforced composites, as well as the various analysis approaches undertaken. Based on this overview it became evident, that in order to investigate the enumerated effects in an efficient and comprehensive manner, an alternative approach to the computationally intensive finite-element based micromechanics approach is required. Therefore, an investigation is conducted to demonstrate the utility of utilizing the generalized method of cells (GMC), a semi-analytical micromechanics-based approach, to simulate the elastic and elastoplastic material behavior of aligned short fiber composites. The results are compared with (1) simulations using other micromechanical based mean field models and finite element (FE) unit cell models found in the literature given elastic material behavior, as well as (2) finite element unit cell and a new semianalytical elastoplastic shear lag model in the inelastic range. GMC is shown to definitely have a window of applicability when simulating discontinuously reinforced composite material behavior.

Stiffness predictions for unidirectional short-fiber composites: Review and evaluation

Micromechanics models for the stiffness of aligned short-fiber composites are reviewed and evaluated. These include the dilute model based on Eshelby's equivalent inclusion, the self-consistent model for finite-length fibers, Mori–Tanaka type models, bounding models, the Halpin–Tsai equation and its extensions, and shear lag models. Several models are found to be equivalent to the Mori–Tanaka approach, which is also equivalent to the generalization of the Hashin–Shtrikman–Walpole lower bound. The models are evaluated by comparison with finite-element calculations which use periodic arrays of fibers, and to Ingber and Papathanasiou's boundary element results for random arrays of aligned fibers. The finite-element calculations provide E 11, E 22, ν 12, and ν 23 for a range of fiber aspect ratios and packing geometries, with other properties typical of injection-molded thermoplastic matrix composites. The Halpin–Tsai equations give reasonable estimates for stiffness, but the best predictions come from the Mori–Tanaka model and the bound interpolation model of Lielens et al.

Short Fibre Polymer Composites: a Fracture-Based Theory of Fibre Reinforcement

The interphase between reinforcing fibers and polymers is brittle, and does not behave in the way it was assumed to when the standard theory for composite strength was developed. Futhermore, this theory predicts curved stress-strain plots for aligned short fibre composites, yet the evidence for this is unconvincing, and there is much new evidence that these stress-strain curves are straight. The time has therefore come to abandon this approach and take into account, instead, the apparent brittleness and sudden failure of aligned fibre reinforced polymers. This paper presents the evidence, and introduces the new approach. This involves microcrack development in composites from stress concentrations at the fibre ends. Since such failure initiation can occur simultaneously at many sites, the stress required to cause abrupt failure across the whole cross section can be estimated by a simple force balance. This analysis gives the familiar expressions used for short fibre composites, with one important difference. For carbon reinforced polymers, the polymer has to reach its breaking strength before failure, so that there is no minimum volume fraction for reinforcement with these composites. With glass, on the other hand, which has a higher breaking strain than most thermosets used for composites, the matrix appears unable to exert its full strength. Thus low fibre volume fraction glass fibre composites can be weaker than the matrix, and a minimum volume fraction for reinforcement exists.

Aligned short-fibre reinforced thermosets: Experiments and analysis lend little support for established theory

Experiments with epoxy resins reinforced with aligned short carbon fibres give results which disagree sharply with traditional fibre reinforcement theory based on interface yielding and slip and the concept of the critical fibre aspect ratio. Earlier results and evidence from interface studies are therefore reviewed, and it is shown that, as the carbon/polymer interface is brittle, the progressive interface failure process previously envisaged almost certainly does not take place. Furthermore, a careful reading of the sources of data relating to the yielding and slip theory indicates that the evidence in support of it is very weak. Thus, the idea of the critical fibre aspect ratio, borrowed from the metallurgists, may not be appropriate for short-fibre reinforced plastics. Instead, a process involving brittle fibre debonds should be considered. These debonds could trigger matrix cracking and hence explain the anomalously low composite breaking strains observed when the breaking strain of the fibre is greater than that of the polymer, and other properties of aligned short-fibre composites.

An energy method for calculating the stiffness of aligned short-fiber composites

A procedure is presented for calculating the longitudinal elastic modulus of a composite possessing dilute concentrations of aligned cylindrical fibers. For a dilute concentration of fibers, the modulus can be written in terms of the perturbation that a single fiber induces in the displacement field of an otherwise homogeneous body subjected to remote uniaxial tension. A boundary value problem, referred to as the auxiliary problem, is formulated which yields the perturbed displacement field directly. The longitudinal modulus can then be expressed in terms of the potential energy of the auxiliary problem; thus, energy-based approximate methods (e.g. FEM) can be exploited to calculate the modulus accurately. With a small modification, this procedure is shown to be applicable to the case of fibers which are imperfectly bonded to the matrix. Moduli for a few representative cases are calculated using a finite element method. Finally, a new means of defining and computing the ineffective length of a fiber is introduced.