Aligned Discontinuous Fibre Research Articles

Stretch-steering of highly aligned discontinuous fiber tape with automated fiber placement

Highly aligned discontinuous fiber (ADF) composites have been developed with mechanical properties comparable to continuous fiber composites. These materials can be stretched in the fiber direction during processing enabling the production of complex geometries. This study utilizes 57% fiber volume fraction ADF tape comprised of a thermoplastic Polyetherimide (PEI) matrix with 3 mm long IM7 carbon fibers. The Laser-Assisted Automated Fiber Placement (LA-AFP) process is used to study the limits of stretch-steering of ADF tape at small radii. An experimental technique based on photogrammetry was developed to quantify the effect of stretch parameters on the tape strain fields and path placement accuracy. The measurements captured the local in-plane strain tensor (longitudinal, transverse, and shear) across the width and along the length of steered tapes. The results demonstrate that placement path accuracy can be achieved when the tool center point (center of rotation) of the AFP head is located at the nip-point versus the roller centerline, which is typically used for steering continuous fiber tapes at large steering radii. Accuracy of the in-situ tape stretching was demonstrated up to 60% applied stretch strain. The measurements also confirm that when stretch-steering ADF tape, the resulting strain across the tape width is on average the superposition of the in-situ applied tensile strain and in-plane bending computed from the tape width and steering radius. A statistical-based methodology is presented to select the average tensile stretch levels to minimize the probability of defects forming during steering on the inner radius of the tape. The experiments show that a 12.5 mm wide tape can be accurately placed on a 50 mm radius of curvature without defects. This represents a two order of magnitude improvement in steerability over continuous fiber tapes of the same tape width found in peer-reviewed literature.

A Review on the Modelling of Aligned Discontinuous Fibre Composites

Aligned discontinuous fibre-reinforced composites are becoming more popular because they have the potential to offer stiffness and strength comparable to their continuous counterparts along with better manufacturability. However, the modelling of highly aligned discontinuous fibre composites is still in its infancy. This paper aims to provide a comprehensive review of the available literature to understand how modelling techniques have developed and consider whether all aspects which could affect the performance of aligned discontinuous fibre composites have been addressed. Here, for the first time, a broad view of the advantages, perspectives, and limitations of current approaches to modelling the performance and behaviour of aligned discontinuous fibre composites during alignment, forming, and mechanical loading is provided in one place as a route to design optimisation.

Micromechanical modeling of discontinuous long fiber reinforced composites

The performance of discontinuous fiber composites is intricately linked to the microstructure of the composite material. Therefore, micromechanics prove valuable for the analysis and design of these materials. Two micromechanics theories, the high fidelity generalized method of cells and Carrera Unified Formulation, are used to model a repeating unit cell (RUC) of the Tailorable Universal Feedstock for Forming, a highly aligned discontinuous fiber carbon fiber composite system. The models are evaluated for their ability to predict the effective properties and local stress fields within the RUC. The stress fields between the fiber ends and the fiber-matrix interfacial stresses are of particular interest. The models are used to predict the effective properties of single fiber and multiple fiber RUCs with varying fiber-to-fiber end gap lengths, as well as the local stresses in the fiber and matrix. Both models produced similar results and demonstrated that they could capture the complex local load transfer. A statistical study is also undertaken to understand the effect of gap clustering on the effective properties and local stress fields.

Microstructural evolution of highly aligned discontinuous fiber composites during longitudinal extension in forming

The longitudinal extensional viscosity of a highly aligned discontinuous fiber (ADF) thermoplastic matrix composite is investigated to develop a model and validate microstructural evolutionary mechanisms. Samples stretched at constant temperature and strain rate are shown to exhibit a strain softening behavior. X-ray CT analysis and optical micrographs show that the composite microstructure deconsolidates before forming and evolves with deformation. The conventional unit cell micromechanical model includes the effects of matrix viscosity, fiber aspect ratio and fiber volume fraction. This model is modified to include the stiffening effect of fiber spacing variability, and the softening effects of porosity and decreasing fiber overlap length with elongation. Calibration of the model reveals that matrix shear strain rate is an order of magnitude higher than previously predicted due to local fiber spacing. This effect is captured by a fiber spacing variability parameter which scales average spacing down by an order of magnitude. The observed strain softening behavior is described and a combinations of fiber overlap length reduction and local fiber spacing increase.

Methodology to establish a forming process window for thermoset aligned discontinuous fiber composites

Highly aligned discontinuous fiber composites have been pursued with the goal of achieving high performance properties with the formability of short fiber composites. The feedstock consists of a highly aligned discontinuous fiber preform which can be combined with thermoplastic or thermoset resins to create prepreg that can extend in the fiber direction. With this added benefit of lamina extensibility in the fiber direction, a methodology is needed to determine the degradation of material quality with applied forming strain. A framework is established to define a forming process window which sets the optimal process conditions—temperature and strain rate—for maintaining lamina uniformity after deformation. Digital image correlation was used to measure strain uniformity for samples stretched in uniaxial tension at constant temperature and strain rate. The experimental results show that forming strain rate should increase with increasing process temperature. The process window established in this work will be used to ensure that material uniformity can be optimized in manufacturing trials and other material characterization evaluations.

DcAFF (Discontinuous Aligned Fibre Filament) – Mechanical properties investigation on multilayer 3D printed parts

DcAFF (Discontinuous Aligned Fibre Filament) is a novel thermoplastic filament developed for 3D printing. This filament is reinforced with highly aligned discontinuous fibres and is based on the High Performance Discontinuous Fibre (HiPerDiF) method which produces thin tapes suitable for a range of different composite manufacturing processes. The HiPerDiF, using fibres longer than the critical length, provides mechanical performance comparable to continuous fibre composites with the high formability typical of short fibre composites. Thanks to the development of the third-generation HiPerDiF machine and the DcAFF filament forming method, circular DcAFF filaments can be produced consistently and at high rates. In this paper, both the physical properties and the internal architecture of the produced filament were investigated. In particular, μCT scanning and image post-processing were used to quantify fibre alignment. The designed filament-forming process ensures that the large fraction of the fibres in the final product are well aligned with the longitudinal axis of the filament. The mechanical properties of the multilayer DcAFF 3D printing part are presented for the first time in this paper with tensile, short beam shear (SBS), and open-hole tensile testing. The comparison with the previous studies and data in the literature shows comparable or indeed superior stiffness of DcAFF over existing methods for 3D printing composite parts; however, to be able to consider this material as a viable candidate for high-performance 3D printing further improvements are required in term of strength, e.g. increasing fibre-matrix adhesion or substituting the proof-of-concept PLA matrix with a high performance one.

Steering Potential for Printing Highly Aligned Discontinuous Fibre Composite Filament.

DcAFF (discontinuous aligned fibre filament) is a novel material for fused filament fabrication (FFF) 3D printing made of highly aligned discontinuous fibres produced using high performance discontinuous fibre (HiPerDiF) technology. It reinforces a thermoplastic matrix to provide high mechanical performance and formability. Accurate printing of DcAFF poses a challenge, especially for complex geometries, because: (i) there is a discrepancy between the path where the filament experiences the adhering pressure from the filleted nozzle and the nozzle path; and (ii) the rasters display poor adhesion to the build platform immediately after deposition, which causes the filament to be dragged when the printing direction changes. This paper explains the implication of these phenomena on steering capabilities and examines the techniques for improving DcAFF printing accuracy. In the first approach, the machine parameters were adjusted to improve the quality of the sharp turning angle without changing the desired path, but this showed insignificant effects in terms of precision improvements. In the second approach, a printing path modification with a compensation algorithm was introduced. The nature of the inaccuracy of the printing at the turning point was studied with a first-order lag relationship. Then the equation to describe the deposition raster inaccuracy was determined. A proportional-integral (PI) controller was added to the equation to calculate the nozzle movement in order to bring the raster back to the desired path. The applied compensation path is shown to give an accuracy improvement in curvilinear printing paths. This is particularly beneficial when printing larger circular diameter curvilinear printed parts. The developed printing approach can be applied with other fibre reinforced filaments to achieve complex geometries.

Developing aligned discontinuous flax fibre composites: Sustainable matrix selection and repair performance of vitrimers

Sustainability of fibre reinforced polymer composites has become vital for reaching the global sustainable development goals. Natural fibres, particularly flax, and bioderived matrices are possible sustainable solutions for the composites industry, due to the constituents’ embedded environmental impact reduction. According to the circular economy paradigm, sustainability can also be achieved by delaying the disposal of materials. This work reports the interfacial properties of flax fibres with three potentially sustainable advanced matrices, i.e., a vitrimer that combines the beneficial properties of both thermosets and thermoplastics, an entirely bio-based thermoset, and an advanced thermoplastic resin. Each of the selected matrices offers the potential for either recyclability, repairability, reusability, or the use of renewable sources and a reduction in the emissions of volatile organic compounds. Microbond tests were used to evaluate the interfacial shear strength and critical fibre length. It was found that the vitrimer and the bio-based thermoset matrices had a higher level of adhesion with flax fibres (∼20 and ∼24 MPa, respectively) compared to a traditional epoxy matrix (∼12 MPa); the advanced thermoplastic resin (∼6 MPa) shows the poorest adhesion. The vitrimer matrix was selected as a candidate for a sustainable and repairable discontinuous flax fibre reinforced composite. Mechanical and low-temperature rapid repair performance of an aligned discontinuous flax fibre composite, produced using the HiPerDiF method, were investigated. End-to-end and single patch repair methods were performed: vitrimer matrix composites show the potential for a mechanical strength recovery (∼%50-70) that would allow them to be reused over several life cycles, enabling a circular economy.

Batch production and fused filament fabrication of highly aligned discontinuous fibre thermoplastic filaments

Fused filament fabrication (FFF), which is a thermoplastic layer-by-layer additive manufacturing technique, called as 3D printing, can quickly build complex geometries, reducing design limitations and production costs of conventional manufacturing methods. Using a fibrous reinforcement strengthens the thermoplastic matrix, allowing FFF to be used in more structural complex-shape small-size applications in engineering. Aligned discontinuous fibre composites (ADFRC) incorporate a high-performance reinforcement architecture that, owing to a sufficient fibre length and high level of alignment, results in mechanical performance comparable to those of continuous fibre composites. Moreover, it can offer higher formability and, fewer manufacturing defects. In this paper, ADFRC preforms, produced with the High Performance Discontinuous Fibre (HiPerDiF) technology, were impregnated with poly(L-lactic acid) (PLA) and then reshaped to a circular-shaped 3D printing filament using a customised manual moulding and pultrusion method. Two different mould types were compared to maximise the filament quality and reduce the amount of trapped voids. The printability of the circular filaments was investigated by varying three parameters: deposition speed, processing temperature, and layer thickness. Finally, the tensile properties of samples from each manufacturing step: moulded, filament, single raster printed, and full height rectangular part, were investigated. Overall, the HiPerDiF-PLA filament can be printed using an 3D printer. Although the tensile properties of filament in each stage reduced as the result of the shape transformation process, the deposited layer of HiPerDiF-PLA part has greater stiffness and strength than other PLA composite materials. At this stage of technology development, the material has lower mechanical performance yet than the nylon continuous fibre composites printed with commercially available machines. However, there are clear routes forward for the improved performance of the HiPerDiF FFF printed filaments.

Gauge length and temperature influence on the tensile properties of stretch broken carbon fiber tows

Continuous carbon fibers are premium reinforcing material for aerospace composites. The challenge with continuous carbon fibers is their difficulty to form deep drawn parts requiring intricate manufacturing techniques that increase manufacturing time, cost and material waste. Stretch broken carbon fiber (SBCF), is a form of aligned discontinuous fiber, has been proposed as an alternative to overcome this challenge. SBCF provide flexibility to form complex shapes while maintaining comparable strength and stiffness. This study compared the tensile behavior of sized continuous fiber and SBCF tows at seven different gauge lengths and two temperatures (24 °C and 100 °C). Materials included Hexcel IM7-G continuous carbon fiber and Hexcel IM7-GP SBCF. The results of this study show that SBCF demonstrate decreased tensile loads when tested at elevated temperatures compared to continuous fibers. The maximum load capacity of SBCF tow increases as gauge length decreases. The load bearing capacity of heated SBCF tow is linked to fiber continuity. These findings suggest superior forming properties of SBCF as compared with continuous fiber tow.

Natural Fibres as a Sustainable Reinforcement Constituent in Aligned Discontinuous Polymer Composites Produced by the HiPerDiF Method.

Sustainable fibre reinforced polymer composites have drawn significant attention in many industrial sectors as a means for overcoming issues with end-of-life regulations and other environmental concerns. Plant based natural fibres are considered to be the most suitable reinforcement for sustainable composites since they are typically from renewable resources, are cheap, and are biodegradable. In this study, a number of plant based natural fibres-curaua, flax, and jute fibres-are used to reinforce epoxy, poly(lactic acid) (PLA), and polypropylene (PP) matrices to form aligned discontinuous natural fibre reinforced composites (ADNFRC). The novel HiPerDiF (high performance discontinuous fibre) method is used to produce high performance ADNFRC. The tensile mechanical, fracture, and physical (density, porosity, water absorption, and fibre volume fraction) properties of these composites are reported. In terms of stiffness, epoxy and PP ADNFRC exhibit similar properties, but epoxy ADNFRC shows increased strength compared to PP ADNFRC. It was found that PLA ADNFRC had the poorest mechanical performance of the composites tested, due principally to the limits of the polymer matrix. Moreover, curaua, flax (French origin), and jute fibres are found to be promising reinforcements owing to their mechanical performance in epoxy and PP ADNFRC. However, only flax fibre with desirable fibre length is considered to be the best reinforcement constituent for future sustainable ADNFRC studies in terms of mechanical performance and current availability on the market, particularly for the UK and EU.

High-Performance Structural Supercapacitors Based on Aligned Discontinuous Carbon Fiber Electrodes and Solid Polymer Electrolytes

This paper presents an investigation of the potential to use aligned discontinuous carbon fiber dry prepregs as electrodes in structural supercapacitors (SSCs). The high fiber-matrix interfacial bonding of the structural composite was achieved by adopting a solid polymer electrolyte, consisting of poly(vinylidene), lithium triflate, and epoxy. Processing of the SSC was carried out via dip-coating of the polymer electrolyte and then cured using a vacuum bag. The electrochemical performance of the SSCs was measured before and after mechanical loading. The microstructures of the SSCs as-fabricated and damaged under flexural loading were identified by μ-CT imaging. An SSC with a specific capacitance of 0.128 mF/cm2 (11.62 mF/g), a flexural strength of 47.49 MPa, and a flexural modulus of 8.48 GPa has been achieved, demonstrating significant improvements in mechanical properties over those of SSCs based on woven carbon fiber fabric-based electrodes. The mechanical behavior of the supercapacitors was evaluated by both quasi-static and cyclic flexural loading tests. The excellent electrochemical stability of the supercapacitors was validated by a capacitance retention of above 96% under galvanostatic charge-discharge cycling tests. The knowledge gained in this work will benefit future research in the optimization of SSC performance.

SPH Simulation for Short Fibre Recycling Using Water Jet Alignment

ABSTRACT This work presents a computational model for a discontinuous fibre composite manufacturing process. The alignment mechanism of this novel process, called the High Performance Discontinuous Fibre (HiPerDiF) method, involves highly coupled fluid–structure interactions. Fibres with a length on the order of a few millimetres are placed in a water suspension, sprayed between two parallel plates and deposited on a moving belt to make an aligned discontinuous fibre tape. This technology can be used as part of a composites recycling process to remanufacture reclaimed fibres into valuable recycled composite feedstock by ensuring a high level of alignment. This work aims to model the alignment mechanism using smoothed particle hydrodynamics in order to inform the design of the industrial machine. The results reveal the influence of jet angle and fibre length on the overall quality of fibre alignment.

Remanufacturing of Woven Carbon Fibre Fabric Production Waste into High Performance Aligned Discontinuous Fibre Composites

The composites industry generates considerable volumes of waste in a wide variety of forms, from the production of by-products to end-of-life parts. This paper focuses on the remanufacturing of dry fibre off-cuts, produced during the composite fabric weaving process, into highly aligned discontinuous fibre prepreg tapes with High Performance Discontinuous Fibre (HiPerDiF) technology. Unidirectional laminate specimens are prepared using various combinations of fibre lengths and tested in tension, obtaining a stiffness of 80 GPa, a strength of 800 MPa, and a failure strain of 1%. Several applications are envisaged for the produced tape: adhesive film, feedstock for filament winding, and tow for weaved fabrics. This work demonstrates the possibility to extract value from what is currently considered manufacturing waste.

Characterisation of Natural Fibres for Sustainable Discontinuous Fibre Composite Materials.

Growing environmental concerns and stringent waste-flow regulations make the development of sustainable composites a current industrial necessity. Natural fibre reinforcements are derived from renewable resources and are both cheap and biodegradable. When they are produced using eco-friendly, low hazard processes, then they can be considered as a sustainable source of fibrous reinforcement. Furthermore, their specific mechanical properties are comparable to commonly used, non-environmentally friendly glass-fibres. In this study, four types of abundant natural fibres (jute, kenaf, curaua, and flax) are investigated as naturally-derived constituents for high performance composites. Physical, thermal, and mechanical properties of the natural fibres are examined to evaluate their suitability as discontinuous reinforcements whilst also generating a database for material selection. Single fibre tensile and microbond tests were performed to obtain stiffness, strength, elongation, and interfacial shear strength of the fibres with an epoxy resin. Moreover, the critical fibre lengths of the natural fibres, which are important for defining the mechanical performances of discontinuous and short fibre composites, were calculated for the purpose of possible processing of highly aligned discontinuous fibres. This study is informative regarding the selection of the type and length of natural fibres for the subsequent production of discontinuous fibre composites.

Validation of a smoothed particle hydrodynamics model for a highly aligned discontinuous fibre composites manufacturing process

This work presents a computational model for a discontinuous fibre composite manufacturing process. The alignment mechanism of this novel process, called the High Performance Discontinuous Fibre (HiPerDiF) method, involves highly coupled fluid-structure interactions. Fibres with a length on the order of a few millimetres are placed in a water suspension, sprayed between two parallel plates and deposited on a moving belt to make an aligned discontinuous fibre tape. This technology can be used as part of a composites recycling process to remanufacture reclaimed fibres into valuable recycled composite feedstock by ensuring a high level of alignment. In order to industrialise this process, the throughput must be increased whilst maintaining the high level of alignment. This work aims to assist this development by modelling the alignment mechanism using smoothed particle hydrodynamics (SPH). It is shown through comparison to experiments that SPH models the process well and captures the influences affecting fibre alignment.

Improving Dispersion of Recycled Discontinuous Carbon Fibres to Increase Fibre Throughput in the HiPerDiF Process.

In order to increase the material throughput of aligned discontinuous fibre composites using technologies such as HiPerDiF, stability of the carbon fibres in an aqueous solution needs to be achieved. Subsequently, a range of surfactants, typically employed to disperse carbon-based materials, have been assessed to determine the most appropriate for use in this regard. The optimum stability of the discontinuous fibres was observed when using the anionic surfactant, sodium dodecylbenzene sulphonate, which was superior to a range of other non-ionic and anionic surfactants, and single-fibre fragmentation demonstrated that the employment of sodium dodecylbenzene sulphonate did not affect the interfacial adhesion between fibres. Rheometry was used to complement the study, to understand the potential mechanisms of the improved stability of discontinuous fibres in aqueous suspension, and it led to the understanding that the increased viscosity was a significant factor. For the shear rates employed, fibre deformation was neither expected nor observed.

A closed-loop recycling process for discontinuous carbon fibre polyamide 6 composites

The effects of a closed-loop recycling methodology are evaluated for degradation using a discontinuous carbon fibre polyamide 6 (CFPA6) composite material. The process comprises two fundamental steps: reclamation and remanufacture. The material properties are analysed over two recycling loops, and CFPA6 specimens show a total decrease of 39.7% (±3.5) in tensile stiffness and 40.4% (±6.1) in tensile strength. The results of polymer characterisation and fibre analysis suggested that the stiffness reduction was likely due to fibre misalignments primarily caused by fibre agglomerations, as a result of incomplete fibre separation, and by fibre breakages from high compaction pressures. The ultimate tensile strain was statistically invariable as a function of recycling loop which indicated minimal variation in polymer structure as a function of recycling loop. To the authors’ best knowledge, the mechanical performance of the virgin CFPA6 is the highest observed for any aligned discontinuous carbon fibre thermoplastic composites in the literature. This is also true for recycled specimens, which are the highest observed for any recycled thermoplastic composite, and, for any recycled discontinuous carbon fibre composite with either thermosetting or thermoplastic matrices.

Hybrid composites of aligned discontinuous carbon fibers and self-reinforced polypropylene under tensile loading

Highly aligned discontinuous fiber composites have demonstrated mechanical properties comparable to those of unidirectional continuous fiber composites. However, their ductility is still limited by the intrinsic brittleness of the fibers and stress concentrations at the fiber ends. Hybridization of aligned discontinuous carbon fibers (ADCF) with self-reinforced polypropylene (SRPP) is a promising strategy to achieve a balanced performance in terms of stiffness, provided by the ADCF, and ductility, delivered by SRPP. The current work focuses on interlayer hybridization of these materials and their tensile behavior as a function of different material parameters. Effects of the carbon layer thickness, carbon/SRPP layer thickness ratio, layer dispersion and interface adhesion are investigated. The carbon fiber misalignment is characterized using X-ray computed tomography to predict the modulus of the aligned discontinuous carbon fiber layer. The hybrids exhibit a gradual tensile failure with high pseudo-ductile strain of above 10% facilitated by multiple carbon layer failures (layer fragmentation) and dispersed delaminations. At the microscopic scale, the carbon layer fails mainly through interfacial debonding and fiber pull-out.

Reclaimed Carbon and Flax Fibre Composites: Manufacturing and Mechanical Properties

The feasibility of using the HiPerDiF (high performance discontinuous fibre) method to manufacture highly aligned discontinuous fibres intermingled hybrid composites with flax and reclaimed carbon fibres (rCF), and the potential benefits of so doing, are investigated in this paper. It is demonstrated that, despite their hydrophilic nature, flax fibres are not affected by this water-based process. Intermingled flax/rCF hybrid composites are characterised in terms of their tensile and vibrational response. It is concluded that natural/rCF fibre hybrid composites can be a viable solution for those applications where a reduction in primary mechanical properties, e.g., stiffness and strength, is an acceptable trade-off for the enhancement of secondary properties, e.g., noise, vibration, and harshness (NVH) mitigation, and the reduction of monetary costs.