L.T. Harper

Low-cost carbon-fibre-based automotive body panel systems : a performance and manufacturing cost comparison

Carbon—;fibre—;based composite manufacturing processes have been considered for automotive body panel applications. A full—;scale front wing—fender component was produced using two composite manufacturing processes (a semi—;impregnated (semi—;preg) system and a novel directed fibre preforming—resin transfer moulding process) and compared with an existing stamped steel component for mechanical properties, weight saving, and cost, using a technical—;cost—;modelling procedure. Mechanical testing demonstrates that the carbon fibre composite solutions can provide 40—50 per cent weight saving for an equivalent bending stiffness to steel panels and greatly improved dent resistance. For the part studied, carbon fibre semi—;preg systems offered the lowest cost process up to around 500 parts/annum and directed fibre preforming technologies were cheaper between 500 and 9000 parts/annum. The steel component was seen to be more cost effective at volumes above around 9000 parts/annum.

Net shape spray deposition for compression moulding of discontinuous fibre composites for high performance applications

Abstract Details are presented of a novel carbon/epoxy spray deposition process for producing high performance, net shape charges for low flow compression moulding. The Bentley–Raycell automated carbon composite charge deposition (BRAC3D) process sprays powdered epoxy and chopped carbon bundles onto three-dimensional (3D) tools, offering a fully automated process with no touch labour. It has been demonstrated that fibre volume fractions of up to 54% are achievable for random discontinuous fibre architectures, with low void content (1·6%). This extends the volume fraction range currently offered by liquid moulding/preforming processes and potentially reduces part scrap rate, since the resin flow direction is through thickness rather than in-plane. Results from an experimental programme are presented, which aims to benchmark the BRAC3D material against commercial advanced moulding compounds. Ultimate tensile strength, tensile modulus and Charpy impact values are reported to be 272 MPa, 44·4 GPa and 128 kJ m–2 respectively, for the random fibre architecture at a fibre volume fraction of 54%. This equates to a 99% stiffness retention and a 59% strength retention compared to a continuous fibre, quasi-isotropic counterpart. Observed trends for increasing fibre volume fraction and fibre length have been compared against finite element predictions and an analytical inclusion model. Simulations from a parametric cost model indicate that the BRAC3D process is cost effective for production volumes exceeding 1100 ppa for a structural demonstrator component, compared with prepreg and resin transfer moulding.

Fiber Alignment in Directed Carbon Fiber Preforms — A Feasibility Study:

The directed carbon fiber preforming (DCFP) process has the potential to create discontinuous fiber architectures with significant levels of fiber alignment. This article investigates the achievable levels of alignment and identifies the compromises inherent in the production of aligned preforms. The tensile properties of laminates produced from 6 and 24 K filament counts are compared at three different fiber lengths (28, 58, and 115mm). Experimental characterization indicates that up to 94% of fiber bundles can be aligned within ±10° using the current DCFP alignment method. Consequently, tensile stiffness and strength can be increased by 206 and 234%, respectively, over the random fiber case, as a result of the high concentration of aligned fibers in the loading direction. These alignment properties equate to maximum stiffness and strength retention values of 83 and 31% compared to continuous unidirectional material.

Influence of screw holes and gamma sterilization on properties of phosphate glass fiber-reinforced composite bone plates

Polymers prepared from polylactic acid (PLA) have found a multitude of uses as medical devices. For a material that degrades, the main advantage is that an implant would not necessitate a second surgical event for removal. In this study, fibers produced from a quaternary phosphate-based glass (PBG) in the system 50P2O5-40CaO-5Na2O-5Fe2O3 were used to reinforce PLA polymer. The purpose of this study was to assess the effect of screw holes in a range of PBG-reinforced PLA composites with varying fiber layup and volume fraction. The flexural properties obtained showed that the strength and modulus values increased with increasing fiber volume fraction; from 96 MPa to 320 MPa for strength and between 4 GPa and 24 GPa for modulus. Furthermore, utilizing a larger number of thinner unidirectional (UD) fiber prepreg layers provided a significant increase in mechanical properties, which was attributed to enhanced wet out and thus better fiber dispersion during production. The effect of gamma sterilization via flexural tests showed no statistically significant difference between the sterilized and nonsterilized samples, with the exception of the modulus values for samples with screw holes. Degradation profiles revealed that samples with screw holes degraded faster than those without screw holes due to an increased surface area for the plates with screw holes in PBS up to 30 days. Scanning electron microscope (SEM) analysis revealed fiber pullout before and after degradation. Compared with various fiber impregnation samples, with 25% volume fraction, 8 thinner unidirectional prepreg stacked samples had the shortest fiber pull-out lengths in comparison to the other samples investigated.

Finite element modelling of the flexural performance of resorbable phosphate glass fibre reinforced PLA composite bone plates.

A finite element method is presented to predict the flexural properties of resorbable phosphate glass fibre reinforced PLA composite bone plates. A novel method for meshing discontinuous fibre architectures is presented, which removes many of the limitations imposed by conventional finite element approaches. The model is used to understand the effects of increasing the span-to-thickness ratio for different fibre architectures used for PBG/PLA composites. A span-to-thickness ratio of 16:1 is found to be appropriate for materials with randomly orientated fibres, which agrees well with the test standard. However, for highly aligned materials the model indicates that a span-to-thickness ratio of 80:1 is required, in order to minimise the effects of shear deflection. The model is validated against flexural stiffness data from the literature for a range of polymers, fibres and fibre volume fractions. Generally there is less than 10% error between the FE predictions and experimental values. The model is subsequently used to perform a parametric study to understand what material developments are required to match the properties of PGF/PLA composites to cortical bone. It is concluded that alignment of the fibre is necessary to exceed the 20 GPa target, since the current manufacturing methods limit the fibre length to ∼10 mm, which consequently restricts the flexural modulus to ∼19 GPa (at 50% volume fraction).

Fiber Alignment in Directed Carbon Fiber Preforms - Mechanical Property Prediction

A finite element method is presented for predicting the mechanical performance of discontinuous fiber mesostructures typically produced by directed carbon fiber preforming. High-filament count bundles are modeled using beam elements to enable large representative volume elements to be studied. The beams are attached to a regular grid of 2D continuum elements, which represent the matrix material, using an embedded element technique. The model is validated by comparing simulations with experimental data for random and aligned fiber architectures produced with different tow sizes (6 and 24 K) and fiber lengths (28, 58, and 115 mm). Stiffness and strength predictions are generally within 10% for 6 K preforms, but this error increases up to 40% with increasing tow size because of the assumption that the fiber bundles are circular in cross-section.

Magnesium Coated Bioresorbable Phosphate Glass Fibres: Investigation of the Interface between Fibre and Polyester Matrices

Bioresorbable phosphate glass fibre reinforced polyester composites have been investigated as replacement for some traditional metallic orthopaedic implants, such as bone fracture fixation plates. However, composites tested revealed loss of the interfacial integrity after immersion within aqueous media which resulted in rapid loss of mechanical properties. Physical modification of fibres to change fibre surface morphology has been shown to be an effective method to improve fibre and matrix adhesion in composites. In this study, biodegradable magnesium which would gradually degrade to Mg2+ in the human body was deposited via magnetron sputtering onto bioresorbable phosphate glass fibres to obtain roughened fibre surfaces. Fibre surface morphology after coating was observed using scanning electron microscope (SEM). The roughness profile and crystalline texture of the coatings were determined via atomic force microscope (AFM) and X-ray diffraction (XRD) analysis, respectively. The roughness of the coatings was seen to increase from 40 ± 1 nm to 80 ± 1 nm. The mechanical properties (tensile strength and modulus) of fibre with coatings decreased with increased magnesium coating thickness.

Mechanical, degradation and cytocompatibility properties of magnesium coated phosphate glass fibre reinforced polycaprolactone composites

Retention of mechanical properties of phosphate glass fibre reinforced degradable polyesters such as polycaprolactone and polylactic acid in aqueous media has been shown to be strongly influenced by the integrity of the fibre/polymer interface. A previous study utilising ‘single fibre’ fragmentation tests found that coating with magnesium improved the fibre and matrix interfacial shear strength. Therefore, the aim of this study was to investigate the effects of a magnesium coating on the manufacture and characterisation of a random chopped fibre reinforced polycaprolactone composite. Short chopped strand non-woven phosphate glass fibre mats were sputter coated with degradable magnesium to manufacture phosphate glass fibre/polycaprolactone composites. The degradation behaviour (water uptake, mass loss and pH change of the media) of these polycaprolactone composites as well as of pure polycaprolactone was investigated in phosphate buffered saline. The Mg coated fibre reinforced composites revealed less water uptake and mass loss during degradation compared to the non-coated composites. The cations released were also explored and a lower ion release profile for all three cations investigated (namely Na+, Mg2+ and Ca2+) was seen for the Mg coated composite samples. An increase of 17% in tensile strength and 47% in tensile modulus was obtained for the Mg coated composite samples. Both flexural and tensile properties were investigated and a higher retention of mechanical properties was obtained for the Mg coated fibre reinforced composite samples up to 10 days immersion in PBS. Cytocompatibility study showed both composite samples (coated and non-coated) had good cytocompatibility with human osteosarcoma cell line.

3D geometric modelling of discontinuous fibre composites using a force-directed algorithm:

A geometrical modelling scheme is presented to produce representative architectures for discontinuous fibre composites, enabling downstream modelling of mechanical properties. The model generates realistic random fibre architectures containing high filament count bundles (>3k) and high (∼50%) fibre volume fractions. Fibre bundles are modelled as thin shells using a multidimensional modelling strategy, in which fibre bundles are distributed and compacted to simulate pressure being applied from a matched mould tool. Finite element simulations are performed to benchmark the in-plane mechanical properties obtained from the numerical model against experimental data, with a detailed study presented to evaluate the tensile properties at various fibre volume fractions and specimen thicknesses. Tensile modulus predictions are in close agreement (less than 5% error) with experimental data at volume fractions below 45%. Ultimate tensile strength predictions are within 4.2% of the experimental data at volume fractions between 40 and 55%. This is a significant improvement over existing 2D modelling approaches, as the current model offers increased levels of fidelity, capturing dominant failure mechanisms and the influence of out-of-plane fibres.

Three-dimensional numerical modelling of discontinuous fibre composite architectures

Abstract A three-dimensional geometrical model is presented for generating discontinuous random fibre architectures consisting of high filament count bundles. The fibre network model randomly distributes fibre bundles in a three-dimensional volume using a non-contact algorithm, together with Catmull–Rom spline interpolation, to provide a physically representative material. Only the spines of the fibre bundles are modelled, using truss elements to permit high fibre volume fractions of up to 60%, with no restriction on the fibre bundle aspect ratio. ABAQUS/Standard is used to predict the tensile performance for coupons with varying levels of out-of-plane fibre curvature. The effect of fibre curvature was found to be insignificant for tensile modulus, but a 34% reduction in ultimate tensile strength (UTS) was observed when adjusting the maximum permissible out-of-plane angle of fibres from 1 to 35°. A novel method for characterising the degree of out-of-plane fibre curvature within experimental test coupons is also discussed.