T.A. Turner

Effect of resin properties and processing parameters on crash energy absorbing composite structures made by RTM

The effects of resin properties and resin processing parameters on the crush behaviour of thermoset composite tubes manufactured using resin transfer moulding are considered. The aims were to quantify the performance of tubes produced over a broad spectrum of conditions and to correlate this performance to the properties of the material. The effects of the mould temperature, post-cure time and resin composition were investigated for random and engineered reinforcement fabrics. Relationships between Youngs moduli and ultimate stresses of the material in tension and compression were established from plaques moulded under similar conditions. Random reinforcement fabrics gave higher specific energy absorption (SEA) levels than engineered fabrics, but were more sensitive to processing conditions. Epoxy absorbed more energy than vinyl ester. Vinyl ester absorbed more than polyester, and additions of vinyl ester resin to unsaturated polyester gave a linear increase in SEA. The ultimate compressive strength of the composite proved the best indicator of performance for the selected materials and processing conditions.

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.

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.

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.

Development of high value moulding compounds from recycled carbon fibres

Abstract A route for the recovery and reuse of carbon fibres is presented with a summary of technological advances and areas requiring further development. Critical issues in size reduction of recyclate are presented along with results from a study considering comminution of a variety of time expired prepregs. High quality recovered carbon fibres have been incorporated in moulding compounds and preimpregnated composite materials. Fibre alignment is shown to be a critical factor in attaining high mechanical properties and high recovered fibre utilisation. A number of demonstrator materials have been developed and used to manufacture automotive parts which have shown excellent mechanical properties when compared with commercial glass fibre based moulding compounds.

Effect of resin formulation on crash energy absorbing composite structures made by RTM

Abstract Crush experiments have been performed on polyester and vinyl ester composite tubes. The preforms were made of random glass mat and the tubes were produced by resin transfer moulding.The effects ofdifferent processing parameters were investigated. Flat plaques were also produced under similar conditions in order to measure in plane properties of the composite material. The two main objectives of the study were to quantify the effect of industrial manufacturing conditions on the crush performance of composite structures and to correlate the performance to a number of in plane laminate properties. The manufacturing parameters considered are resin related:the mould temperature, post-cure time, and resin composition were varied according to a full factorial experimental plan. In addition to crush experiments, the tensile and compressive moduli and ultimate stresses were determined; the degree of conversion was also measured. The results demonstrate that while relationships between all in plane properties and the crush performance can be observed, the ultimate compressive stress is the most reliable indicator of this performance. The results also show clear advantages associated with the vinyl ester resin, and the many intricacies pertaining to the modelling of the effect of processing parameters on crush performance.

Applications for carbon fibre recovered from composites

Commercial operations to recover carbon fibre from waste composites are now developing and as more recovered fibre becomes available new applications for recovered fibre are required. Opportunities to use recovered carbon fibre as a structural reinforcement are considered involving the use of wet lay processes to produce nonwoven mats. Mats with random in-plane fibre orientation can readily be produced using existing commercial processes. However, the fibre volume fraction, and hence the mechanical properties that can be achieved, result in composites with limited mechanical properties. Fibre volume fractions of 40% can be achieved with high moulding pressures of over 100 bar, however, moulding at these pressures results in substantial fibre breakage which reduces the mean fibre length and the properties of the composite manufactured. Nonwoven mats made from aligned, short carbon fibres can achieve higher fibre volume fractions with lower fibre breakage even at high moulding pressure. A process for aligning short fibres is described and a composite of over 60% fibre volume fraction has been manufactured at a pressures up to 100 bar with low fibre breakage. Further developments of the alignment process have been undertaken and a composite of 46% fibre volume fraction has been produced moulded at a pressure of 7 bar in an autoclave, exhibiting good mechanical properties that compete with higher grade materials. This demonstrates the potential for high value applications for recovered carbon fibre by fibre alignment.

Environmental Aspects of Use of Recycled Carbon Fiber Composites in Automotive Applications

The high cost and energy intensity of virgin carbon fiber manufacture provides an opportunity to recover substantial value from carbon fiber reinforced plastic wastes. In this study, we assess the life cycle environmental implications of recovering carbon fiber and producing composite materials as substitutes for conventional and proposed lightweight materials in automotive applications (e.g., steel, aluminum, virgin carbon fiber). Key parameters for the recycled carbon fiber materials, including fiber volume fraction and fiber alignment, are investigated to identify beneficial uses of recycled carbon fiber in the automotive sector. Recycled carbon fiber components can achieve the lowest life cycle environmental impacts of all materials considered, although the actual impact is highly dependent on the design criteria (λ value) of the specific component. Low production impacts associated with recycled carbon fiber components are observed relative to lightweight competitor materials (e.g., aluminum, virgin carbon fiber reinforced plastic). In addition, recycled carbon fiber components have low in-use energy use due to mass reductions and associated reduction in mass-induced fuel consumption. The results demonstrate environmental feasibility of the CFRP recycling materials, supporting the emerging commercialization of CF recycling technologies and identifying significant potential market opportunities in the automotive sector.

Cold-Cure Adhesives for Use in Structural Aluminium Bonding

Some vehicle manufacturers rely solely on adhesives as their primary form of joining, moving away from more conventional methods such as mechanical fasteners and welding. This joining technique has proven to be very effective, resulting in stiff, lightweight structures that are cost affective for low-volume production (<10,000 units per year). The adhesives used are usually one-component (1 K) epoxy-based adhesives that require a high-temperature cure (∼190°C) in order for them to reach their most favourable performance. This requires large ovens that not only limit the material selection of the bonded parts but also results in high energy bills. The introduction of a cold-cure adhesive (cure of <70°C) would eliminate the need for the costly cure-ovens, resulting in a direct cost saving from energy consumption, and reduced thermal distortion in the curing under-body, while also allowing for a wider range of materials to be cured together in each assembly. With the introduction of a cold-cure adhesive, there are questions of how a cold-cure adhesive could be used as a replacement to the current hot-cure adhesive, what design alterations would need to change or could now be changed, and how would this adhesive impact the final vehicle performance. This paper investigates some of the issues that arose from selecting a cold-cure, two-component (2 K) adhesive for use in the automotive industry as a primary structural joining technique. Testing was performed in lap-shear and T-peel, on three adhesives chosen from screening tests performed prior to project. Emphasis is made on the bonding of direct-current sulphuric-acid-anodised (DC SAA) aluminium, with comparison with alternative coatings and pre-treatments.

The shear viscosity of carbon fibre suspension and its application for fibre length measurement

The viscosity of short carbon fibre suspensions in glycerol aqueous solution was measured using a bespoke vane-in-cup viscometer, where the carbon fibre has an aspect ratio from 450 to 2209. In the semi-concentrated regime, nL3 ranging from 20 to 4400, the suspensions demonstrated strong shear-thinning characteristics particularly at higher concentrations. The shear-thinning characteristic is strongly related to the crowding factor proposed by Kerekes, indicating that non-hydrodynamic interactions occur in the suspensions. The influence of fibre bending on viscosity emerges when the bending ratio is lower than 0.0028. An empirical model based on transient network formation and rupture was proposed and used to correlate the relative viscosity with fibre concentration nL3 and shear rate. Based on the model, a viscosity method is established to analyse the fibre length by measuring the viscosity of the fibre suspension using a bespoke vane-in-cup viscometer.