Pre-preg | EPFL Graph Search

Pre-preg is a composite material made from "pre-impregnated" fibers and a partially cured polymer matrix, such as epoxy or phenolic resin, or even thermoplastic mixed with liquid rubbers or resins. The fibers often take the form of a weave and the matrix is used to bond them together and to other components during manufacture. The thermoset matrix is only partially cured to allow easy handling; this B-Stage material requires cold storage to prevent complete curing. B-Stage pre-preg is always stored in cooled areas since heat accelerates complete polymerization. Hence, composite structures built of pre-pregs will mostly require an oven or autoclave to cure. The main idea behind a pre-preg material is the use of anisotropic mechanical properties along the fibers, while the polymer matrix provides filling properties, keeping the fibers in a single system. Pre-preg allows one to impregnate the fibers on a flat workable surface, or rather in an industrial process, and then later form the

Optimisation of low pressure processing for honeycomb sandwich structures

Sandra Sequeira Lopes Tavares

Vacuum-bag processing of honeycomb sandwich structures faces particular challenges compared to autoclave sandwich fabrication for aeronautics, a consequence of using, at the most, atmospheric pressure for their manufacturing. In particular, the adhesion of the skin to the honeycomb core is reduced and the skins tend to have higher values of porosity. On the other hand, autoclave processing is expensive, in particular for large structures, and a growing interest has been observed for vacuum-bag only processing of sandwich structures. During honeycomb sandwich cure, the skin represents a barrier to air flow between the honeycomb and the vacuum-bag so the air transport through the prepreg, adhesive and consumables or other additional materials may be a crucial factor to increase skin debulking, reduce void content and extract air from the honeycomb cells. The pressure inside the honeycomb is, therefore, a result of the through thickness permeability to air of the skins and vacuum pump work. The goal of this thesis is to develop methods for improving the low pressure processing of honeycomb sandwich structures by quantifying the through thickness air permeability of fibre reinforced composites during cure and analysing related processing issues and governing phenomena. An experimental method based on a falling pressure measurement was proposed to evaluate the through thickness air permeability of prepregs during cure, with two set-up variants. One concerns the measurement of the inherent air permeability and the other applies to processing conditions, implying the use of both consumables and vacuum-bag. The methods were successfully tested and fulfil the criterions used in soil science for the evaluation of the air permeability. Prepreg and adhesive permeabilities were determined separately and in skin combination. Prepreg permeability to air during cure varies between 10-19 m2 and 10-17 m2 and was found to depend on the evolution of the resin viscosity, which shifts from an initially immobile phase to a mobile one and then back to an immobile phase. The adhesive film was found to have very low initial permeability, at least two orders of magnitude lower than that of the prepreg. A range of initial skin through thickness air permeability could be achieved by perforating the prepreg plies, the adhesive layer, or a combination of both. An adhesive deposition method which results in the centre of honeycomb cells being free of adhesive was also tested. A corresponding range of achievable pressure was measured inside the honeycomb. The evolution of the through thickness permeability with the curing cycle was determined for each case. The adhesive layer was identified as the element that reduces the initial through thickness air permeability the most in skin manufacturing. This suggests that one of the ways of increasing the initial through thickness air permeability of the skin is to modify the permeability of the adhesive layer. The role of the resulting pressure inside the honeycomb on skin-core adhesion and skin quality was then evaluated. Skin permeability was found to control skin-core adhesion through the pressure drop in the honeycomb cells and potential outgassing of the adhesive layer. An optimal range of pressure inside the honeycomb was found to be between 40 kPa and 70 kPa. A process window is proposed for achieving the optimal pressure inside the honeycomb, as a function of the skin permeability and time of vacuum application. A semipreg alternating dry and resin impregnated areas along the fibre bed surface was also investigated as an alternative to prepreg in skin manufacturing. The semipreg through thickness air permeability before cure is approximately three orders of magnitude higher than that of a unidirectional prepreg impregnated with the same resin, thus allowing the reduction of air pressure in the honeycomb. A model was proposed for the air permeability change during cure, as dry areas get infiltrated. Due to resin pouring inside the honeycomb cells, this type of semipreg is viable as a skin only if combined with a material that has low permeability to resin, e.g., a prepreg. The resulting initial pressure in the honeycomb cells might then be tailored to specific needs. Finally, it was possible to produce sandwich samples combining a consolidation system, such as a prepreg, with an infiltration system, as wet lay-up or with a semipreg. The combination of a prepreg with a semipreg revealed to be a very practical and clean system, where it is possible to have the same resin in both materials. Moreover, it allows to tailor the initial honeycomb pressure as well as during cure, and to have the initial permeability assessed.

EPFL2009

Economic and environmental assessment of alternative production methods for composite aircraft components

Fabrice Gaille, Jan-Anders Månson, Véronique Michaud, Rémy Teuscher, Robert Andrew Witik

The use of carbon fibre reinforced plastics is steadily increasing in the aerospace industry as rising fuel costs and concerns over the environment push airframe manufacturers to improve aircraft efficiency. The high costs associated with manufacturing carbon fibre reinforced components in autoclaves have prompted interest in alternative out-of-autoclave processing methods. In this study a combined cost modelling and life-cycle assessment approach is applied to selected out-of-autoclave production scenarios. Out-of-autoclave specific "prepregs" and resin infused fabrics are cured in thermal and microwave ovens of comparable volume and assessed against a benchmark autoclave scenario. Results showed that materials, in particular carbon fibres, contributed most significantly to component cost and environmental impacts. Resin infusion processes were effective at reducing costs, as reinforcement fabrics and resin were less expensive. Due to the small contribution of energy to total cost, reductions in energy use did not lead to significant savings, although they did improve the environmental performance of the manufacturing process. Out-of-autoclave specific prepregs did not perform as well due to their higher costs, longer associated cycle times and the need for lengthy de-bulking operations. Microwave oven curing offered little in terms of cost reduction and environmental improvement as investment costs were comparable to those of an autoclave, and energy consumption was relatively high compared with traditional thermal oven use. Opportunities for improvement exist if investment costs can be reduced and additional work carried out to promote more efficient transfer of energy. Improvement of the carbon fibre production process would be the most effective approach for reducing impacts and costs from carbon fibre components. (C) 2012 Elsevier Ltd. All rights reserved.

Elsevier2012

Hybrid processing of thick skins for honeycomb sandwich structures

Jan-Anders Månson, Véronique Michaud

Carbon-epoxy prepregs are generally used to form the skins of honeycomb sandwich structures used in aerospace or racing yachts. For some applications, it is desirable to increase the thickness of the skins. In order to achieve an ideal core pressure level during cure for maximal skin-core bonding, the issue of air extraction from the honeycomb cells through the skin during processing thus becomes critical, in particular if vacuum only processing is used. In the present work, partially impregnated prepregs, called semipregs, having high initial transverse permeability to air, are combined with traditional prepregs to form a hybrid skin. Results are presented on the pressure change inside the honeycomb cells and the skin permeability to air during cure, as well as on skin-core adhesion. The final sandwich quality is assessed and found to be comparable to that obtained with prepreg skins. © 2010 Elsevier Ltd.

Elsevier2011