JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010 - 52

KNOW-HOW Design & Manufacturing g…/… Advanced composite racing yacht construction component and thus reduce the volume of air to be heated. It is usually possible to achieve post-curing temperatures of at least 60-70°C in this manner. Styrene acrylonitrile (SAN) core materials For almost a decade now, SAN became the thermoplastic foam core material of choice when used in conjunction with prepregs. This type of foam has not ceased to improve and there is currently a range of grades to cover most applications. We have used thermoforming extensively with SAN core materials with excellent re¬sults. Material and process choices Prepreg carbon vs. dry fibre (infusion and wet-preg) Therefore, in today's advanced marine structures, the choice of materials comes down to low-temperature prepregs with the use of dry fibre being limited to specialised infusion processes as mentioned before, and in secondary bonding as a wet-preg or wet lay-up process. We see a declining use of glass and aramid fibres in such structures with the recent ACCs, VOR70s, TP-52s, Open 60s, etc., being mostly built with prepreg carbon fibres and using honeycomb and foam cores of different configurations and weights. We also see an increase in the popularity of carbon/epoxy advanced infusion using foam cores. Infusion was initially a process made popular for health and safety reasons as a substitute for open-mould GRP production methods, but it is now being looked at in a new light and may be a successful competitor of prepreg technology derivatives in series production projects. Honeycomb: Al, Nomex®, Kevlar® Honeycomb core materials have been used for many years in advanced structures. Aluminium honeycomb does have excellent mechanical properties but is prone to corrosion and cannot really be considered for structures that need to last. Phosphoric acid anodised (PAA) and epoxy-dipped AL honeycomb is available, which resists corrosion better but at a premium price. Nomex® paper honeycomb is by far the most popular material of this type, although we have seen a higher grade of aramid (Kevlar®) honeycomb used in some ACC yachts and VOR70s with good results in spite of the very difficult processing of this material in anything but flat surfaces. Release agents Carbon fibres The most common grades of carbon fibre used in advanced marine structures fall in the following range: - High-strength (HS) < 265 GPa modulus fibres for hull and deck shell skins and most internal structures, - Intermediate-modulus (IM) 265-320 GPa modulus fibres for spars and for specific stiffening of structures where allowed, - High-modulus (HM) 320-440 GPa modulus fibres use is limited to spars and very stiff structures like the forward cross arms of certain catamarans. Commercial high-strength fibres with a modulus of 240 GPa or lower have become a popular material in the last few years and have seen applications in less demanding marine structures mostly for economic reasons and the shortfall in world carbon fibre production, which has made the typical advanced grades more expensive and difficult to obtain. Release agents and their application are still an area where the most diverse opinions abound. This goes to show that if you dominate the application of a particular system and it works, stick with it. Trimarine’s experience is with a popular proprietary semipermanent release coating specifically formulated for use with elevated temperature cured composites. Surface preparation and sealing is of critical importance. As this class of release agents is based on polymer technology, intermediate elevated temperature cures of the sealer and the release agent are recommended so as to properly form the polymer release film. The ideal number of release agent coats to be applied should be tested to reach a good level of component final release, without risking premature pre-release in the intermediate phases of the process. Good release agents do not transfer to the component surface or at least should not affect the component surface finish application. Weave and UD styles/weights The majority of prepreg carbon production used in marine structures is supplied as unidirectional collimated non-woven 100 to 500 mm wide tapes with surface dry fibre weights of 150 to 400 g/m2. Lower areal weight material is available at a higher cost as it requires thinner tows for uniform processing. Woven materials are usually in the range of 200 to 300 g/m2 and up to 600 g/m2 or more for tooling purposes. Biaxial and multiaxial stitched fabrics are available in the range of 400 g/m2 to 800 g/m2. Here again, the lower weights are used for secondary bonding or for laminating small components with intricate shapes. The higher weight materials are used for tooling or to bulk up solid area laminates. Vacuum ancillaries and disposables Most of these materials are common to aerospace composite practice and their selection depends on the process temperature. Lower-grade materials are used in room-temperature procedures like debulking prepregs and wet-preg RT-cured vacuumed composites. Such materials, being more economical, are also ideal for infusion processes although we have discovered that some lower-grade vacuum bagging films can be porous, forcing the operator to use a double bag. jec composites magazine / No55 February - March 2010

JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010