A GENERAL LOOK AT COMPOSITE CONSTRUCTION
The use of fibre reinforced plastic or composites in the construction of Fireball hulls is certainly on the increase at present, with a lot of discussion about the advantages and disadvantages
taking place. Having been involved with aircraft composite structures, I thought it might be worth explaining about some of the basic concepts and problems associated with this ever expanding
technology. At this stage i must point out that the word "composite" is used for structures made from FRP with or without a sandwich, whereas in dinghy building we think of a composite hull as
one of FRP with wood decks.
When I was first shown around the composite facility at Westland Aerostructures I could not understand why the operators did not apply the resin by hand. Back in the days of Reliant I went on a factory visit and watched car bodies being produced using "wet lay up" or what some people call "bucket and brush". It was very quickly pointed out to me that the aircraft industry does not manufacture composite structures using that method - "pre-pregs" are used. I obviously had a lot to learn ! Other new words were also being used - reinforcements, resin system, filler materials, lay-up, consolidation, mould tools, autoclave and many more.
There are three main types of fibre reinforcement used, all can be produced as woven cloth of various weaves or as single filaments (unidirectional). They are also mixed together within a reinforcement to compliment each other.
All of the above mentioned fibres are of little use without a resin system to bond them together. there are many new resins being developed which can be used at higher temperatures but for general use polyester and epoxy are the most popular.
This is the most commonly used for everyday applications and is not used with pre-pregs or for high tech applications. They are cheap, easy to use and not so toxic as epoxy in the un-cured state. they become unstable with hot/wet cycling (don't leave your boat out in the sun !)
Mostly used in more demanding conditions, requires some calorific imput even at room remperature. Has superior structural properties and will withstand higher temperatures. Has good adhesion and moisture resistance. Can also be weakened by hot/wet cycling. With all resin systems care must be taken when using un-cured resin and hardeners due to toxicity.
There are basically two most commonly used types - foams and honeycomb. Foams are easily cut to shape or machined into complex forms but will not bend easily and are better for flat surfaces. Honeycomb on the other hand is very easy to bend but difficult to cut to shape. Due to the structure of honeycomb, the voids are liable to fill up with water in the event of skin damage or porosity. This could also ba a problem if the wrong type of foam is used.
One of the most important requirements of a composite structure is to achieve consistency so that duplicate components will have the same characteristics. Particularly in aircraft manufacture where safety is of prime importance. Pre-pregnated laminates are used, the resin being held within the cloth and is activated at elevated temperature. This will give a much better control over the resin content. They are a lot easier to handle. Being tacky, a pre-preg is less trouble to lay-up and will stay where it's put. Low temperature epoxy pre-pregs are baked at about 80 degrees C and high temperature at typically up to about 190c. New generation, lower temperature, pre-pregs are being developed which will have an obvious advantage to boat building. Unfortunately pre-pregs need to be stored in fridges and have a limited life, when at room temperature, ovens or autoclaves are requires for the baking process.
To achieve the desired physical properties the laminates should be laid up in a pre-determined pattern which must be repeated every time a new component is made. Essentially the pattern should have the least number of joints between laminations and not laid at random as tends to be the case with boat manufacture. To ensure consistency the laminations are cut with the aid of templates and in many composite facilities by computer controlled machines, especially designed for the purpose.
Consolidation and Baking
Before the composite structure is baked, the lay-up had to be "consolidated". This is done by a process commonly referred to as "vac-bagging", where, as suggested by the name, a plastic bag (heat resistant) is placed over the structure. This is sealed around the periphery of the mould tool. A vacuum is then applied which allows air pressure to compress the laminates and removes air pockets. This "de-bulking" may be done a couple of times with complex structures as the laminates are built up. Between the lay-up and vac-bag is placed a release ply and breather cloth. The structure can now be baked. Depending on requirements, either a low temperature oven or an autoclave can be used. Autoclaves are used when extra pressure and heat is required in the baking process. Pressure can be 80/90 PSI for a typical aerostructure, although autoclaves can be used at more than twice this level. When pressure is used there is no need for a gel coat as the lay-up is compressed against the mould face. Consequently the finished component is finally painted unlike a conventional FRP build where the colour is the gelcoat.
These become more critical when they are subject to pressure and vacuum as well as high temperature. Consequently mould tools become very expensive and are normally made from steel, aluminium alloys or carbon pre-preg. The metal tools are machined after fabricating which costs tens of thousands of pounds ! The carbon tools are made by laying them up on a master plug, similar in someways to FRP boat building. There are two more composite manufacturing techniques which are worth a mention :
RTM (Resin Transfer Moulding)
This technique uses a tool made from matched moulds where the reinforcement is placed within and resin is then forced under pressure into the closed tool. The tools are heated which helps the curing process. Many small complex components can be made this way and it is a system that is gaining favour.
"Threads" of reinforcement are wound onto a mandrel, with resin, a bit like a ball of string, until the required thickness is achieved. After curing the mandrel is removed. This would be a manufacturing technique suitable for spar manufacture once you work out how to remove the mandrel !
Although this article is based on materials and manufacturing techniques as used by the aircraft industry, they are now finding their way into dinghy building. Composite structures can easily out-perform existing metal ones in a number of cases - helicopter rotor blades are commonly being made from composites. A high tech Fireball hull made to these standards would, without doubt, be very stiff, strong, light and last for many years but at what cost ? My own opinion would be to resist the expensive aircraft route and concentrate on getting the basics of composite construction right. The importance of the laminate pattern, consolidation and consistency is I feel more important than being paranoid about the materials used. Having said that I would certainly consider the use of a low temperature glass/epoxy pre-preg with a foam sandwich for a Fireball hull. It's debatable whether to use Kevlar on the outside skin of the hull to improve impact resistance or for weight reduction if that is a problem. I would finish it off with wooden decks, centreboard case etc. Not just for the looks but it's easier to attach fittings if for no other reason. Of course there is nothing wrong with a FRP top, which should be cheaper, you pays your money and makes your choice!