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Attwater

Design Philosophy

Imagine for a minute that composites do not exist and then consider what one would need from a material for any product, which has to work in an aggressive environment such as the sea, for example. This material we seek needs to be easily shaped, it has to be happy in a hot or cold salty environment and ideally it needs little equipment to turn it from a raw material into a product. It would also help if the material is low in weight, relatively inexpensive and can be tailored with regards to strength and stiffness. And by the way, as we may be making a car or an expensive yacht, we would like the material to be any colour and glossy.

It is easy now to see why composites (fibre reinforced polymers or plastics) have become the mainstay material in the marine designers’ drawer of materials.

However, a designer in the 1940’s would now be extremely surprised to see the proliferation of fibres, resins, sandwich core materials and manufacturing processes, which abound in the composites industry. Since the first boat was made in glass reinforced polyester resin, now some seventy years ago, we have transgressed to high impact resistance aramid fibres (Kevlar) to high strength and stiffness carbon fibres using heat cured resin systems.

Not that this progress has been without problems. Most people know about blistering, no more so than the unfortunate boat owner whose boat is looking like a bad attack of mini-mumps. Also, because it is a material that requires little in the way of equipment, (a bucket and a brush) it has been so often used by the inexperienced, leading to poor products and unfortunately, problems. This gave the material a bad press on many occasions. But progress has been made and composites are now used widely in almost every application.

So let us take a look at the real benefits of composites and basically how they work.

Fibre reinforced polymers are what this says – a fibre of some sort held within a resin matrix. The most common fibres are glass, aramid (Kevlar) and carbon. The most common resins are polyester, vinyl ester and epoxy. Phenolic resins are also available and incidentally, the oldest type. They have better fire resistance, but because they are more difficult to use, they are not common within the marine industry.

The fibres may be random or directional. Because of the variation in strength and stiffness of the fibres, an immediate advantage can be seen – it is possible to ‘engineer’ the required strength or stiffness and the direction in which these properties are required.

Glass reinforced polyester is the cheapest and the most widely used composite. The basic manufacturing process is simple – a bucket of mixed resin (resin, accelerator and catalyst), a brush to apply the resin and some fibre. The more sophisticated manufacturing methods now include resin infusion, where the resin is drawn into a closed mould under vacuum. The mould already contains the fibre, in thicknesses and direction to suit the load or stiffness requirement. Whole boat hulls are now made by this method – one-shot manufacture.

At the more expensive end, and the higher property end also, we have carbon and aramid fibres. These are often in pre-impregnated (pre-preg) form, that is the fibre has been coated with a heat curing resin. To prevent curing before use, the pre-preg is kept at low temperature.

It is this huge range of fibres, resins, manufacturing processes and supporting sandwich core materials, which give composites the real advantage over other materials. It is also the real advantage that is not always immediately appreciated by those new to the material.

A misconceived disadvantage is the apparent high cost of the higher strength and stiffness carbon and aramid fibres. This tends to make designers believe that the end product will be expensive when compared to glass reinforced polyester or even metals. But, carbon and aramid have much improved strength and stiffness over glass fibre. Aramid is also very tough. Furthermore, when the specific strength and stiffness (ie property divided by material density) is compared to metals, the composites are significantly better. More strength or stiffness per kilogram of material. Also less weight means less material. The higher material costs are then compensated.

If we now consider the fact that the moulded surface will be very smooth and fair (as opposed to look of welded aluminium alloy which requires filling and fairing), the fact that the material will not degrade in the salty environment and there will be no painting required, we will equate costs to the more conventional metal structure. Despite therefore the apparent high raw material cost, we end up with a cost-effective product.

It is this philosophy of composite material selection that is used by the experienced designers to create many of the other well-designed and engineered products in composites. However, a small word of warning – if the materials are not fully understood and they are used by inexperienced designers, errors can be dramatic. When they are used correctly, composites can be shown to be the designers’ path to the optimum structure and composites are very happy to be used in a wide variety of applications.

Published courtesy of Anthony Marchant, CETEC Consultancy Limited