From medieval village huts to interplanetary scientific probes, composite materials have been widely used for constructions of all kinds. The actual reinforcing materials vary tremendously from wattle and daub to balsa cored GRP, from concrete to carbon fiber/epoxy. Modern reinforcements are available in a wide variety of compounds and a huge range of forms. We have reached the stage of ‘designer materials’ – and not only for fashion.
The main types of flexible sheet reinforcements are cloths, rovings and fabrics. These are typically ‘laid up’ and ‘wetted out’ with an epoxy or polyester polymer which then cures to give the final properties required. They may be woven or non-woven, with non woven forms having their filaments randomly oriented or aligned. Non woven cloths and fabrics require a binder to hold the filaments in the cloth format until they are impregnated with resin in the final product.
The most common base materials for these sheets of reinforcement are glass filaments, carbon fibers and aramid fibers, each of which offers specific properties:
Glass filament: High strength, tolerant to flexing, inexpensive. On the downside, it is relatively heavy when compared with alternatives.
Carbon fiber: High strength, light weight but relatively low flexing tolerance.
Aramid fiber: High flexing tolerance, high fire resistance and insulation properties, but low resistance to abrasion so has to be ‘buried’ in epoxy if the final surface is to be sanded or faired.
Rovings are simply defined as bundles of continuous fibers which have been drawn from the source chemical mix.
Cloths are woven from these threads (rovings). With glass, carbon fiber and aramids; the weaves may vary in warp and weft designed to accommodate specific strain directions to suit the application. The range of constructions available includes unidirectional, bidirectional, multiaxial and quadraxial.
Rovings are also used to manufacture products of complex shape using filament winding, multi-axial weaving and pultrusion techniques (to name a few).
For glass fiber based cloths, many DIY fans are familiar with the mat format – ‘CSM’ – chopped strand mat which is technically a ‘short fiber’ format, non-woven. There is also another format known as continuous-strand mat. Both these formats have randomly oriented fibers. Glass reinforcement is also widely user in a woven cloth format.
Another format which offers interesting properties is stitched fabric. A sheet of unidirectional carbon or aramid fibers is held together by stitching the rovings in their various directions together. This is a ‘non-woven fabric’ (though the fibers are aligned) and is typically delivered as a rolled fabric. Compatible with epoxy and polyester resins, it offers simple local reinforcement capability.
Some manufacturers are now offering hybrid cloths which combine various fibers, such as carbon fiber and aramid fibers to offer the high stiffness of carbon combined with the impact resistance of Kevlar (aramid fiber Tradmarked by DuPont) fibers. With the aramid being dyed in a range of bright colors to contrast with the black of carbon, the cloth is eye-catching and adds a ‘style’ aspect to the finished product as well as the enhanced physical properties.
Other hybrids include cloths with 50/50 carbon fiber and glass which offers high strength with enhanced impact resistance without the weight penalty an all-glass cloth would bring. With an 80/20 glass/carbon fiber blend, then a more economic cloth results.
We have arrived almost at the stage of designer reinforcements for composites. The range of cloth/fabric formats is enormous, with tailorable properties (a great feature of composite technology) being able to deliver precise engineering characteristics in a way unimaginable twenty years ago. But design goes beyond that. We now have carbon cloth finishes for iPhone covers being augmented by colorful carbon/armid style statements.