This article by Dan Fleetcroft, Technical Director at Performance Engineered Solutions (PES) Ltd, was originally published in F1-Monitor Magazine.
In the perpetual search for improved mechanical properties and reduced mass, Formula One has a reputation for being the earliest adopter of new materials and technologies, such as composites. Some of these are developed internally by the teams while the Formula One fraternity is often the first port of call for a company with a new performance-enhancing product, feeding new developments into the industry.
The competitive advantage of any performance gain – combined with the large budgets, a pioneering engineering attitude and potential global exposure – makes racing an obvious customer for the latest technologies.
The development of composite materials is relentless, with new fibres and resin systems offering improved mechanical and thermal capabilities, among other advancements coveted by Formula One engineers. However, one less obvious area of development has been in the design of the weave for composite fabrics. Most Formula One components are traditionally manually laminated using continuous pre-impregnated (pre-preg) carbon fibre reinforcements with either unidirectional (UD) fibres or woven fabrics, often referred to as cloth.
Conventional cloth weave designs commonly include plain weaves, 2 x 2 twills and satin weaves. Each of these have different draping properties (the desirable ability of the pre-preg to conform to the mould tool’s surface geometry), different levels of crimp (the undesirable distortion of the fibres produced by the interlacing of the warp and weft tows detrimental to mechanical performance) and provide varying degrees of ‘wet out’ (resin impregnation of the fibre reinforcements) and surface finish smoothness. The combination of characteristics in each individual weave style all have some level of compromise, and this has driven the research and optimisation of weave design.
Advances in textile engineering and manufacturing have resulted in the development of spread tow fabric (STF) materials. Instead of ‘bundling’ the carbon fibres in narrow and thick tows, spreading the fibres into thin and wide tapes and then weaving these together allows ultra-lightweight fabrics to be produced. This offers a number of benefits over the more traditional cloth designs.
The flat structure of STFs reduces the crimp angle and frequency while improving the resin wet-out (cover factor). This results in high fibre volume fractions with straighter fibres, increasing the mechanical properties of the laminate while reducing the amount of excess resin, therefore minimising weight.
Weight savings of 20-30% are achievable over conventional woven composites with thinner laminate thicknesses*, giving mechanical performance similar to a cross-ply construction made using UD tapes but with improved drapability and delamination resistance. The tables here give a comparison of crimp ratio between a conventional plain weave fabric and a spread tow material.
STFs deliver improved surface smoothness by reducing interlacing points, increasing fibre float and minimising crimp. This improves the aesthetics of the composite and, more important, offers the potential elimination of ‘print through’ of the weave pattern when the moulded surface is lacquered or painted. Figs. 1 and 2 show a standard plain weave fabric and a spread tow fabric which have been consolidated under a vacuum to highlight the benefits of the reduced crimp on surface smoothness. It is clear to see the flatter surface produced by the spread tow material.
Ultimately the significant weight savings, improved mechanical properties and thinner laminates are why STFs have found a home in Formula One racing cars, with many components – including the monocoque, bodywork and floors – benefiting from the superior performance this composite offers.
The constant pursuit of performance from both the racing teams and the companies who supply them drives the exploration of all potential advantages. STFs highlight how an obsessive attention to detail and comprehensive understanding of composites can take a seemingly small development and deliver significant performance returns.
See the full article, including tables here.