Advanced techniques to improve the design of tennis ball cores

Over the past century tennis balls have seen little development, despite issues with durability and recyclability. Over the same period rackets have seen several development iterations through the use of wood, aluminium and carbon fibre reinforced composites frames. Players physical capabilities have dramatically improved and even line-calling has been automated. In professional tennis, balls are used for as little as nine games before being discarded, whilst recreational players demand a long-lasting product at minimal cost. Balls are comprised of a vulcanised rubber core, which is pressurised, and woven felt covering. Similarities in materials, combined with strict performance limits defined by the International Tennis Federation (ITF) and consumer pressures culminates in a product with low profit margins and little market differentiation. The work presented in this thesis focussed on the elastomeric material used to manufacture the core of tennis balls, presenting a scientific means of assessing alternative ball core materials that could benefit players and brands alike. Ball tracking data collected during professional tournaments, spanning the major court surfaces used in professional tennis, was analysed and used to determine the impact frequency and conditions a ball is subjected to during play. The range of impact conditions determined were replicated in the laboratory and subjected to digital image correlation techniques (GOM Correlate Professional), which were applied to measure the surface strains and strain rates present during impact. The results of which enabled the transformation of typical ball impact conditions in professional tennis into mechanical testing conditions representative of what occurs during impact. Current pressurised and pressureless ball core rubber were subjected to tensile testing, matching as closely as was possible, the strains and strain rates measured during impact. Dynamic mechanical analysis (DMA) was also utilised to characterise the viscoelastic properties of current ball core rubber. The characterisation of current materials provided a benchmark against which alternatives could be compared and enabled the implementation finite element (FE) simulations of ball cores during impact. FE modelling utilised advanced viscoplasticity material models (Bergstrӧm-Boyce model) enabling the viscoelastic and strain rate dependent behaviour of rubber to be incorporated, eradicating the need to artificially tune model coefficients, as seen in previous examples of tennis ball modelling. Having quantified the conditions required for materials characterisation testing and developed a methodology for the simulation of pressurised ball core impacts, alternative materials were identified and assessed. Thermoplastic elastomers (TPEs) were identified as materials with potential for replacing vulcanised rubber. TPEs offered potential improvements to pressure retention properties, recyclability as well as the opportunity to utilise thermoplastic manufacturing processes. When subject to the same materials characterisation testing and FE modelling as ball core materials, TPEs exhibited, in part, comparable properties to ball core rubber, with simulations estimating similar ball core performance for melt processible rubber TPE. The work presented in this thesis implies TPE materials are worthy of further investigation for use as tennis ball cores.