Advanced modelling of soccer balls

Soccer is the most popular ball sport in the world. With an estimated 247 million active players world-wide, the game generates an annual turnover of approximately 200 billion dollars, which is far in excess of corporate leaders such as General Motors. The soccer ball represents the fundamental equipment requirement with ball sales estimated at 40 million units per year. The market is extremely competitive and manufacturers strive for superior product performance to enable commercial advantage. The Federation Internationale de Football Association (FIFA), soccer's world governing body use quasi-static testing to scrutinise ball designs however there is a need for greater understanding of the dynamic performance characteristics of soccer balls. This thesis is directed toward the development of a modelling methodology using finite element (FE) technology supported by a set of dynamic test standards. The modelling activity has been predominantly concerned with two different ball types, the traditional manually stitched ball (MSB) which contains 32 textile reinforced composite panels pressurised by an internal rubber bladder and two new generation ball types (NGB) which incorporate an underlying 12-panel stitched fabric carcass, onto which outer panels are adhered. FE models have been created with homogeneous and isotropic material properties to simulate ball impact behaviour. Experimental measurements of coefficient of restitution, deformation, and contact time, representative of play conditions, were used to validate the models. Each ball model was developed to capture material heterogeneity, which included the valve, stitching seam for the MSB, the influential carcass structure and softer outer panel arrangement within the NGBs. Material anisotropy has been modelled in order to replicate the deformation behaviour of soccer ball s at high speeds. Anisotropy was shown to affect impact characteristics including significant variations in ball oscillations, spin, and ball motion. The validated carcass FE model was subsequently used as a predictive design tool within an optimisation process to reduce the effects of material anisotropy. This has resulted in the development of a ball with more uniform impact characteristics and a set of design guidelines for future sports ball development. Dynamic material test data has been used to define material damping for the description of kinetic energy loss throughout impact. The model combines static and dynamic force hysteresis measurements to accurately represent energy loss characteristics throughout impact. The visco-anisotropic-hyperelastic ball modelling strategy described in this thesis accurately represents the complex deformational behaviour and energy loss characteristics for soccer balls undergoing dynamic impacts. The modelling strategy has also been successfully used in a design optimisation process for the development of soccer balls with more uniform impact characteristics.