Christian Fischer
Controlling the static and dynamic properties of sports equipment has since long been of great concern in the sports engineering industry. More recently there has been interest in better understanding and improving the interplay between such properties and the perception of the athlete using the equipment. The aim of this thesis was first to develop a new approach to identifying the corresponding requirements for a particular type of sports equipment. The tailoring of a novel material with the potential to fulfill these requirements, and the integration of this material into composite structures was then investigated. Alpine skiing was used as a case study for this thesis. During this first project phase, the importance of specific ski properties was identified. Perceived characteristics were linked to physically measurable properties, and the level of differences in mechanical properties discernable by skiers was examined. For this purpose, four pairs of slalom skis that differed in flexural rigidity by a maximum of 20 % and in torsional rigidity by 60 % were tested by advanced and expert skiers under specific snow conditions. Among other results, Spearman correlations showed that expert skiers considered a high torsional rigidity to have a positive effect on characteristics such as quietness at high speed or energy restitution in carved turns, and advanced skiers judged stiff skis in torsion to positively influence the grip during short turns. Furthermore, it was also shown that athletes are able to discern small differences in damping and vibration of the skis. These results demonstrated that a quantitative approach to perceived properties is possible, and that it should be considered for the future design of sports equipment. Stiffness and damping usually counter-balance one another. The need for high values of both properties, as for example in skis and other types of sports gear where shocks induce large deformations, presents a considerable challenge for materials scientists. Shear-thickening fluids (STFs) are exceptional in this perspective, because they reversibly increase their moduli by 2-3 orders of magnitude, and simultaneously increase their energy absorption, when a critical shear stress or strain is reached. The dynamic shear-thickening behaviour of two different types of STFs was investigated, with the aim of integrating them into dynamically loaded structures. Dynamic frequency sweeps showed that the complex viscosity at different imposed strain amplitudes followed a unique power-law type behaviour up to the onset of strain thickening. Similar behaviour was also observed in the post-transition state, i.e. the viscosities again superimposed at frequencies beyond the transition frequency. Furthermore, the shear-thickening effect considerably decreased with increasing frequency. Viscosity increased by 3 orders of magnitude at 0.1 rad/s, and by 1 order of magnitude at 100 rad/s, suggesting that shear-thickening would totally vanish at frequencies beyond those measurable by conventional methods. Depending on the composition of the STFs, they either thickened at a constant shear stress throughout the frequency domain (monodisperse STFs) or required a minimum shear strain to be activated at high frequencies (polydisperse STFs). Results from dynamic rheological measurements for the STFs were correlated with results from vibrating beam tests (VBT) on model sandwich structures containing layers of the same STF sandwiched between solid beams. At the highest STF concentration, continuously increasing the excitation amplitude led to a 9-fold increase of the damping of the 1st resonance frequency. Since the increased viscosity of the STF interlayer resulted in improved stress transfer between the stiff polymer layers, the flexural rigidity simultaneously increased by over 20 %. Also, it was shown that the extent of damping and stiffening of the structure could be tailored by varying the type of STF. Finally, methods to integrate STFs into composite structures at different scales have been developed. At macro-scale, a STF damping device showed that the latter could strongly affect the dynamic behaviour of a ski. At meso-scale, the most promising design was based on glass fibre-epoxy rods integrated into the structure such that they function as small pistons when the structure is bent. For a tailorable deformation and frequency domain, the STF at the rod-matrix interface thickened, increasing the stress transfer between the rods and the matrix, and thus leading to changes in the structure's dynamic response. Dynamic mechanical analysis of soft matrix composites, containing STF at the rod-matrix interface, demonstrated that STFs could be integrated at the micro-scale of composite materials, which was confirmed by a micro-mechanical model. In summary, this thesis has provided a tool to include human perception into the design of sports equipment, and has built up a broad knowledge of the dynamic behaviour of an adaptive material, which can be included into a material-structure-athlete loop with the aim of evaluating its potential for any specific application in sports where comfort, feel and control of the performance should be improved.
EPFL2007
