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10:20   Ball-racket and ball-surface interaction Chair: Kim Blair 10:20 20 mins NON LINEARITY OF THE BALL/RUBBER IMPACT IN TENNIS TABLE: MODELING AND EXPERIMENTS Renaud Rinaldi, Lionel Manin, Clément Bonnard, Adeline Drillon, Nicolas Havard, Hugo Lourenco Abstract: A table tennis racket is composed of a wood blade on which are glued two elastomeric coverings. a foam (sponge) and an architectured rubber (array of cylinders). On a general standpoint, countless complex systems comprise polymeric elements bringing their peculiar intrinsic properties (lightweight, compliance, high damping …) to the designed multi-functional element. For the practical case of tennis table racket, the polymeric elastomeric layers have a crucial effect on the “in-use” performance often described in terms of “control” and “speed”. This study focuses on the influence of these layers on the “speed” performance alone, characterized by the restitution coefficient (CR) of the ping-pong ball impacting the systems under normal incidence for different launching speed corresponding to the in-use conditions (10 to 30m/s). Numerical and experimental studies are carried out on different foams and rubbers. A three dimensional finite element model was developed: in which the exact geometry of the rubber with the pimples is considered. Furthermore, the foam is modeled as an homogeneous media with account for its real density experimentally measured. It is worth mentioning that both the racket polymeric constituents and the ball (made of celluloid) require the implementation of constitutive equations with account for rate yet dissipative effects. More precisely, the ball, the foam and the rubber constitutive equations add a Prony’s serie (experimentally identified) in parallel with a hyperelastic law to consider the high speed and large strain effect involved. The deformed shape of the ball during the impact corresponds to a ring with its mean radius increasing till the maximum crushing. The friction between the ball and the rubber plays an important role in the dissipation of energy. The effect of the ball-rubber friction coefficient on the restitution coefficient is highlighted numerically. Overall, fair agreement is obtained with the experimental measurements. Finally, the numerical model is used to identify the influence of key design parameters, such as layer geometries, constitutive equations ... 10:40 20 mins SETTINGS ADJUSTMENT FOR STRING TENSION AND MASS OF A TENNIS RACKET DEPENDING OF THE BALL CHARACTERISTICS: LABORATORY AND FIELD TESTING Piotr Baszczynski, Cyril Chevrel-Fraux, Solène Ficheux, Lionel Manin, Sylvain Triquigneaux Abstract: The sensations of power for a tennis racket may change drastically when used with different balls. The aim of this work was to determine the optimal string tension and mass distribution of a tennis racket, based on its initial configuration and the type of ball used. This study may be also helpful nowadays, when during each tennis tournament the use of particular balls is imposed. First, an experimental analysis is carried out on six different balls, the tennis ball hardness and restitution coefficient are measured and compared. Then the power of a racket is analyzed versus the ball used and the racket settings (string tension, added mass), a dedicated test bench was used. Balls are thrown with a canon, the restitution coefficient is the observed parameter. It is shown that the racket settings are more influent than the ball characteristics in terms of power. A detailed analysis of the influence of each parameter is given. The second part of the work was concerned with the field tests where the performances of the rackets were evaluated in terms of players’ sensations. The tests were blind as the players did not know which ball among the six different they were hitting. Each ball was hit four times by each player following specific instructions. The players were asked to qualify the balls depending of the power and the general sensation (comfort) they had felt during the stroke. A global synthesis of the results obtained is presented, it shows that during a stroke, the power felt by the players is related to the general sensation. Finally, using the experimental design theory, an interactive program was written in order to provide to its user the optimal settings for his racket in terms of string tension and mass to add. The input parameters are the ball hardness, the racket mass, the string tension, the balance of the racket. 11:00 20 mins NOVEL METHODOLOGY FOR MEASURING THE COEFFICIENT OF RESTITUTION FROM VARIOUS TYPES OF BALLS AND SURFACES Federico Colombo, Karoline Seibert, Hugo Espinosa, David Thiel Abstract: The determination of the coefficient of restitution is of major interest in the design of balls and surfaces. Tennis courts are required to be resurfaced every five years. Players that slide on the court trust the surface to be uniform. Tennis court surfaces change as the ball fluff builds up, the heavily used playing areas are compacted more, the surface on clay is scuffed, and the sun and rain degrades the surface. Injuries can be caused by a player losing footing because of surface variability. With bouncing balls, the ball type and pressure are variable and depend on temperature and age. An investigation on the bounce of various balls (diameter less than 150 mm) from surfaces using an accelerometer on a novel, low cost, portable apparatus is presented. The mechanical structure holds both the moving ball and an inertial sensor. The quality and age of balls and the wear on playing surfaces is particularly important for reflex actions of elite athletes. Courts, pitches and other sports surfaces can be routinely quantified using sport specific balls and this simple, low cost method. Good agreement was observed between the coefficient of restitution using the portable device and a vertical drop test using a high speed camera. The error obtained using the portable device on various types of sports balls with the variation in CoR ≤ 0.01 which falls within the standards of the International Tennis Federation. There is a significant difference (p = 0.0003) between a hardcourt tennis CoR and a synthetic grass tennis court.