Damping of Moisture-Absorbed Composite Rackets

This work characterizes influence of moisture absorption on damping of tennis rackets. Three thermoset and one thermoplastic matrix composites were used for making the rackets. The first is a carbon/epoxy system, and the second is similar except that the epoxy is a polyurethane-modified type. The third is an interleaved composite that adds Nylon 6 non-woven fabrics in between the carbon/epoxy plies. The last is a thermoplastic composite made from commingled yarns that mix carbon and Nylon 6 filaments. Based on these materials, four prototypes of tennis rackets with identical shape were fabricated. Two environments representing extreme hygroscopic conditions have been applied to the rackets. One is a dried condition that forces the rackets to expel moisture, and the other is a wetted condition that allows the rackets to be fully saturated. Under these conditions the vibration tests were conducted by using the impulse method. The time-response of the rackets excited by a hammer was acquired, and the corresponding frequency response function was calculated in real time. The resonant frequency and the damping ratio were measured for each racket under the dried and wetted conditions. How the vibration behavior depends on the material systems and how the rackets respond to the hygroscopic conditions are discussed in detail.