There are some features for an approved soccer ball by Fédération Internationale de Football Association (FIFA), such as properties of the material, mass, pressure, stitches, etc. Many of these features up to now have been studied; nevertheless, the mechanical properties of the soccer balls to date have not been well reported. The chief purposes of the current research, hence, were to calculate the mechanical properties of the soccer balls, i.e., linear elastic, nonlinear hyperelastic, and viscoelastic, at two different sizes, including 4 and 5 which are using for football and futsal, respectively. To do this, compressive and stress-relaxation loading were applied to 38 approved soccer balls to quantify the stress–strain as well as reduced relaxation function of the balls. The strain/displacement of the balls was also measured via a high-speed camera using Digital Image Correlation (DIC) technique. The results revealed the mean elastic modulus of 66 and 67 kPa for the football and futsal balls, respectively. In addition, the maximum stresses of the football and futsal balls were 16 and 13 kPa, respectively. The nonlinear mechanical response of the soccer balls were analyzed using hyperelastic material models, i.e., Mooney-Rivlin and Ogden. A Finite Element (FE) model was also developed to verify the hyperelastic data compared to the experimental ones and, remarkably, the numerical data were in consistence with the experimental data. Finally, Prony- series was employed to quantify the viscoelastic properties of the balls. During the game, a soccer ball can reach to a speed of 210 km/h that can damage the human eye; however, the injury detail still has not been studied.
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