In the dynamic arena of sports technology and engineering, engineers pay special attention to the energy stored and lost in equipment. Inflatable (and solid) balls are often governed by a coefficient of restitution that describes the interaction between the ball and a rigid plate. Proper dynamic characteristics are achieved by careful selection of materials, construction, and inflation pressure. After the ball leaves the factory, materials and construction remain relatively consistent, but the internal pressure is left up to the discretion of the athlete, who receives only a recommendation. A study was conducted to observe the relationship between over- and under-pressurization and energy loss at various levels of deformation.
An ElectroPuls™ E3000, fitted with light-weight compression plates, was used to compress entire balls. During the study, many different types of balls were tested (football, volleyball, futsol, netball, volleyball, rhythmic gymnastics), and the results from a basketball are presented here. Each ball was compressed cyclically at a constant frequency with a 5 mm amplitude around mean levels of initial displacement that we subsequently increased by 5 mm increments. The motion was prescribed with digital encoder control within WaveMatrix™ Dynamic Testing Software. Each ball was tested at three pressures:
- The ‘nominal’ pressure was at the middle of the range specified on the ball
- Under-pressure was 25 % lower than the nominal
- Over-pressure balls was 25 % above the nominal
Force-displacement curves were taken from the final cycle (20th) at each initial displacement. The difference between the numerically integrated loading and unloading curves gave the energy lost in each compression cycle. The graph presented shows the ratio of the energy lost to the energy introduced to the system in the various cycles. The larger of the bars indicate higher relative energy loss.
The properly inflated ball, represented by the green bars, loses the least amount of energy of the three pressurizations tested. The higher energy loss of the under-inflated ball may be attributed to the interplay between bending and spherical in-plane deformations (hoop strains) of the panels. At high pressures, the ball is essentially pre-loaded and the panels experience a higher range of strains, maybe departing the elastic zone. With hyperelastic materials, it is no surprise that more energy is lost at higher strains. Full-ball dynamic compression tests provide insight on the role of internal pressure in sports balls. Quick studies like this help quantify global ball behavior and suggest new areas of further research.
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