Poof Ball Dynamics

[John Neun]

If anyone is interested, we have done some simple compression testing on the Nerf balls. We just squeezed the balls to a controlled thickness and measured the required force. We compressed with a 1-inch diameter rod, and a flat plate. We found that:

- There isn't much difference in loading with a rod or plate (a surprise to me).
- Old, beat up balls are not very different from new balls (phew!). They are slightly softer.
- The balls are decidedly viscoelastic, relaxing about 1/3 of the compressive load after 30 seconds of stasis, where they are essentially stable.

This last finding is an important one for us. It doesn't matter much on the "long" end, (long duration), but when the load is impulsive, viscoelastic materials behave very differently then when the load is static. I have been very puzzled about our shooting tests. Why does the ball perform better when it is highly compressed through the shooter? We are zeroing in on a single wheel pitching machine, by the way. When a ball drops to the floor, only about 10% of its original height is recovered. This implies a very inefficient bounce, and thus, energy invested in compressing the ball should be mostly lost. It is not. The viscoelastic nature of the material probably explains this. Such materials often behave much more elastically when load rates are fast. In other words, the material modulus is not a constant, but a function of time. This makes the ball a much more efficient projectile.

Does anyone have any experience to share, or further thoughts?