Projectile rebound:good or bad?

[redbelly52]

I am involved in a project that includes testing potential pad systems of ballistic helmets for impact force absorption. This does not involve a projectile, such as a bullet, piercing the shell, but rather an object striking the helmet with such force that traumatic brain or neck injuries will occur. Our goal is to identify materials that would reduce, as much as possible, the impact force that is transferred to the head of the wearer, however that can be achieved. A question has arisen as to whether rebound of a test projectile off of the pad material is desirable or not.
The below tests are not representative of a high force impact, and are not intended to collect data, they are merely an in-house developed way to give us a direction to look for materials.
Our first test involved low force drop testing of a 0.5 oz lead ball from a distance of 2 feet onto 1/4" thick pad material and measuring the rebound height of the ball and the penetration, if any, of the ball into a layer of clay beneath the padding.
My thought is that the best results in this test would show little or no rebound, and would not allow the clay beneath it to be disturbed at all, or very little.
The second test involved dropping a helmet from a height of 4 feet, with the pad system inside, so that the crown of the helmet will strike the concrete floor. A tennis ball is lightly affixed to the crown pad, so that when the crown of the helmet hits the floor, the impact force that is not absorbed by the helmet shell, and the pad, will be transferred to the tennis ball, causing it to rebound into the air. The height of the rebound of the tennis ball was then measured on a background scale.
Again, my thought is that the best result would be the least rebound height. The other train of thought is that rebound is a redirection of the force and may be desirable.

Any thoughts on this would be greatly appreciated.

 

[mgb_phys]

What you are measuring is the total energy transferred through the pad.
what you probably want is the rate of accelaration - that's what does the damage to the person, to do this you need to hookup accelerometers inside the helmet, you also want a dummy head of the same mass as a real head in the helmet.