How to Measure Impact Force / Energy Absorption

[MikeJee]

Hi, I am a bit out of my league here, but had a question. I have 5 pieces of .25" thick foam/padding of different densities, and I wanted to measure which one absorbs the most energy (weakens the impact) when struck by a fist/punch or even a ball, how would I do that?

I ran across a something called a "force plate", but wasn't sure if that is what i needed. Or perhaps if there are businesses that offer "impact testing" services, albeit I would like to do it myself though so I can learn.

Any feedback is surely appreciated.

 

[kuruman]

If you are only interested in ranking the padding, here is a simple setup that might work. Suspend two identical masses (wood blocks, billiard balls, etc.) with strings from a fixed support so that they barely touch when just hanging at rest. Attach a piece of padding on one of the masses so that it fills the gap between them. You may have to adjust the weight of the other mass to be equal to the weight of the other mass plus the padding. Move the first (unpadded) mass back to some angle, release and observe how far it swings after the collision. The farther it swings on the other side, the more energy is absorbed by the padding.

Reasoning: At one extreme no energy is lost (perfectly elastic collision), the first mass will stop and the second mass will start its swing with the speed of the first mass just before the collision. This effect is seen in the Newton's cradle demonstration - observe the "Vee" suspension of the masses. At the other extreme the maximum energy that can be lost is lost (perfectly inelastic collision), the masses will stick together, they will move as one and the first mass will travel as far as it could possibly travel. Having the padding should provide situations in-between these two extremes.

Repeat with all your samples making sure that you always start the first mass from the same angle reproducibly, e.g. hold it against a vertical post before releasing.

Disclaimer: Of course this is a crude measurement and you may not be able to tell the difference if the pads are close to each other in springiness and your way of measuring how far the first ball travels after the collision is not very accurate. Nevertheless, it's something to think about.

 

[MikeJee]

Cool! Thank you kindly for the reply. That's a test that would be fun to try. So if I am understanding correctly, the farther the first (unpadded) mass travels past the center line after impact, that would indicate more impact absorption/protection by the pad - correct?

Now that you have my wheels are turning, as an alternate test, I could even put a piece of glass under each pad and drop a cue ball from a fixed height, albeit more messy.

My first thought was to simply hang the pads on a brick wall and throw a baseball at them, and the one that bounced back the farthest has the worst impact protection... but I guess that wouldn't measure anything really but the springyness of the pad. The energy could still be getting through the pad and absorbed by the wall if I am understanding correctly?

 

[sophiecentaur]

Now that you have my wheels are turning, as an alternate test, I could even put a piece of glass under each pad and drop a cue ball from a fixed height, albeit more messy.

A nice extrovert demonstration but there would be no useful data from it apart from when the glass actually shatters. Measuring actually how far the plate moves after each impact would give a continuum of values. Much more useful.
But a more comprehensive study would involve different moving masses and plate masses. The amount of energy absorbed by the padding will be different for different combinations of masses according to the mechanical modulus (spring constant) of the padding.

 

[MikeJee]

Ah I see @sophiecentaur . Thank you.

In addition to the question in my previous post (to @kuruman), is it possible to substitute weight/mass for speed? For example, if the unpadded mass is 6 oz and drops at 10 mph, can we imitate 20 mph by increasing the weight of the mass to say 12 oz (in this scenario)? I realize it may not be a 1-to-1 correlation, but wondering if that correlation exists at all. Thank you.

 

[sophiecentaur]

Ah I see @sophiecentaur . Thank you.

In addition to the question in my previous post (to @kuruman), is it possible to substitute weight/mass for speed? For example, if the unpadded mass is 6 oz and drops at 10 mph, can we imitate 20 mph by increasing the weight of the mass to say 12 oz (in this scenario)? I realize it may not be a 1-to-1 correlation, but wondering if that correlation exists at all. Thank you.

You have spotted the fact that is often the Mass times Velocity that counts. That's referred to as Momentum and it's important in collisions. However, the Kinetic Energy is also relevant and that's mv²/2. Similar but not the same. When you want to cushion an impact, you have to reduce the KE with a 'lossy' material, rather than just a spring. A spring would conserve the energy and the projectile would bounce off at the same speed to do more damage.
I was pointing out that the mass of the absorber is important. Afaik, all designs for absorbing impact relay on a lot of actual measurements. One thing you can be certain of is that a massive 'bumper' will provide meter protection than a lightweight one. (Sort of obvious)
It all depends on what your absorber is designed to protect and how massive it can be, in practice.

 

[jbriggs444]

It is complicated.

Neither momentum nor energy is the be-all and end-all. If one is attempting to defend against penetration then peak pressure may be the figure of merit. One would want lateral stiffness that can spread a blow out over an area. By contrast, if one is attempting to defend against concussion then one may want to mitigate peak acceleration or, perhaps, jerk. One would want to tune the stiffness to the range of expected impact velocities. Too hard and you transmit the blow. Too soft and you fail to absorb it.

 

[MikeJee]

Thank you all for the feedback and explanations. I will attempt a couple of experiments and hopefully post video results back here.