by Stephen A. Mayfield
This project started because I wanted to find an easy way to determine the blunt trauma forces on a person wearing a Kevlar vest when struck by a projectile. In order to do this I needed a pressure sensor to measure the forces, so I decided to use eggs. Eggs are very convenient because they break at a fairly consistent pressure, and give a very good indication when that pressure has been reached. What I discovered was that the force required to break an egg was not something I could simply look up on the Internet, but something I had to determine for myself.
To determine how much force is required to break an egg, I built an egg crusher from wooden boards and dowels. Once I knew how much force was needed to break an egg on its own I needed to test how much force would be needed to break an egg embedded in ballistics gel. The egg needed to be embedded far enough back so that the projectile wouldn’t just break the egg on impact but would break from the shockwave. The problem was that the egg crusher wouldn’t create a shockwave, so it couldn’t be used to measure the forces.
To test the forces to break an egg in ballistics gel I first tried dropping heavy weights on the gel block, but I quickly learned that I couldn’t get the velocity needed to break the egg. I then decided to try using a large slingshot. This allowed me to shoot a lighter weight at high velocities, which gave higher kinetic energies and more closely matched a bullet.
However I soon discovered that how forces and pressure waves travel through ballistics gel is not very well understood. So I came up with an idea to treat the ballistics gel like a spring being compressed from one direction. I then built a box to hold it and keep it from expanding laterally. Then if I had the Young’s modulus I could compute the potential elastic energy necessary to stop the slingshot projectile of a given kinetic energy by the gel. I was able to use the egg crusher to compute the Young’s modulus of the ballistics gel.
I realized then that I needed to build a frame to protect the box holding the gel, because otherwise if I missed the gel and hit the box it would shatter. Using the velocity and mass of the projectile I could get the kinetic energy and then I was able to calculate the force of impact using the elastic potential energy formula to get depth of penetration.
Slingshot setup with chronograph.
Raw egg embedded 5 cm inside gel. Ball traveling 43.8 ft/s. Egg breaks.
After I had collected all of this data I discovered how difficult it is to compare these forces to injury on a person. I tested using Kevlar in front of the gel to calculate how much energy different numbers of layers dissipated. I then looked up bullet masses and velocities to see if Kevlar would stop the force. I finally found papers that showed how to determine likely injury from non-penetrating projectiles. These papers showed the likelihood of serious injury based on the force and diameter of the projectile as well as the weight of the impacted target. They also showed that seven layers of Kevlar would be sufficient to stop a .38 caliber bullet.
