Spalling

When I was six years old, someone shot a large window in my school with a BB gun. The hole was in the shape of a cone, with the smaller end on the exterior, where the BB had impacted. Many of us were fascinated with why the window had a hole that was smaller than the BB. At the time we incorrectly reasoned that the BB must have somehow squeezed through the hole. Much later I would learn that the BB had not passed through and that this was called spalling.

Spalling is not limited to glass, but is a common type of armor failure. Spalling can pose an injury hazard to people riding in armored vehicles. The BB striking the glass is illustrative of the armor problem as well. When the BB struck the exterior surface of the glass a series of waves emanated from the contact point. The waves travelled in the direction that the BB was originally moving. When the waves reached the interior surface of the glass they reflected back the other way, much like waves in a bathtub.

Before the waves reached the interior they were trying to compress the glass. They were compressive waves. After reflecting the waves were trying to pull the glass in tension. They became tesile waves. The sudden change from push (compressive wave) to pull (tesile wave) literally ripped the glass apart.

Even though I've learned why the BB hole looked the way that it did, it still fascinates me.