In a demented kind of way, when either a missile or a meteor strikes Earth, as much havoc as it can cause, it is pretty exciting. While the destruction it can cause above ground is fairly apparent, there is a whole three-ring circus going on underground that is a bit more difficult to see. But physicists at Duke University have come up with special techniques that have fitted them with the means to simulate high-speed impacts in artificial soil and sand, and observe the underground ramifications in slower-than-slow motion.
One discovery that they have come up with via their lab experiments is that upon such forceful impact, soil and sand indeed become stronger the harder they are struck. This unearthing serves to explain why efforts to force ground-penetrating missiles deeper underground by just shooting at them more quickly and with greater impact don’t really pan out. In reality, projectiles come against resistance to a greater extent and will actually stop before their strike speed has a chance to reach full throttle.
THE EXPERIMENT
In order to replicate the occurrence of a missile or meteor thrashing into soil or sand, the scientists plummeted a metal projectile with an orb-shaped tip from 7 feet above into a pit of beads. Upon impact, the kinetic energy of the projectile was taken on by the beads and dissipated as the beads bumped into one another below the surface, absorbing the energy and force of the collision.
To visualize this force as it moved away from the point of the crash, the researchers employed beads that were made of clear plastic which transmit light differently when compacted. When observed through polarizing filters such as those found in regular sunglasses, the portions of greatest stress showed up as branching chains of light referred to as “force chains” that move from one bead to the next during the impact, akin to lightening bolts that zig-zag their way across the sky.
The metal projectile plunged into the vat of beads at a speed of 6 meters/second, or close to 15 MPH. Via the use of beads of varying hardness, the researchers made it possible to trigger pulses that rippled through the beads at speeds ranging from 67 to 670 MPH. At low speeds, a small number of beads carried the brunt of the force, and at higher speeds the “force chains” grew more extensive, resulting in the energy of the crash to move away from the point of the collision a lot more quickly than predicted by previous models. New contacts are generated between the beads at higher rates of acceleration as they are pressed together, and that is the cause for strengthening the material.
TO SUM IT UP
Said co-author Abram Clark, currently a postdoctoral researcher in mechanical engineering at Yale University:
“Imagine you’re trying to push your way through a crowded room…If you try to run and push your way through the room faster than the people can rearrange to get out of the way, you’re going to end up applying a lot of pressure [to] and ramming into a lot of angry people!”