When a missile or meteorite strikes the earth, the destruction caused can be observed on the surface easily but what happens underground is harder to detect. To study the same, a new technique has been developed by the physicists of Duke University in which they stimulated high speed impacts using artificial sand and soil and used high speed cameras for recording the strikes. The impacts were then played back in super slow-motion or frame by frame. The research was funded by Defense Threat Reduction Agency.
Researchers observed that the harder the soil and sand are struck, the harder they become. They explained that this is the reason why the ground penetrating missiles which strike the land harder and faster achieve limited success, as they experience resistance and finally stop. This research may help in building better earth penetrating missiles for destroying buried stockpiles of underground weapons and buried targets.
For the study, the researchers dropped metal projectile, having a rounded tip, over beads from a seven foot high ceiling. When the projectile struck the beads its kinetic energy transferred to the beads. These beads absorbed the energy and struck against each other. The beads used for the experiment were composed of clear plastic which transmitted light differently when they were compressed due to the impact. Polarizing filters were used to observe the impact. Researchers found that the areas of greatest stress showed branching chains of light, known as force chains, which travelled from one bead to another during the impact.
The projectile was dropped with a speed of 15 miles per hour. Beads of varying hardness were used due to which the team was able to stimulate impacts at a variety of speeds ranging from 67 to 670 miles per hour. The camera used in the experiment was capable of shooting 40,000 frames per second. It was found that the force chain grows at high speed causing the energy to move away from the point of collision much faster. Owing to the high speed, the beads get pressed together and new contacts are formed between them, which increases the strength of the material.
Study co-author Abram Clark, postdoctoral researcher in mechanical engineering at Yale University said “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 and ramming into a lot of angry people.” The study has appeared in the journal Physical Review Letters.