Research into making more effective American "bunker buster" bombs that explode deeper underground has shed more light into what goes on when high-speed objects slam into the earth.
Physicists at Duke University simulated the effects of a fast moving object penetrating the ground by developing artificial soil and sand in a lab then simulating the high-speed impact of a bomb or meteor on this artificial soil. They used a close up camera to capture the impact in slow motion.
Results from these experiments led physicists to conclude that materials like soil and sand actually get stronger when they're struck harder.
The findings help explain why attempts to make ground-penetrating missiles or bombs penetrate deeper by simply shooting them harder and faster have had limited success, said researchers. Projectiles actually experience more resistance and stop sooner as their strike speed increases.
Funded by the Defense Threat Reduction Agency, the research may ultimately lead to better control of earth-penetrating missiles designed to destroy deeply buried targets such as enemy bunkers or stockpiles of underground weapons.
The largest of these operational ground penetrating munitions or "bunker busters" in the U.S. arsenal is the GBU-57 Massive Ordnance Penetrator or MOP. This 30,000 pound bomb that can destroy deeply buried bunkers has been upgraded and can be carried by the U.S. Air Force B-52 and B-2 bombers.
The GBU-57 is a 20 foot-long GPS-guided bomb believed capable of penetrating 200 feet before exploding. It's been equipped with new fusing and tail fins needed to give it a greater ground penetration.
The bunker buster bomb can be used to destroy nuclear facilities, as well as bunkers that protect command centers, chemical, biological and nuclear weapons and submarine pens.
To simulate a missile or meteor slamming into soil or sand, the Duke researchers dropped a metal projectile with a rounded tip from a seven-foot-high ceiling into a pit of beads.
During collision, the kinetic energy of the projectile is transferred to the beads and dissipates as they butt into each other below the surface, absorbing the force of the collision.
To visualize these forces as they move away from the point of impact, researchers used beads made of a clear plastic that transmits light differently when compressed. When viewed through polarizing filters like those used in sunglasses, the areas of greatest stress show up as branching chains of light called "force chains" that travel from one bead to the next during impact, much like lightning bolts snaking their way across the sky.
The metal projectile fell into the beads at a speed of six meters per second, or nearly 15 miles per hour. But by using beads of varying hardness, researchers were able to generate pulses that surged through the beads at speeds ranging from 67 to 670 miles per hour.
Each impact was too fast to see with the naked eye, so they recorded it with a high-speed video camera that shoots up to 40,000 frames per second. When they played it back in slow motion, they found that the branching network of force chains buried in the beads varied widely over different strike speeds.
At low speeds, a sparse network of beads carries the brunt of the force, said study co-author Robert Behringer, a professor of physics at Duke.
But at higher speeds, the force chains grow more extensive, which causes the impact energy to move away from the point of impact much faster than predicted by previous models.
New contacts form between the beads at high speeds as they are pressed together, and that strengthens the material.
"Imagine you're trying to push your way through a crowded room," said study co-author Abram Clark, currently a postdoctoral researcher in mechanical engineering at Yale University.
"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."