What happens underground during an impact

[dragon]

hen a missile or meteor strikes Earth, the havoc above ground is obvious, but the details of what happens below ground are harder to see.
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Duke University physicists have developed techniques that enable them to simulate high-speed impacts in artificial soil and sand in the lab, and then watch what happens underground close-up, in super slow motion.

In a study scheduled to appear this week in the journal Physical Review Letters, they report that materials like soil and sand actually get stronger when they are struck harder.

The findings help explain why attempts to make ground-penetrating missiles go deeper by simply shooting them harder and faster have had limited success, the researchers say. 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.

To simulate a missile or meteor slamming into soil or sand, the 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, the 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, the 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."

Story Source:

The above story is based on materials provided by Duke University. The original article was written by Robin A. Smith. Note: Materials may be edited for content and length.

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[Lord of the Abyss]

Interesting. It's somewhere between cool and amusing that they are still finding out significant new things about something as basic as "what happens if you throw something at the ground real hard?"

[Sea Skimmer]

This would really help explain why high caliber artillery shells are known to ricochet back out of the ground, emerging tens of yards away or sometimes further from the original impact.

[KroLazuxy_87]

Very interesting post. I wasn't aware this was even something science hadn't fully figured out yet. Please correct me if I'm mistaken, but isn't this similar to how water reacts to being struck? Jump in from the edge of the pool, you just get wet. Jump in from 10 stories up(without proper technique), you get dead. Are the underlying mechanics much different? Or is this simply the far end of a spectrum spanning water, non-newtonian liquids, loose sand, then soil?

[Sea Skimmer]

You die from an impact with water based on your increased speed, not because the water got harder. In fact water will do the opposite and compress under impact, which would soften a blow not strength it. Good form simply reduces your impact area, and thus the maximum shock impulse. If water hardened under impact a tight form would probably make life worse, not better for you. The hardening would be concentrated and intensified!

Now at a velocity of say 30mph the compression effect is almost irrelevant and water can be considered close to a Newtonian fluid at those kind of impact speeds and pressures, which is why school children are wrongly taught that water does not compress. For say, high pressure pressure hydraulic systems the compression can be as high as 10-15% of volume which is a reason why engineering these systems for precision movement (such as in aircraft controls) can be very difficult and expensive.

Some strongly non-Newtonian fluids do shear harden, such as quicksand. This ground behavior is something linked into that sort of behavior.

[KroLazuxy_87]

school children are wrongly taught that water does not compress. For say, high pressure pressure hydraulic systems the compression can be as high as 10-15% of volume which is a reason why engineering these systems for precision movement (such as in aircraft controls) can be very difficult and expensive.

I was also operating under this false assumption. I love learning new things.