Missile and meteorite impacts are more complex at the granular level than scientists previously thought

December 11, 2012 by Ashley Yeager
Grains Gang Up to Bear Brunt of Missile and Meteorite Impacts
The way a meteorite or missile transfers the energy of its impact to sand and dirt grains is far more complex than scientists thought. Impact illustration courtesy of NASA.

(Phys.org)—High-speed video of projectiles slamming into a bed of disks has given scientists a new microscopic picture of the way a meteorite or missile transfers the energy of its impact to sand and dirt grains.

The transfer is jerky, not smooth. "It was surprising just how unsmooth the slow-down of the intruding object was," Duke physicist Robert Behringer said. His team describes their new videos and impact analysis in the Dec. 7 . The research may change the way scientists model and missile impacts and their effects. 

Scientists previously assumed that the slowing down would be smooth and that any sound wave would travel through a in a regular, uniform pattern, similar to the way noise from a clap of the hands diffuses evenly in all directions through the air. But using high-speed video, Behringer, his graduate student Abram Clark and Lou Kondic of the New Jersey Institute of Technology have shown a very different behavior for the and during a collision. 

In the study, supported by the Defense Threat Reduction Agency, the team shot bronze disks into a narrow bed of photoelastic grains and used an ultrafast camera to track the collision energy as it shifted from the disk to the beads. The footage shows that the bronze disk loses most of its energy in intense, sporadic acoustic pulses along networks of grains, or force chains, in the bed of beads.

"This phenomenon was so hard to observe before because of how fast the force chains travel," Behringer said. The standard movie rate is about 30 frames per second. To capture the path of energy down the force chains, the scientists had to use a camera that could capture 40,000 frames per second, 1300 times faster than a normal video, because the sound pulses move at such high speeds.

The scientists shot the intruding disks into the photoelastic grains at speeds up to 6.5 meters per second, about 15 miles per hour. On impact, the force chains in the disks started moving the energy away from the intruding object, dumping it down deep in the bed of disks like the drainpipes of a septic system carrying water and waste away from a house, Behringer said.

The speed of the bronze disk was well under sonic or super-sonic speed, which could make the patterns of energy transfer substantially different, the team noted in the paper. "For supersonic speeds, it's kind of like the car chases that happen in markets in movies. People can't get out of the way fast enough. Similarly the pulses wouldn't clear the chain networks and the forces would back up rather than get carried away from the intruder," Behringer said. 

Studying the impacts at sonic and supersonic speeds, however, is a set of experiments that requires different grain particles, Behringer said, adding it's one the team may try soon. He also explained that once a missile or meteor drops below sonic speeds, the grains absorbing its impact would carry the and momentum away jerkily and sporadically, just as the team's new microscopic picture shows.

Explore further: How granular material becomes solid: Stress causes clogs in coffee and coal

More information: Clark A., Kondic, L., and Behringer, R. Particle Scale Dynamics in Granular Impact. Physical Review Letters 5:137 (2012).  DOI: 10.1103/Physics.5.137

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2.5 / 5 (4) Dec 11, 2012
There may also be an element of fluid dynamics involved as the surface of a natural impactor may be liquified (or near that) when it strikes.
5 / 5 (6) Dec 11, 2012
would love to see the video for this
1 / 5 (4) Dec 11, 2012
There may also be an element of fluid dynamics involved as the surface of a natural impactor may be liquified (or near that) when it strikes.

Perhaps even gaseous.

A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures

3.5 / 5 (11) Dec 11, 2012
would love to see the video for this

found it for you

5 / 5 (1) Dec 11, 2012
Its completely obvious from studying the movie that the lifetime of these "force chains" increases with depth. I am wondering what experimental parameters cause this, or its simply a natural phenomena of the experiment. In other words, I am wondering if this is an experimental artifact.

One way to see if this is so would be to vary some of the parameters, like bead size, or experiment with randomly shaped particles, like sand for example.
3.7 / 5 (3) Dec 11, 2012
The results of this experiment are fairly intuitive. Imagine a pool (billiard) table nearly packed with balls, and how a cue ball would initiate chains of interactions. But just how well would these results apply to a material where the particles are welded tightly together, or where the pore space is filled with a cement, there's enough grains of varying size that there is no pore space?
1.8 / 5 (5) Dec 11, 2012
from a first impression, this seems like the mechanical analog of how electrical discharge in lightning propagates. The projectile's mass acts as a velocity source where power/mechanical waves propagate through the force-chains to a reference point in the ground where the velocity is zero.

charge separation between clouds and the ground act as a voltage source. After the voltage rises to a certain point, charges (current) begin to do a gradient search, ionizing the gas to a plasma state as they find a path to the reference ground.

Air is a bad conductor. Free ions in a plasma act as an excellent conductor. Therefore current then flows in surges through this plasma filament until it reaches the reference ground where the voltage gradient is zero.

These conductive filaments are the electrical analog to the force-chains seen in this study. The voltage/velocity gradient drops the least along these paths and we see power flow carried in waves through their respective domains
1 / 5 (3) Dec 11, 2012
I wouldn't be surprised if this is also related to the process of liquefaction resulting from earthquakes, which turn soil into a fluid allowing heavy object on its surface to sink. And, doesn't it look like lightening? One wonders if there's a relationship there, too, in the way energy distributes itself in a fluid medium...
5 / 5 (1) Dec 12, 2012
@Parsec. "Its completely obvious from studying the movie that the lifetime of these "force chains" increases with depth. I am wondering what experimental parameters cause this".

Nice catch. FWIW, the movie ends with the text "We can correlate the deceleration of the intruder with the acoustic behavior we observe". Increased deceleration = increased acoustic force?

@ Argiod: Meet infinion, in the comment above yours...

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