Scientists led by Dr Abram Clark of Duke University and Yale University developed a novel technique which enabled them to simulate accurately high-speed meteorite impacts.
This image shows an asteroid hitting Earth. Image credit: Don Davis / NASA.
To simulate a meteorite slamming into soil or sand, Dr Clark and his colleagues from the Duke University’s Department of Physics & Center for Nonlinear and Complex Systems, and the New Jersey Institute of Technology’s Department of Mathematical Sciences, 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 scientists 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 15 miles per hour (6 m per second). But by using beads of varying hardness, they were able to generate pulses that surged through the beads at speeds ranging from 67 to 670 miles per hour.
Frames from a video of a metal object slamming into a bed of artificial soil; shown at slow (top), medium (middle) and high impact speeds, the changing impact forces illuminated in each frame help explain why soil and sand get stronger when they are struck harder. Image credit: Abram Clark.
Each impact was too fast to see with the naked eye, so they recorded it with a 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 Prof Robert Behringer of Duke University, a co-author on the study published in the Physical Review Letters.
“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.
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Abram H. Clark et al. 2015. Nonlinear Force Propagation During Granular Impact. Phys. Rev. Lett. 114, 144502; doi: 10.1103/PhysRevLett.114.144502
