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High-Strain-Rate Behavior and Impact Dynamics
Strain rate testing is a great concept when the details of the impact, environment, or wear conditions are well-understood and can be reproduced and modeled. But often even getting that far stretches what we know. Prof. Ramesh also uses the equipment in his LIDAR laboratory to study impact dynamics—the deformation, flow, and failure of materials during the very first milliseconds of an impact event.
Strain rates that occur during an impact vary from as gentle as 10-5 per second to as fierce as 10+8 per second, depending on the event. A bat hitting a baseball is about 10+2 per second. A projectile hitting a tank or a bullet hitting a bulletproof vest is about 10+5 per second. An extreme event like a micro-meteorite hitting the space station, a meteorite hitting the Earth, or a nuclear explosion would be about 10+8 per second. As the impact event proceeds, the strain rate usually falls off quickly. A meteorite travels at tens of thousands of miles per hour; at the moment of impact, huge shock waves are generated that propagate away from the impact site, generating most of the damage. At the moment the shock wave arrives at your location, the strain rate is about 10+8 per second. After 10 microseconds, it’s decayed to 10+5 per second, and in a matter of milliseconds, we’re back to baseball, at 10+2 per second.
Pressure during impact also varies dramatically, going as high as 1 million atmospheres in some events. Huge pressure changes like this can do all sorts of interesting things to materials. Glass is created at meteorite impacts. Liquids can turn to solids because the high pressures effectively lock the molecules in place. Cracks that might have lurked in ceramics are forced closed under high pressure, making them stronger than before.
To test material behavior in the initial milliseconds of an impact event, Prof. Ramesh takes small pieces of a material and deforms them very, very quickly. From measurements taken during the test, he can predict what would happen in an actual event. If we understand extreme events well enough, he argues, perhaps we can make modifications to armoring materials or somehow alter the impact event in an effort to minimize damage. Perhaps he will find an answer to the nagging question that keeps some of us up late at night: Would it be better for us to get hit by a big meteorite, or to break the meteorite up in space and get hit by lots of little meteorites?