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The blast wave that comes with an explosion does far more damage—to bodies, buildings and vehicles alike—than any projectiles or fragments that come along with it. Thickening an exterior with armor or anything else will cut down the impact of a blast, but it comes with heavy economic and weight costs.
Now, researchers at the City College of New York’s Experimental Fluid Mechanics and Aerodynamics Laboratory have shown that a thin coat of porous nickel or copper, carefully designed at the nano-level, can cut down the force of an explosion by more than 30%. The material is designed to be as porous as possible so that the surface area to volume ratio is as large as possible. In essence, the elasticity of the porous nanomaterial bounces a portion of a blast’s energy back in the direction from which it came.
Normally, when a blast hits a wall all of its energy is transferred into and beyond the wall. After it’s hit, there is no longer any flow in whatever gas the blast wave has passed through. “The flow behind the shock is zero velocity, because that’s the velocity of the wall,” says Yiannis Andreopoulos, a professor of mechanical engineering at the school, who lead the project. “But now there is a velocity to the wall, so the flow goes in the opposite direction. Without the nanoparticle there’s no reversal.”
Stress and Strain
To test the effectiveness of their nanoparticle material, Andreopoulos ran both a real world macro scale experiment and a model. The later abandoned Navier-Strokes equations usually used to look at the flow of fluid and instead turned to molecular dynamics simulations. “When we go to very small sizes, the classical continuum mechanics doesn’t work anymore,” says Andreopoulos. “We have to come down to individual interactions between atoms and molecules.”
The model allowed the team to look at stress and strain, usually bound by constitutive law, separately.
The nano-coating also shows high strain rate effects. Compress a dog bone slowly and it will endure a certain amount of stress and strain. But if the same force is applied suddenly the bone becomes temporarily stronger. Andreopoulos’s material exhibits this effect to the tune of 30%.
Right now the layer that Andreopoulos is experimenting with is a single micron thick. Once it’s become a commercial product he expects that the layer will be 50 microns. The increase in effectiveness that might come with an increase in thickness has yet to be explored.
The fruits of Andreopoulos’s research are not limited to potential applications. His model and experiments offer insight into how waves propagate through porous material. They may help explain how an ultrasound’s waves are dissipated by small pores in human tissue and help us better understand the effects of underground gas stored in the tiny pores of rocks.
But the high performance of the material, its relative low cost of production, and the ease with which it can be applied, does suggest that much of the world will soon be coated in the new material.
Put it on the outside of a bus, for instance, and the vehicle and people inside will be more likely to survive an explosion. “It’s going to dissipate,” says Andreopoulos. “There are some ideal cases where the subject would be protected by it.” Fuel tanks or other parts susceptible to shockwaves could be coated as well. Passengers wearing coated clothes would be more greatly protected from an internal explosion.
The military applications are myriad. “They can coat everything,” says Andreopoulos, who has indeed received calls from the army. The next generation of airplanes, which are already slated to have a porous coating for sound absorption, could combine it with the nanomaterial to help mitigate potential blasts.
It’s understanding elasticity at the smallest scale that will protect us from dangers at the largest.
برچسب ها : Nano Stops the Shock Wave ,