Concrete Security
Ramesh and his collaborators at other universities are looking at some of the basic issues raised by protective materials, including the concrete in blast walls. What’s the best way to design these materials for the greatest safety and efficiency?
Specifically they’ve asked if a barrier, say, should break up into small pieces, large pieces, or not break up at all? If it should break up, how can it do it in a way that limits damage and injuries? “That’s the fragmentation problem,” Ramesh says.
Engineers studying blast effects previously could study the behavior of only a few dozen of the largest pieces of a material hit by a bomb; the accompanying blizzard of rubble and dust, they assumed, could be treated like a cloud of debris rather than fragments. There were just too many things happening in a blast—too fast at every scale, large and small, to identify and track.
“In the past, we could never understand the little pieces,” Ramesh says. “We couldn’t track them. We couldn’t see them. We couldn’t calculate them. They were too small. So we had to ignore them.”
But now, advanced cameras can track something being blown to bits microsecond by microsecond, from the first instant of the explosion until the dust has settled. Microscopes can show the impact of a blast on a material at the scale of several atoms. And fast computers can model the physical forces of the blast and simulate its effects on different materials.
The working assumption that the small pieces didn’t matter was wrong. “It turns out the little pieces matter a great deal,” Ramesh says. “The little things are where a lot of the action is.”
These fragments soak up a lot of the blast’s energy, so they play a major role in limiting its reach. On the other hand, they account for many of the injuries in bombings, so blast walls can’t be designed just to shatter into rubble. Ramesh’s institute is working with labs at CalTech, Rutgers, and the University of Delaware to develop new computer models as part of a 10-year $90 million grant from the U.S. Army Research Laboratory.
The aim is to subject new materials, which at first may exist only in cyberspace, to these environments and predict how they act. The result, they hope, will be some tough new stuff. Something as seemingly simple as a lighter, more efficient blast wall, Ramesh says, could make “a huge difference.”
“In practical terms, it’s the difference between saying you need a big concrete wall in front of your building and saying you need a smaller wall and a certain kind of glass in the windows,” he says.
While you may not be able to eliminate blast walls entirely, he says, you may be able to make them easier to live with.
Visionary Defense
An improvised explosive device (IED) triggers a blast wave that travels at 1,600 feet per second— faster than the speed of sound—and can shatter glass, splinter wood, and mangle steel. But its most precious and vulnerable target is the human body.
An explosion can do terrible damage to all the body’s tissues, including the eyes—which are difficult to protect because we need to be able to see. A blast can pick up debris that will cut or puncture the eye. Soldiers hit by an IED can also suffer internal bleeding in their eyes, as well as damaged lenses and detached retinas.
A blast can pick up debris that will cut or puncture the eye. Soldiers hit by an IED can also suffer internal bleeding in their eyes as well, as well as damaged lenses, and detached retinas.
“What is not really known is what damage is caused just by that blast wave itself, not by fragments or being thrown by the blast,” says Thao Nguyen, assistant professor of mechanical engineering at the Whiting School.
So Nguyen, a member of Ramesh’s HEMI team, is building a digital model of an eye inside a typical male head, and has started to test it using computer programs that simulate the pressure of an explosion hitting the skull from various angles.
She’s begun by measuring the effect of the blast on the eyeball. That’s the tough external shell that protects the very delicate internal structures including the retina, lens, and choroid, the vascular layer of the eye.
One of the first things she’s looked at is how the anatomy of the face handles the pressure wave from a blast. “How does the blast wave scatter about the orbital features— your brow, and your nose ridge, your cheekbones? How do those facial features serve to either deflect or focus the blast wave onto the eye?” asks Nguyen, who is working with a grant from the U.S. Army Medical Research and Materiel Command in Fort Detrick, Md.
Some of her initial findings could help explain why 10 percent of injured soldiers have eye injuries, a much higher proportion than would be expected given the eyes’ size relative to the body.
It turns out that the design of the human face, which is good at deflecting blows from rocks and sticks, is lousy at protecting the eyes from a head-on blast wave. In fact, it can make things worse. “What we find is that if the blast is directly in front of you, your brow and nasal ridge together act as a reflector, as a focuser, so it takes the incident pressure wave and it focuses onto the eye,” she says. “And the eye becomes the largest region of pressure on the face.”
But when a blast comes from below, facial features can offer partial protection.
So far, Nguyen’s computer model of the eye includes only the fluid-filled shell and external tissues—the protective transparent cornea and the sclera, or white part of the eye. Her model does not include the delicate internal parts, including the retina, lens, and blood vessels.
But her research suggests a blast wave fluid inside it, potentially causing injuries.
Eventually, Nguyen and her team hope to test the military’s protective eyewear to determine if it really protects the eyes or if, as has been suggested, the blast wave may bypass the lenses through a process called “underwash.”
They also plan to build a physical model of the head and subject it to real-life explosions, to double-check the reliability of their digital soldier.
“Computational models are great,” she says. “You can make anything happen in a computer. But whether or not it reflects reality has to be validated.”
A higher proportion of gravely wounded in Iraq and Afghanistan are surviving than in previous wars, thanks to advanced armor and trauma medicine. Nguyen says she hopes her work will one day help develop better protective equipment, as well as new methods of diagnosis and treatment for those already injured.
“If I can help surgeons understand, or help the designer of armor understand, how these mechanical loadings [from the blast wave] can effect injury, then that can go into directly helping the soldier,” she says.