We’ve all seen this scenario in a movie: the hero is trapped on the roof of a burning building or drowning in a stormy sea. A helicopter swoops in, drops a long steel cable down to him, and plucks him from his quandary. This sort of rescue does happen in real life. Just recently helicopter rescues saved lives during Hurricanes Harvey, Irma, and Maria; but something is missing in the movies.
In reality approximately one out of every five times a steel cable is lowered from a helicopter, the human who touches the cable experiences a huge shock of static electricity caused by the rotating helicopter blades. The United States Navy is determined to fix this problem by finding a new material for the cables. The Navy contacted Cornell University for help, and the Cornell Center for Materials Research (CCMR) immediately directed the Navy team to Stuart Leigh Phoenix, Sibley School of Mechanical and Aerospace Engineering.
Solutions—from Cornell Research to Spinoff Company, and Back Again
Phoenix’s expertise is in materials with extremely strong fibers in a polymer or epoxy matrix. His research involves theoretical and experimental studies of the long-term reliability of these fibrous composites under high stresses. In response to the Navy’s request to develop a new fiber cable, Phoenix joined together with Advanced Design Consulting USA Inc. (ADC)—a developer and supplier of complex scientific components and instruments founded and run by Phoenix’s former student, Alex Deyhim, MA’93, MEng/MBA’98—and the Cortland Company (part of Actuant Corporation), which designs and manufactures synthetic ropes and cables.
ADC funded the project through CCMR’s Industrial Collaboration Program. CCMR also supplies funding for postdoctorate Muhammad M. Rahman, PhD’16 Fiber Science, to work with Phoenix on the project.
New Cables, Old Winches
“The Navy had the idea to make the cable out of a high-strength polymer material like Kevlar®, with an outer jacket of polyurethane,” Phoenix says. However, the Navy also wants to continue to use the same winches for the new cable as they used for the old. This means the new fibrous cable has to work well on winching equipment that was designed for steel cables, resulting in limitations on the new cable’s thickness and lateral stiffness properties, particularly in keeping it round when wound onto a drum under tension. “Building up a jacketed cable with enough fiber on the inside to have a breaking strength of 3,200 pounds yet with a thick and tough enough jacket on the outside has been problematic,” says Phoenix.
Knowing that Kevlar® was unable to fulfill the need, the researchers turned to another fiber material Phoenix was familiar with through his work on bulletproof vests known under the trade names Dyneema® and Spectra®. Dyneema® fibers (manufactured by Royal DSM in the Netherlands) and Spectra® fibers (manufactured by Honeywell International Inc.) are made of highly drawn and stretched polyethylene, also called ultra-high molecular weight polyethylene (UHMWPE). Pound for pound, the resulting UHMWPE fiber is 12 to 15 times stronger than steel, and it weighs slightly less than water.
“We wanted to see if we could pack these fibers more tightly in the cable and only use half the cross-sectional area that Kevlar® needed,” Phoenix says. “That would still leave room for a thicker outer layer for the jacket. The more jacket you can have, the more robust the cable will be against abrasion and the more stable it will be when wrapped and squeezed around the drum.”
The researchers asked Cortland to create prototype cables. One of these, made using Spectra®, had the strength the Navy was seeking. “That specimen was very successful,” Phoenix says. “We got tremendous strength out of it, but the jacket wasn’t thick enough. I’d like to see it improved, but they were very happy with it because no one else had succeeded in giving them something that passed the first test.”
The original Navy grant has ended, and Phoenix and ADC have delivered a successful prototype, but Phoenix continues to work on the cable with Rahman—supported by CCMR’s Industrial Collaboration Program—and with ADC, which is building special devices to test the strength and fatigue of future versions of the cable. “Right now, we’re focusing on understanding how the cable works,” Phoenix explains. “We’re looking at all its properties besides strength. For instance, we want to understand what kind of stiffness we can get for wrapping around the drum. I don’t want to go back to ask for more funding until we’ve got something to show. That’s how I operate; you go in when you’ve got the answer.” The limit on UHMWPE fiber strength has not been reached. There’s still room for up to a 30-percent improvement.
Bulletproof Vests: Putting Theory to Work
This combination of practical and theoretical work is what Phoenix is known for. “I’m in the trenches,” he says, “but I’ve got papers that I’ve published in math journals, physics journals; I span the gamut. Every good practical problem usually has a mathematical question, a physics question, somewhere in there.”
“People had sort of empirically figured it out, but I wanted to solve the problem. Some 200 equations later, I had it. That got me into the business of body armor.”
Back in the early 2000s Phoenix caused a stir in the ballistics world by modeling the physics of how bulletproof vests work. He published a paper in 2004 that was the first to show mathematically the reasons why all the data available on bullet penetration plotted on a master curve the way it did. The paper has now been cited almost 100 times—a rarity for papers on ballistics. “People had sort of empirically figured it out, but I wanted to solve the problem. Some 200 equations later, I had it. That got me into the business of body armor.”
Understanding the physics of bullets and armor allows researchers to create better bulletproof material. To further Phoenix’s research in this area, the National Institute of Standards and Technology awarded him a five-year grant, now beginning its third year, to continue modeling how bulletproof vests work. Phoenix is creating computer models built on the ballistics principles he helped define.
“There are all kinds of things a computer model built on principles can do that you can’t do with mathematics and a pencil,” he says. “I’m creating a model that can be done on a laptop. You enter in parameters and produce models to get some understanding about whether an idea will work. Often they show that it won’t; you realize a lot of ideas don’t have a chance of working once you really understand what you have.”
It was through his early ballistics work, too, that Phoenix first became involved with Royal DSM, the company in the Netherlands that makes Dyneema®. In his office at Cornell, Phoenix has multiple examples of Dyneema® fibers woven into airy sheets of material, each slightly heavier than the previous, all of them seemingly more like summer clothing than armor. By making body armor out of multiple layers of the material, he says, they can even stop a bullet from an AK-47 assault rifle. “It’s just a question of how many layers you need,” he says.
Body armor research also requires both the practical and theoretical sides of Phoenix’s expertise. “You have to learn the nitty-gritty details of how they make it,” he explains. “It’s not just a math problem, but the mathematics of how things work is very useful to see why something might be particularly successful or to pinpoint its Achilles heel. That’s what I really like about my work: it’s a blend of theory and real applications.”