To cover this dome, Kouba wanted a material that was lightweight, affordable and provided excellent ballistic performance. He investigated dozens of options, including a steel dome sprayed with ballistic fiber, but that proved to be too heavy. His search eventually led him to Innegra Technologies, which manufactures Innegra™, a very high-modulus polypropylene (HMPP) fiber with good ballistic properties. Innegra is also hydrophobic, so it doesn’t easily absorb moisture.
Light weight was very important for the Tornadopod application. Innegra is already a light material, and the extrusion process used for its production makes it even lighter. (Kevlar weighs 1.44 grams per cubic centimeter, while Innegra weighs .84 grams per cubic centimeter.) Other materials that Kouba had previously tested required a laminate so thick (1¼ inch) that labor costs were astronomical. With fabric woven from Innegra fibers, however, the part was slightly under ⅜-inch thick and 20 percent the weight of the other laminate.
The properties that Innegra brings to a composite depend on where the material is placed within the laminate. A client who wants a lighter part may use Innegra as a core material. “Added to a composite structure close to the surface of the laminate and it all but eliminates stress fractures,” says Russ Emanis, Innegra Technologies’ chief composites engineer.
The Tornadopod’s composite stack consists of the Innegra fiber layered with a number of other traditional ballistic materials, including specialized glass mats that absorb energy and help slow down the projectile before it gets to the Innegra material. The stack is then infused with a ballistic resin using a vacuum process.
After modeling and testing various composite designs, Innegra developed a material that could pass the Federal Emergency Management Agency’s stringent tests for approved tornado structures. The Tornadopod’s dome was undented after it withstood the impact of a 16-pound, 2 x 4-foot piece of lumber fired at more than 100 mph.
Kouba staged other dramatic tests to demonstrate the strength and effectiveness of the Tornadopod’s composite ballistic shield. His team dropped a full-size SUV on it, first from a height of 20 feet and then from a height of 35 feet. (Kouba was inside the Tornadopod for the second drop). “The pod just flexed with it, absorbed the energy and literally pushed the SUV off the pod,” he says. “The Innegra absorbs the energy, flexes and comes back without breaking.”
No breaking. No shattering. Complete protection. That’s the ultimate goal of all ballistic composites. It’s a lofty one, but manufacturers and suppliers devise new materials and methods each day to provide greater protection.