In previous methods, a symmetric set of differently oriented layers were required. "For example, the top four layers were oriented in one direction, and the bottom four in another," said Kelkar. "So you needed eight layers of composites to avoid bending and warping. Now you don't need as many layers or even an even number of layers, and the composite is much flatter."
Also, in previous composite structures, resin collected in large pockets. Resin is weak, and fibers are strong, so resin pockets must be minimized to produce a stronger material. "With this structure and process, we can minimize the resin pocket areas, so they are almost five times less than in previous composites," he said. "This has made the new composite's strength uniform."
Bowers said the resin pockets are reduced by using a nonwoven material created with a small, fine carbon thread. "All these carbon fibers are laid down flat-oriented in one direction and stitched together. Then another layer of fibers is laid down on top of the first at a slightly different angle and stitched together and then infused with resin. This process lets the composite be flatter than can be achieved by weaving."
The proper angles for layup are critical, and so are the number of plies, he said.
Advaero's HVARTM process reduces the viscosity of the resin infused into the carbon fiber material, so it doesn't move the threads, according to Bowers. "Instead, it flows evenly into the carbon fibers, so the space between fibers is kept to a minimum, allowing everything to lie flat."
Kerkal said conventional composites that are subjected to enough loading fail where the large resin pockets are located. Once failure starts, the composite breaks into two pieces. "In the new fabric, the resin pockets are very small and distributed all over the composite, and it's at those many points that failure starts." Instead of breaking into two pieces when it fails, the new composite breaks into hundreds of small pieces -- somewhat like safety glass.