Friday, August 1, 2008
Composites and Combat
This article focuses on composite materials and their contribution to survivability in combat aircraft applications.
Composite materials, specifically fiber-reinforced polymers, provide manufacturers with amazing improvements producing structural strength and flexibility. These composite panels vary from cowlings and integral structural members to all-composite rotor blades replacing formerly all-metal components. Composites offer strength increases, weight reductions (depending on compounds and thicknesses), and flexion tolerance for bending, compression, tension, and shear loads that would work-harden or fatigue-weaken metals in the same application. Their manufacture is often complex, costly, and time-consuming, but well worth the investment for the survivability benefits derived.
In combat there are four structural "kill" modes that affect aircraft: pressure overload, penetration, structure or area removal, and thermal weakening.
Composite structures and panels allow manufacturers to design and build passive protection into panels and cowlings using the inherent provision for integral armor plating. In fact, the most common seat armor, ceramic-Kevlar, is a composite. Integral armor provides built-in protection for penetration prevention or ballistic-velocity reduction. Strength and component life is enhanced through fiber-weave patterns designed with layering and bias techniques to reduce harmonic vibrations and to distribute vibration and flexion loads in ways that can make component life unlimited.
When combined into a structural panel, this produces structure and armor in one panel that protects occupants and systems from pressure overload and penetration. One can see the benefits for all-composite rotor blades, which eliminate the older metal laminate design and associated fatigue limits of solid-metal parts in a highly repetitive flexing and twisting environment.
When structure is removed by kinetic or explosive force, strength is also removed. The difference is that composites are layered, woven materials whose ply biases are multidirectional (much as plywood grain is alternated 90 deg for each layer). Composites spread the stress loading around the damaged, missing material in a way that allows the surrounding, fiber-reinforced material to absorb the loads uniformly. In solid materials like metal, damage like this tends to propagate cracks from the damage area under load and can cause catastrophic failure starting at the hole. However, composites aren’t a panacea. If structural material is removed from a key joint or edge, strength can be reduced to failure. The likelihood that composites will catastrophically fail in this way, though, is significantly lower than solid metals. Thermal weakening is a condition in which fire or intense convective heat causes material to soften and weaken. Metals, with a few high-tech exceptions, are adversely affected by this vulnerability. Composites can be designed to tolerate extreme heat without melting or softening. Phenolics are the resins most commonly used in these applications due to their low heat-transfer rate. Designers often choose composites tolerant of high heat for structure surrounding fuel cells to provide both ballistic protection (and thus prevent hydraulic-ram effects inside fuel tanks) and thermal. In this way, penetration, overpressure and thermal protection are provided.
Composites that are ignited in combat can pose problems if they support or promote combustion or release toxic chemicals as they burn. The U.S. FAA Office of Aviation Research’s report "Health Hazards of Combustion Products From Aircraft Composite Materials" (DOT/FAA/AR-98/34) expands on these issues. "The primary fire hazards of interior and secondary composites used in aircraft cabin and fuselage components are the heat-release rate and the toxicity of the gaseous combustion products from the burning polymer matrix. The aircraft cabin occupants are exposed to this hazard during an impact-survivable accident."
Are the vulnerability-reduction benefits of structural redundancy, flexion tolerance and load distribution worth these risks? Absolutely, if the materials keep the aircraft structure from catastrophically failing in flight.
Today’s composites are designed for relatively simple battlefield damage repair. Field kits include vacuum-formed membranes to emulate the surrounding surface and help blend repairs using room-temperature curing resins. To learn about these repair techniques, readers may contact SURVIAC at 937-255-4840 or DSN 785-4840. For U.S. military units, the video "Battle Damage Repair of Composite Structures" is available for $75. Requests from non-U.S. agencies must be forwarded to the air attaché’s office in their country’s embassy in Washington.
Composites are a promising and ever-developing technology that are pushing performance and survivability to previously unimaginable levels. Future combat aircraft will likely use predominantly composite structures for all of the benefits mentioned above.
Lt. Col. Steve Colby, USAF (retired), is the former commander of the 34th Weapons Sqdn at Nellis AFB, Nev., which trains combat search-and-rescue helicopter instructor pilots. He can be reached at firstname.lastname@example.org.