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1 Introduction Revolutionary advances in structural materials have been responsible for revo- lutionary changes in all fields of engineering. These advances have had and are still having a significant impact on aircraft design and performance. Early aircraft con- struction involved wood, fabric, and wire, which later gave way to metals, notably aluminum. Aluminum has given way to selected use of other higher-strength metals (titanium, steel, and superalloys), and both are giving way, to a significant degree, to composite materials. Composites are engineered materials. Their properties are tailored through the use of a mix or blend of different constituents to maximize selected properties of strength and/or stiffness at reduced weights. A common composite approach is to use a matrix or host material reinforced by a fibrous second material. These composites can be ceramic, polymer, or metal based, or mixtures of these materials. Of special interest in this study are filamentary (organic) polymer systems, herein commonly referred to as advanced organic composites. More than 20 years have passed since the potentials of filamentary composite materials were identified. In a report dated July 1964, the Scientific Advisory Board of the U.S. Air Force recommended the intense development of boron filaments. The board identified significant gains in aircraft weapon-system performance through application of boron composites because of their low densities and high strengths and stiffnesses per unit of mass. During the 1970s, however, much lower-cost carbon filaments became a reality and gradually designers turned from boron to carbon composites. By 1971, there was so much unfettered enthusiasm for carbon epoxy that 16 suppliers were marketing over 50 brands of carbon-epoxy preimpregnated (prepreg) materials. The boron- epoxy material system was developed with substantial assistance and direction from the government through the Air Force Materials Laboratory, but the carbon-epoxy material system received only limited government assistance and direction.
The list of composite achievements over the past two decades is long and im- pressive. Two high-performance military airplanes, the F-18 and AV-8B, currently in production, utilize carbon-epoxy for 10 percent and 26 percent of their structural weight, respectively. These carbon-epoxy percentages include appreciable portions of the primary structural elements of the wings, empennages, and control surfaces of these aircraft. Two new transports, the Boeing 757 and 767, each use about 3,000 pounds of carbon-epoxy in rudders, elevators, and spoilers. Two aircraft under de- velopment, the U.S. Navy Osprey V-22 and the Beech Aircraft Starship, merit the appellation "all-composite" because nearly all of the structural components that can gainfully use composites are made of composites. Despite these and other examples, filamentary composites still have significant unfulfilled potential for increasing aircraft productivity; the rendering of advanced organic composite materials into production aircraft structures has been disappoint- ingly slow. This report addresses why and recommends research and technology de- velopment actions that will assist in accelerating the application of advanced organic composites to production aircraft.
