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		Polymer Matrix Composites The airframe (essentially the structure of the plane without the engines) of a plane can be made up of many materials. Historically, the structure was made almost exclusively of aluminum, with steel used in key support areas such as engine mounts and landing gear. As jet engines were adopted, titanium was introduced to combat the high temperatures imposed on the airframe. Following titanium were advanced composites - graphite and boron filaments in a cured epoxy matrix. These composites were significantly lighter, and could be manufactured to improve strength in a particular direction (thus designing a part for a specific purpose). Today's planes use combinations of these materials, and are constantly being improved with new materials. When selecting materials for a part, design engineers must think in terms of "design tradeoffs". This is a careful balancing act of choosing the lightest material which still meets part strength requirements, all the while staying within cost budgets. Graphic courtesy of ASM But there is more to airframe development than just selecting material types; choosing manufacturing methods can have a huge impact. Powder metallurgy can be employed to increase strength over the same material made through traditional methods. Diffusion bonding of superplastic metals (a mouthful, no?) creates a bond that is indistinguishable from the parent metal. Shown here is a diagram of a military transport jet, the Air Force C-17, illustrating part locations which have been manufactured from polymer matrix composites. The parts have been replaced where corrosion protection and weight reduction is possible and cost effective. The composites used here are various mixes of carbon, aramid, glass, and DuPont Nomex fibers. Why are composites made of fibers? This is a good question - why not just stick several sheets of a material together with the matrix 'glue'? The answer has to do with imperfections (it is, after all, an imperfect world out there). Almost all high strength/high stiffness materials fail because of the propagation of flaws. A fiber of such a material will be stronger than the bulk material because the flaw is limited by the small diameter of the fiber. Even if the flaw does make the fiber fail, it will not cause the whole composite to fail, since there would be many other separate fibers still intact. Try that with a flawed solid sheet! Shown are two photos dealing with composites. The top photo shows the weave of the fibers in a diagnal (notice the diamond pattern). The other photo shows a key part to composited materials, the core. This core is a honeycomb structure, used for its strength and light weight. For more information, read "Polymer composites for aerospace", in Advanced Materials & Processes, Volume 149, No. 3, March 1996, p. 37-39, by ASM International. Or you could read "Introduction to Polymer-Matrix Composites", a chapter in the Desk Edition of the Engineered Materials Handbook, published by ASM. Site related questions or comments? Contact crc@tms.org