A MONOCOQUE (MOW-no-coke) structure is one which uses its outer skin as a structural member to bear shear or load force due to bending which is accomplished through the distribution of these forces. They can be divided into three groups - monocoque, semi-monocoque, and reinforced shell. Monocoque comes from a French word meaning "single shell".
Monocoque design is primarily facilitated by placing planes perpendicular to the direction of load. This translates load into shear, which is distributed fairly evenly throughout the structural member. It allows a very lightweight and low-volume piece to handle a great deal of load.
Any material can be used to create monocoque structures - A cardboard box is actually an example of monocoque design, as is the honeycomb structure used in many lightweight materials, including the skins of modern military aircraft. The box frames used in motorcycles, and the combination chassis/body of the McLaren F1 Road Car are also monocoque designs. Monocoque designs first came into vogue in aircraft, as a way to save weight by transferring load from a heavy frame to the skin of the craft, which is a required component.
While monocoque design has seen limited use over the years in favor of more widely used designs like ladder chassis and tubular steel frames, a resurgence in interest has come with the development of inexpensive carbon fiber. Carbon fiber is especially well-suited for monocoque designs because it can be designed for shearing loads in specific directions, and be made semiflexible in others, while it has an extremely light weight. This fits in with the design goals of monocoque systems nicely. Monocoque designs also require somewhat arbitrarily shaped pieces to be used, and it is easier to craft carbon fiber into odd shapes than to use steel.
Mossman, Ralph W. and Russell G. Robinson. Bending tests of metal monocoque fuselage construction. NACA TN 357, Nov 1930.
Webpage: Harcourt Academic Press Dictionary of Science and Technology, monocoque (http://www.harcourt.com/dictionary/def/6/6/3/8/6638900.html)
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