Government/Military Trends
October 2004
The Basics of Composites
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Composite materials are found in everything from office buildings to space ships. They are an attractive material for aviation and aerospace applications because of their strength-and-stiffness-to-density ratios.
What is a composite?
Composite materials consist of combinations of two or more components. One serves the role of matrix, the component that holds all the materials together. Composites are made by embedding reinforcements into the matrix. A matrix gives the composite shape and form and can be a polymer, metal, or ceramic. The strength of the material comes from reinforcements — fibers, flakes, particles, or fillers. The resulting material has better mechanical properties than the individual components.
An example composite made with this combination process is glass fiber reinforced plastic, a common composite used in the aerospace industry. Glass fibers are strong, but damage easily. Plastics are tough and malleable, but most do not offer the same strength properties as glass fibers. Embedding glass fibers in plastic protects the fibers from damage and produces a superior material.
Different makeup, different properties
Individual materials have different qualities, and so does each composite type. Particulate composites have the same properties in every direction, so it responds equally if you apply a force to it from the back, front or side. This property is the same for conventional metals like aluminum or steel. In metals, this quality has a cost: weight, which is why composites are attractive.
Fibrous composite has property variations depending on the direction. Because all the reinforcements of a fibrous composite run in the same direction along the fibers, it has more strength against a perpendicular force than a parallel force.
The value of composites
The major advantage for composite use in aviation is the high strength and stiffness to weight ratio. Metal becomes very heavy when you are tying to propel it off the ground. Though metal properties are consistent and unchanging, some of those properties may not be needed or may even be a detriment. Lighter composite materials allow for designs that would not otherwise be possible. Composites also reduce the amount of fuel needed to propel an aircraft.
Composites accommodate complex shapes in manufacture, reducing the number of pieces joined by fasteners and joints in assembly. Fewer fasteners and joints aid in the overall strength of the resulting component by reducing the number of holes. Holes, a stress concentration, are a prime candidate for potential crack-initiation. Fewer fasteners and joints also reduce weight and assembly time.
Points to consider before using composites
The cost of materials and fabrication can make composites costly. There are also maintenance issues. Depending on use, some composites cannot be repaired. For example, a dent in the metal door of an airplane can be banged out. That same door made from a composite material might need to be replaced — certainly a more costly solution.
Even at a higher cost, any material that can equal or surpass the properties of conventional metal and carry less weight is of great use to the aviation industry. The challenge is to ensure these materials meet aviation quality and performance standards. As fabrication processes improve, the costs associated with composite development will go down. This will only serve to expand the role of composite materials in aircraft and spacecraft development.