According to Webster's Dictionary, a composite is: "a solid material which is composed of two or more substances having different physical characteristics and in which each substance retains its identity while contributing desirable properties to the whole".  Adobe, with mud and straw, is an example of a primitive composite.

At Automated Dynamics, when we talk about a composite, we are referring to a material with two components, a "fiber" and a "matrix".  The fiber gives the material its strength (defines many principal mechanical properties). The matrix supports the fibers and transfers the applied loads between them.  In other words, the matrix is the reason that the fibers can work together.  Staying with adobe as the example, the straw is the fiber and the mud is the matrix.  The straw gives the material its strength, while the mud gives the material its shape and holds the straw in position.

The image to the left is a photomicrograph taken of the end of a section of continuous carbon fiber, thermoplastic composite tape.  In this case the tape was ½" wide and 0.006" thick.  The black, vertical lines indicate the outer edges of the tape and show the tape thickness (0.006").  The whiter circles are the ends of the fiber.  In this case, the fibers would be traveling perpendicular to (i.e. out of) your screen.  The light grey in between the surfaces is the matrix (thermoplastic) and the black areas between the tape surfaces show voids (areas without fiber or resin).

In recent years, the term "advanced composite" has been adopted to most frequently refer to continuous fiber reinforced plastics.  Polymer matrices are the most prevalent (thermoplastic and thermoset) although carbon, ceramic and metal matrix options are slowly gaining in popularity.  Glass and carbon (graphite) fibers have the dominant position in this marketplace, although there are many other options (boron, aramid, etc.).

The image to the left shows a ring made of a continuous glass fiber composite.  The continuous fibers can be seen traveling circumferentially around the outer diameter.  The alternating color bands through the wall thickness show changes in fiber direction (i.e. a changing lay-up).