Although humans have used composite materials for millennia, the concept of composites as a distinct classification of materials was not recognized until the mid-20th century. Composite materials are formed from two or more distinct phases of materials. This is in contrast with metal alloys, which we studied in an earlier lesson. In metal alloys, additional atoms, compounds, or phases are dissolved into the base metal. This solid mixing does not result in distinct phases, which are present in composite materials. Possibly the earliest usage of a composite was by the ancient Mesopotamians (circa 3400 BCE) who realized that gluing wood at angles produced better properties than single-ply wood. Modern five-ply plywood has five plies arranged in steps of 45° (0, 45, 90, 135, and 180 degrees) for better strength. A photo of an unknown type of plywood is shown below.
Around 1500 BCE in the Fertile Crescent, humans began adding straw to strengthen clay bricks. Human structures were no longer limited to wood or the piling of stone. Unreinforced clay bricks, like most ceramics, are strong under compression stress, but unstable when subject to tensile stresses. So, unreinforced clay bricks carry the load but will readily fall apart. Except for its unstable nature under tensile stresses, clay is otherwise an ideal building material. As a raw material, it is available almost everywhere and, before drying, it can be easily worked into the desired shape. Strengthening clay through the addition of straw, gravel, or bitumen greatly enhances its applicability as a building material. Before moving to the next section, please watch this brief introductory video (2:07) on composites.