The basic idea behind a composite structure is that by using two materials together the builder can get better performance than either one is capable of on its own. This is not a new idea. Rebar is a classic example, the concrete is reinforced by steel rods that can better support tensile loads. Concrete itself is a composite, gravel and ash reinforces the cement that holds the material together. The oldest man-made composite material is called
cob, a building material of mud or clay brick that uses sticks and straw for support.
Each of these composite materials is composed of two parts: matrix and reinforcement.
Matrix gives the structure its shape and provides rigidity.
Reinforcement provides strength to the final structure. Reinforcing materials are often much stronger than the matrix, but would make a poor building material because they bend easily like straw, or don't form a structure like the gravel and ash used in concrete.
Long before humans were making cob nature had produced composite structures. The best example is also the oldest composite building material used by man: wood. Wood is composed of long fibers made from the cell walls of the original tree bound together by a glue-like substance called
lignin. In this case lignin is the matrix and the fibers are the reinforcement. The strength of wood comes from the fibers which are supported and held together by lignin. In the direction the fibers run, called the
grain, the wood is stronger than in any other direction. However, because the lignin soaked fibers are more stiff than the lignin itself, against the grain the wood is more flexible.
It's rare that the fibers themselves break. When a piece of wood breaks it is usually in the lignin first, cracks form where it begins to fail or where it separates from the fibers. That's because the fibers are so much stronger than the lignin is. When you glue two pieces of wood together the glue becomes the weakest part. Most glue isn't as strong as lignin. Even if it was there's no fiber in the glue bond for reinforcement, only the glue is carrying any loads.
Materials like wood are very different from those like metal or stone. Metal and stone have nearly the same properties in all directions, given the term
isotropic. Wood is an
anisotropic material, meaning that its properties depend on what direction it is assembled and loaded in. Composites like wood, which act differently in different directions, can often act in ways that seem counter-intuitive to people who aren't accustomed to dealing with them.
