A composite material is an end-use material that is the product of two or more other materials that maintain their individual characteristics. For example, a mud brick with straw filler and a silicon carbide fiber in a tungsten matrix are both composite materials. Each of these two examples represent the low-end and high-end types of composite materials.
An advanced composite material uses high performance fibers or particulates in an advanced polymer system. At the low end of the performance capability is chopped glass fiber in a polyester resin and at the high end of performance is a graphite fiber (a modification to a carbon fiber) in a bismaleimide toughened resin system.
Composites are heterogeneous materials, i.e., they have two or more distinct and separate material phases. There is the fiber (fiber ends shown in the diagram above) for enhanced material performance, and the polymer or matrix that supports the fiber and improves on damage tolerance, toughness and environmental resistance. Note the relationship to fiber volume (fiber diameter area x number of fiber ends x fiber length) against polymer volume (blue background above). This has significant influence on physical and mechanical properties of the composite.
Composite materials behave anisotropically. An anisotropic material, such as composites, have variations in the physical and mechanical properties in all directions. Tailoring the fibre direction will allow specific directional properties to be designed into the composite material.
Material performance is dominated by fiber direction. Specifically, the engineering properties of a composite material are enhanced in the fiber direction.
For example, the stiffness (Young’s Modulus) of a composite material in the fiber direction can be 20 times greater than in the perpendicular direction where the polymer system properties dominate (see the blue curve in the graph).
In the fiber dominant direction, composite materials are more brittle than metals. Metals typically plastically deform near to failure (for example see Ti – Titanium, Al – Aluminum below). In composite materials fiber direction failure is without warning and the load versus deflection behavior is linear as seen in the diagram below (CF – Carbon Fiber). Perpendicular to the fiber direction the polymer system can either be brittle or viscoelastic in behavior (for example PU – Polyurethane in the diagram). Hence, the designs of openings, joints and attachments in a composite material are a critical design detail in the structural performance of the composite component.