Fibers — whether glass, carbon or more esoteric varieties like aramid — are the backbone of composites. A single glass filament, thinner than a human hair, has a tensile strength of approximately 500 ksi/3.5 GPa, but essentially no compressive strength. To achieve a structural building material, fibers are encased in a polymer resin matrix, which binds them together and allows imposed loads to be spread across the fibers. In addition to tensile strength, fiber provides stiffness and impact resistance. Resin protects the fibers from moisture, ultraviolet light and corrosive chemicals.

    Three basic factors must be considered when selecting fiber for an application: the fiber type, the fiber form (roving, tow, mat, or woven fabrics) and the fiber orientation (also known as fiber architecture). Reinforcement can be oriented or aligned in whatever direction best suits the performance requirements of the application. For example, structural parts are designed with the greatest strength in the direction subject to the greatest load, so a beam may have the majority of the fiber oriented longitudinally (i.e., at 0° or parallel to the longitudinal beam axis). However, fiber direction can be so varied that a virtually isotropic material can be produced, with equal strength in all directions. High fiber loading — that is, a relatively high proportion of fibers compared to resin — makes a stronger and lighter part, but usually increases cost, as well.