The mechanical properties of most of the reinforcing fibers are significantly higher than those of non-reinforced resins. The mechanical properties of fiber / resin composite systems are controlled by the presence of fiber composite material. The four main factors determining the contribution of fibers are:
The basic mechanical properties of the fiber itself
The surface of the interaction of fiber and resin.
The amount of fiber in the blend. The orientation of the fibers in the composite material.
The interface of the fiber and the resin is controlled by the degree of bonding that exists between the two. It is strongly influenced by the surface treatment of the fibers. The amount of fiber in the composite is largely determined by the manufacturing technology used. The use of high density fabrics will give a higher fractional fiber volume (FVF) in the laminate than those fabrics made with coarse fibers or having distances between them. The fiber diameter is an important factor, with small diameter fibers being more expensive, providing a larger surface for the matrix distribution of the surface load.As a general rule, the hardness and strength of the laminate will increase in proportion to the amount of fiber in it. However, at about 60-70% FVF, the tensile strength can continue to increase as the strength of the laminate reaches its peak and then begins to decrease due to the lack of enough resin to properly hold the fibers together.Finally, since reinforcement threads are intended to be loaded along their length rather than their width, the orientation of the fibers creates highly "oriented" properties of the composite material. This "anisotropic" characteristic of composite materials can be used for a good design advantage, with the majority of the fibers being positioned along the orientation of the main loading direction. This minimizes the amount of "parasitic" material that is placed in places where there is little or no load.The fiber properties display only part of the picture. The properties of the composite will result from those of the fiber, but also the way they interact with the resin, its properties, the amount of fiber and its orientation.