Functional relationship of polymer matrix to reinforcing
The matrix gives form and protection from the external environment to the fibers.
Chemical, thermal, and electrical performance can be affected by the choice of matrix resin.
But the matrix resin does much more than this. It maintains the position of the fibers.
Under loading, the matrix resin deforms and distributes the stress to the higher modulus fiber constituents. The matrix should have an elongation at break greater than that of the fiber.
It should not shrink excessively during curing to avoid placing internal strains on the reinforcing fibers.
If designers wish to have materials with anisotropic properties, then they will use appropriate fiber orientation and forms of uni-axial fiber placement.
Deviations from this practice may be required to accommodate variable cross sections and can be made only within narrow limits without resorting to the use of shorter axis fibers or by alternative fiber re-alignment.
Both of these design approaches inevitably reduce the load-carrying capability of the molded part and will probably also adversely affect its cost effectiveness.
On the other hand, in the case of a complex part, it may be necessary to resort to shorter fibers to reinforce the molding effectively in three dimensions. In this way, quasi-isotropic properties can be achieved in the composite.
Effects of fiber length on laminate properties—Fiber placement can be affected with both continuous and short fibers. Aside from the structural implications noted earlier in this chapter, there may be part or process constraints, which impose choice limitations on designers.
The alternatives in these cases may require changes in composite part cross section area or shape. Variables in continuous-fiber manufacture, as well as in considerations in part fabrication, make it impossible to obtain equally stressed fibers throughout their length without resorting to extraordinary measures.
Bonding interphase—Fiber composites are able to withstand higher stresses than can their constituent materials because the matrix and fibers interact to redistribute the stresses of external loads.
How well the stresses are distributed internally within the composite structure depends on the nature and efficiency of the bonding.
Both chemical and mechanical processes are thought to be operational in any given structural situation.
Coupling agents are used to improve the chemical bond between reinforcement and matrix since the fiber-matrix interface is frequently in a state of shear when the composite is under load.