The essential quality of a good polymer composite is that the bond between the fiber and the matrix is well established and is continuous both around the fiber and along its length. When a load is applied in the direction of the fiber, the ratio of the load share depends on the relative elastic modulus of the fiber and the matrix. However, the elastic modulus of the polymer matrix is significantly influenced by the temperature. At low temperature the modulus of elasticity increases considerably, and thus load sharing changes between the fibers and the matrix. Also, because of the mismatch of coefficient of thermal expansion (CTE) of matrix and fiber, the matrix is usually stretched in the fiber direction during curing, and develops internal tensile stress, interfacial shear stress, hoop stress, and radial stress. On further cooling during the freezing process, the magnitude of all these induced stresses would usually increase, developing potential microcracks. The change in the radial or clamping stress which controls the crack development and propagation (fracture) both across and along the fiber would also change the composite's fracture behavior in the low temperature regime. More complex stresses are developed when the composites are constructed as laminates with each lamina (layers of fibers) having fiber orientations different from the adjacent ones. Reductions of strength and modulus of composites, following freeze-thaw cycling, as evident in experimental results, support this micromechanical theory of composites degradation.
In polymer composites high elastic modulus fibers are incorporated into a lower elastic modulus matrix to achieve structural reinforcement. Most common fibers are E-glass, carbon or graphite, and aramids (Kevlar). Typically the elastic modulus and strength of these fibers are of a magni- Tude higher than the polymer matrix in which these fibers are embedded.
