Thermo-Mechanical Behavior of Polymer Composites

Abstract

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 magnitude higher than the polymer matrix in which these fibers are embedded. The essential quality of a good composite is that the bond between the fiber and the matrix is well established and is continuous both around the fiber and its length. Thus, a good composite’s performance essentially depends on the interfacial bond quality. When a load is applied in the direction of the fiber orientation of the composite, the load is shared both by the fiber and the polymer matrix. The ratio of this 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 lower temperature the modulus of elasticity increases considerably, and thus it is expected that load sharing between the fibers and the matrix would also change. Another significant aspect of the temperature variation in polymer composites is related to the development of internal stresses. The typical fibers of polymer composites have extremely low thermal expansion coefficients (TEC), wheras, polymers have thermal expansion coefficients which are almost an order of magnitude higher. Thus, when the composites are cooled, the fibers do not shrink as much as the polymer matrix would tend to shrink, but because of the continuous interfacial bond with the fiber the matrix only shrinks as much as the fiber shrinks. Nevertheless, the matrix is stretched, and in the fiber direction, develops internal tensile stress and interfacial shear stress. Cooling also develops hoop stresses and radial stresses around the fiber and thus changes the clamping stress. The clamping stress controls the fracture behavior or crack development and propagation both across and along the fiber direction of unidirectional composites. 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. A large amount of experimental investigations on the effects of temperature on mechanical and fracture behavior of composites including strength, modulus, toughness, failure mode, as well as damage tolerance, have been reported in literature which have primarily aimed towards high temperature behavior, however very little work has been done on mechanism and mechanics of behavior and failure at low temperature. The purpose of this paper will be to briefly discuss on the micromechanical aspects of the low temperature responses expected from the polymer composites.