LOW-TEMPERATURE COMPRESSIVE STRENGTH OF GLASS-FIBER-REINFORCED POLYMER COMPOSITES

Polymeric composites are relatively inexpensive materials of high strength, in which deformation of the matrix is used to transfer stress by means of shear traction at the fiber-matrix interface to the embedded high-strength fibers. At low temperatures, complex stresses are set up within the microstructure of the material as a result of matrix stiffening and mismatch of thermal expansion coefficients of the constituents of the composites. These stresses in turn affect the strength and deformation characteristics of the composites. This is demonstrated by compression testing of an unidirectional glass-fiber-reinforced polymer composite at room and low temperatures. The increase of compressive strength matched the analytical prediction of strength increase modeled from the consideration of increase in matrix stiffness and thermal residual stresses at low temperatures. Additional compression tests performed on a batch of low-temperature thermally cycled specimens confirmed the predictable reduction of brittleness due to suspected increase of microcrack density. The mode of failure characterized by definite pre-fracture yielding conforms more to Budiansky's plastic microbuckling theory than to Rosen's theory of elastic shear or extensional buckling.