FIBER-MATRIX ADHESION AND ITS EFFECT ON COMPOSITE MECHANICAL-PROPERTIES .2. LONGITUDINAL (0-DEGREES) AND TRANSVERSE (90-DEGREES) TENSILE AND FLEXURE BEHAVIOR OF GRAPHITE EPOXY COMPOSITES

An optimum level of interfacial bond strength between reinforcing fiber and a polymeric matrix in which it is placed is essential for acceptable composite mechanical properties and performance. The interfacial bond strength can be optimized only when the relationship between the level of fiber-matrix adhesion and the mechanical and fracture behavior of composites is clearly understood. This study establishes the relationship between the fiber-matrix interfacial shear strength and 0-degrees and 90-degrees tensile and flexure properties of graphite/epoxy composites. A well defined and characterized graphite fiber/epoxy system was chosen in which the level of adhesion between fiber and matrix was changed by using the same graphite fibers through the use of surface treatment and finish. The level of adhesion between the fiber and matrix associated with these changes resulted in an increase of fiber-matrix interfacial shear strength (ISS) by over a factor of two while the fiber and matrix properties remained unchanged. The experimental results demonstrated that the fiber surface modification did not have much effect on the tensile and flexural moduli and on the fiber dominated properties. However, the strengths and maximum strains that are governed by the matrix and interface properties were highly sensitive to the fiber surface modification. In addition, the major failure modes were also found to be affected by the fiber-matrix interfacial shear strength.