Fiber-Matrix Adhesion and Its Effect on Composite Mechanical Properties: I. Inplane and Interlaminar Shear Behavior of Graphite/Epoxy Composites

Abstract

An experimental investigation was performed to establish the relationships between fiber-matrix adhesion as determined by single fiber interfacial shear strength tests with the inplane and interlaminar shear properties of graphite/epoxy composites. ±45°-tension, Iosipescu, and short beam shear tests were conducted on three identical sets of composites differing only in their fiber-matrix interfacial shear strength. The fiber-matrix interphase and consequently the interfacial shear strength was varied by using the same graphite fiber was different surface modifications, namely untreated, surface treated, and surface treated and coated with a thin layer of epoxy. The surface modification changed the interfacial shear strength by more than a factor of two, while the properties of fibers remained unchanged. The experimental results showed that both inplane and interlaminar shear strengths increased approximately in the same ratio as the interfacial shear strength, however, the inplane shear modulus was relatively insensitive to the fiber surface modifications. The fracture surface analysis revealed that when the fiber matrix interfacial shear strength was increased from low to intermediate to high values, the major failure modes changed from primarily interfacial failure to a combination of interfacial and matrix failure to primarily matrix failure, respectively, in a manner identical to that observed with the single fiber fragmentation tests.