Fiber-Matrix Adhesion and Its Effect on Composite Mechanical Properties: IV. Mode I and Mode II Fracture Toughness of Graphite/Epoxy Composites

To optimize the level of fiber-matrix adhesion an understanding of the relationship between fiber-matrix interfacial bond strength and the mechanical and fracture behavior of composites is essential. This study establishes the relationship between fiber-matrix interfacial shear strength (ISS)and interlaminar fracture toughness (both Mode I and Mode II)and failure modes for 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 with different surface treatments. These surface treatments changed the level of adhesion between the fiber and matrix thus resulting in an increase of the fiber-matrix ISS by over a factor of two while the fiber and matrix properties remained unchanged. The Mode I and Mode II tests were conducted by the double cantilever beam (DCB)and end-notch flexure (ENF)tests methods, respectively. The Mode I fracture toughness I()GC of composites having low fiber-matrix ISS could not be determined from the DCB test because of extensive fiber bridging and crack meandering. For the composites having higher values of the ISS, the I GC increased with the ISS. The experimental results demonstrated that there is a strong dependency of Mode II fracture toughness I()GIC on fiber-matrix adhesion. Increased fiber-matrix adhesion in one group of composites significantly improved the I,GIC but the presence of brittle interphase around graphite fibers in another group of composites tended to cancel part of the improvement resulting from increased adhesion. Based on the major failure modes occurring during the Mode I and Mode II loading conditions, a causal linkage between fiber-matrix adhesion and interlaminar fracture behavior of graphite/epoxy composites is established.