FIBER-MATRIX ADHESION AND ITS EFFECT ON COMPOSITE MECHANICAL-PROPERTIES .3. LONGITUDINAL (0-DEGREES) COMPRESSIVE PROPERTIES OF GRAPHITE EPOXY COMPOSITES

An experimental investigation was performed to quantify the relationship between fiber-matrix adhesion as determined by single fiber interfacial shear strength tests with the compressive properties and failure modes of unidirectional graphite/epoxy composites. The compression tests were conducted on three identical sets of composites differing only in their fiber-matrix interfacial shear strength. The interfacial shear strength was varied by using the same graphite fibers with different surface treatments, namely untreated, surface treated, and surface treated and coated with a thin layer of epoxy. The surface treatment changed the interfacial shear strength by more than a factor of two, while the properties of the fibers remained unchanged. The experimental results show that the fiber surface treatment affects the compressive modulus only marginally. However, the compressive strength and maximum compressive strain were highly sensitive to the fiber surface treatment. Both the strength and maximum strain increased with the interfacial shear strength. The fracture surface analysis revealed that when the fiber-matrix interfacial shear strength was low the compressive failure occurred due to delamination and global delamination buckling. At intermediate values of the interfacial shear strength, the compressive failure occurred due to shear failure driven by inphase microbuckling of fibers. The composites having the highest values of the interfacial shear strength yielded "stepped" fracture surfaces showing compressive failure of fibers in several planes perpendicular to the fiber axis.