Initiation and Display of Delamination Failure During Fatigue of Graphite Fiber-Epoxy Composites

Laminar microdiscontinuities which lead to the delamination of two unidirectional layup graphite fiber-epoxy composite materials during fatigue have been characterized, and discontinuity geometry and type related to the fatigue lifetime of specimens in which they occur. Two types of discontinuities were commonly observed: voids and organic inclusions consisting of regions of excess hardener. A stress intensity range based analysis applied to discontinuities found on the fatigue fracture place demonstrates that the several orders of magnitude scatter in fatigue life found for a series of test specimens is a result of a variation in the type and size of discontinuities present from specimen to specimen. A nondestructive acoustic pulse echo system used to study the progress of crack propagation from the discontinuities during fatigue is also described