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The present work examines in a systematic way the development of microdamage in several laminates of graphite/epoxy material subjected to both quasi-static tensile loading and tension-tension fatigue. Emphasis is placed upon discriminating and quantifying matrix and fiber microdamage. Penetrant-enhanced standard- and stereo X-ray radiography were used along with edge replication to follow the progression of matrix damage. The recently developed technique of laminate deply was used to map the development of fiber fracture.
The most significant result, confirmed in both the quasi-static and fatigue tests, was the dominant role played by off-axis ply cracks in the fracture of fibers in adjacent load-bearing plies. By direct observation of fiber fractures in situ, it was established in both cases that fiber fractures do not occur in a random pattern at elevated loads/cycles, but instead occur in narrow bands adjacent to off-axis ply cracks. It is the action of these crack tips upon adjacent fibers which may govern the wearout or overload of laminates under these conditions.
By direct accounting of fiber fractures in deplied laminae taken from damaged but unfailed laminates, the relationship between the density of fiber fractures and the number of cycles at one cyclic stress level is reported. The relationship between fiber fracture density and quasi-static stress level is reported as well.
For the quasi-statically loaded specimens, efforts to characterize and discriminate between fiber fracture and transverse ply cracking on the basis of acoustic emission (AE) amplitude distribution are described. Correlations between attributes of the AE amplitude distribution and the established damage extent and chronology are reported. Common assumptions regarding the contributions of these distinct damage modes to acoustic emission from laminates under tensile loading are discussed in terms of the present evidence.
Finally, in comparing damage resulting from quasi-static tension loading of cross-ply laminates with that observed in tension-tension cyclic loading of laminates of similar graphite/epoxy material, microdamage modes unique to fatigue are identified and discussed.
composite materials, laminates, graphite/epoxy, fatigue, tensile failure, damage, ply cracking, fiber fracture, edge replication, stereoradiography, deply, acoustic emission
Assistant professor, United States Naval Academy, Annapolis, MD