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The objective of this study was to characterize the morphology of interlaminar fractures in unidirectional graphite/epoxy composites. The specific objective was to correlate characteristic fractographic patterns for different pure and mixedmode loadings with the loading parameters and fracture mechanisms. Two graphite/epoxy composite materials were investigated, one with a brittle matrix and the other with a rubber-toughened matrix. The tests were conducted with a specially designed loading system which allows the application of pure Mode I or Mode II loadings or any combination thereof. SEM fractographs, including stereo pairs, were taken of the specimens after failure at various points across the width of the crack and at various distances from the initial crack front. In the case of the brittle matrix composite, the fibers are, in general, covered with a film of matrix. The matrix fracture surface consists of hackles resulting from interfacial tensile or shear failure and tensile failure normal to the maximum tensile stress. The hackle density increases from Mode I, to Mode II, to mixed Mode I and II. In the case of the toughened matrix composite, the fibers are covered with matrix for Mode I and mixed Mode I and II fractures, whereas they are clean under Mode II and mixed Mode II with compression loading. Matrix fracture is granular or consists of hackles with a low degree of orientation and regularity increasing from Mode I to Mode II. Failure mechanisms include interfacial shear and high ductile or plastic elongation of the matrix.
graphite/epoxy, delamination, Mode I fracture, Mode II fracture, mixed mode fracture, test methods, fractography, failure mechanisms, toughened composites
Research physicst, Volcani Research Center, Beit Dagan,
Professor, Tel Aviv University, Tel Aviv,
Professor, Northwestern University, Evanston, IL