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The effect of constraining the crack-tip damage zone on the Mode I fracture of a toughened epoxy resin (BP-907) was studied systematically using the double-cantilever-beam (DCB) adhesive joint specimen and scanning electron microscopy (SEM). While a broad bond thickness range was employed (5 < t < 875 µm), particular attention was given to the lower bond spectrum and to the applicability of the results to interlaminar fracture of toughened composites.
Depending on t, interesting fracture characteristics were found including continuous, unstable, and a combination of both types of growth. For t > 33 µm the fracture energy exhibited dramatic variations with t that were shown to follow from the variations in size of a certain dimple-like morphological feature. In constrast, the arrest energy was fixed, in consistency with the unchanging morphology. The fracture behavior for t < 33 µm was equally interesting, now though because GIC was fixed while GICa varied with t. These changes in failure conditions were accompanied by morphological changes; as t decreased from 33 µm, the failure locus shifted from the bond center to the metal/matrix interface, and the failure itself occurred by shear yielding.
Similarities were found in the failure conditions of butt joint and DCB adhesive specimens, on the one hand, and in that of the latter and composite interlaminar fracture, on the other hand, which suggest an interrelationship among these three failure phenomena.
composite materials, adhesive joints, Mode I, fracture energy, toughened matrix, bond thickness, interlaminar fracture
Visiting scientist, Materials Laboratories, AFWAL/MLBM, Wright-Patterson Air Force Base, Dayton, OH