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Structural configurations in helicopter rotor systems often contain plies of 0° and 45° to resist centrifugal and torsional loads. Delaminations in test specimens that are tested to failure often experience delaminations between a 0°/45° ply interface. However, the standard methods to characterize delamination utilize a unidirectional 0° specimen. The reasons for using a unidirectional layup are that multidirectional beams suffer from ply cracks in the nonzero plies, increased anticlastic bending and other effects. This work investigated the issues for testing a nonunidirectional double cantilever beam (DCB) specimen with a delamination in a 0°/45° interface. Several criteria were developed to minimizethe effects noted above. These included reduction of bend twist coupling and residual thermal stresses, prevention of nonzero ply breakage, and minimized anticlastic bending. The resulting layup consisted of a 64-ply laminate of 0°, 45°, and -45° plies. Quasi-static and fatigue tests were conducted on unidirectional layups and the new configuration fabricated from S2/E773 glass/epoxy. For both the static and fatigue tests the delamination grew and remained in the 0°/45° interface for a significant portion of delamination growth. Crack branching was noted in some specimens as the delamination length became longer. The interlaminar fracture toughness, GIc, of the 0°/45° DCBs had more scatter and a 10% lower mean than the 0° DCB specimens. Both specimens experienced fiber bridging shown by an increase in GIc with delamination growth and observation of the delaminated surfaces. The influence of the fiber bridging was similar in the two specimen configurations. The fatigue delamination onset curves of the two specimen configurations were coincidentally not showing the scatter or lower values seen in the static tests. The static and fatigue results indicate that the use of 0° specimens for characterizing delamination in a 0°/45° interface is satisfactory for the materials tested in this work. However, the presence of fiber bridging in the 0°/45° specimens indicates that using the initiation value for predicting delamination growth may be overly conservative when fiber bridging is present. The same fiber bridging has also been observed in failure surfaces on structural components tested to failure. While the inclusion of fiber bridging in delamination growth predictive models is possible, further work should be conducted on the parameters that affect fiber bridging, such as thickness, processing and crack opening, to ensure that the properties from a DCB specimen are representative of that in the structures. Work on other material systems, other delamination modes and fatigue crack growth is also recommended.
delamination onset, double cantilever beam, fatigue, fiber bridging, interface, interlaminar fracture toughness, multidirectional
CEO and Head, Advanced Composites, Materials Engineering Research Laboratory Ltd., Hertford,
Conference chairman, Bell Helicopter Textron Inc., Fort Worth, TX