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Results are presented from a theoretical investigation of the effects of stacking sequence on the energy release rate in laminated composite end-notched flexure test specimens. Deflections and energy release rates of unidirectional and multidirectional ENF specimens are obtained by classical laminated plate theory, shear deformable plate theory, and three dimensional finite element analyses. It is shown that the distribution of energy release rate varies across the front of an initially straight delamination. The percentage of mode I1 and mode 111 energy release rates for the specimen, as well as the local peak values of the mode 11. mode 111 and total energy release rates that occur at the specimen's free edges are shown to correlate with a nondimensional ratio comprised of the specimen's flexural rigidities. The results of the study are used as a basis for a proposed "ENF test design procedure" that may be used for the determination of appropriate specimen stacking sequences and test geometries for studying delamination growth at interfaces between plies at various orientations. The test design procedure minimizes the contributions to the energy release rate from residual thermal stresses, geometric nonlinearities, local mode I1 concentrations at the specimen's free edges and local mode III effects.

The characterization of interlaminar fracture—with the goal to obtain a database that can be used in design—is demonstrated for a first generation graphite/epoxy composite widely used by European aircraft manufacturers. Critical energy release rates for Mode I and Mode II failure have been obtained from static tests using double cantilever beam, end notched flexure, and transverse crack tension specimens. An interaction criterion for the mixed mode case is formulated based on the results from mixed mode bending tests. Fatigue tests have been carried out to determine the Paris law parameters for pure Mode I, pure Mode II, and mixed mode conditions as well as threshold energy release rates that could be used as a design limit in a no-growth concept. In accordance with the static case, an interaction criterion is formulated for the crack growth rate under mixed mode conditions. Delamination progression in more complex specimens has been measured, and mixed mode energy release rates have been computed along the delamination fronts. Results lie well within the scatter band of the Paris law as obtained by the specimens employed for characterization. This confirms that the data obtained from the characterization of interlaminar fatigue growth can be applied for predictions in design.

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