The singular-field-based mode-mix decomposition procedure is assessed for its ability to accurately predict delamination growth in various test geometries. Initially, toughness versus mode mix is obtained from laminates with delaminations at their midplanes. These results are then used to predict delamination growth in other geometries, and the associated tests of these geometries are performed. A typical graphite/epoxy material system is considered. For this material, it is shown that predictions of delamination growth using a singular-field-based mode-mix decomposition will result in significant errors. That is, when a classical, singular-field-based definition of mode mix is used, tests of different specimen geometries predicted to be at the same mode mix do not display the same toughness. All the test data is then reinterpreted with the aid of a previously developed crack-tip-element analysis. This approach allows for various definitions of mode mix to be evaluated, of which the singular-field-based result is one special case. It is shown that defining mode mix in terms of non-classical parameters results in accurate predictions of delamination growth, and test geometries predicted to be at the same mode mix now display the same toughness. This new definition of mode mix is based on parameters that uniquely characterize the conditions at the crack tip, but which are insensitive to the scale of the near-tip damage. A systematic method for determining the “appropriate definition” of mode mix for arbitrary material systems is described.

Results are presented from an investigation of the effects of mode ratio, interfacial ply orientation and precracking on the delamination toughness of a graphite/ epoxy composite. Delamination toughness tests were performed on specimens that exhibited growth at 00/00, 15°/15°, 15°/- 150, 30°/30° and 30°/-30° interfaces. Three mode ratios were considered. The double cantilever beam test was utilized for pure mode I, the end-notched flexure test was used for pure mode II, and the symmetrically delaminated single leg bending test was used for a mixed-mode condition, G,,IG, equal to approximately 0.4. All five interfaces were tested in both precracked and non-precracked conditions at each mode ratio. Also, for the 30°/30° and 30°/- 30° interfaces, two different stacking sequences were used to assess the effect of remote ply orientation on the perceived mode I and mode II toughness. For any mode ratio and interface angle, it was found that precracked toughness values were always lower than the corresponding nonprecracked result. No other consistent trends were observed in all of the data. For the mode I tests, the 30/30 interface exhibited the lowest toughness, for the mixed-mode tests, the 0/0 interface produced a minimum toughness, and for mode II no significant effect of interface angle on toughness was observed. These results are interpreted with the aid of recent three dimensional finite element analyses for energy release rate distributions along the delamination front for the different stacking sequences and test geometries. The accuracy of various data reduction techniques are also evaluated, and suggestions are made for future testing.

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