EFFECTS OF MODE RATIO, PLY ORIENTATION AND PRECRACKING ON THE DELAMINATION TOUGHNESS OF A LAMINATED COMPOSITE

Results are presented from an investigation of the effects of mode rat io, interfacial ply orientation and precracking on the delamination to ughness of a graphite/ epoxy composite. Delamination toughness tests w ere performed on specimens that exhibited growth at 0 degrees/0 degree s, 15 degrees/15 degrees, 15 degrees/-15 degrees, 30 degrees/30 degree s and 30 degrees/-30 degrees interfaces. Three mode ratios were consid ered. The double cantilever beam test was utilized for pure mode I, th e end-notched flexure test was used for pure mode II, and the symmetri cally delaminated single leg bending test was used for a mixed-mode co ndition, G(II)/G, 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 degrees/30 degrees and 30 degrees/-30 degrees interfaces, two different stacking sequences were used to assess the e ffect of remote ply orientation on the perceived mode I and mode II to ughness. For any mode ratio and interface angle, it was found that pre cracked toughness values were always lower than the corresponding non- precracked result. No other consistent trends were observed in all of the data. For the mode I tests, the 30/30 interface exhibited the lowe st toughness, for the mixed-mode tests, the 0/0 interface produced a m inimum toughness, and for mode II no significant effect of interface a ngle on toughness was observed. These results are interpreted with the aid of recent three dimensional finite element analyses for energy re lease rate distributions along the delamination front for the differen t stacking sequences and test geometries. The accuracy of various data reduction techniques are also evaluated, and suggestions are made for future testing.