Effect of Remote Ply Orientation on the Perceived Mode I and Mode II Toughness of θ/θ and θ/-θ Interfaces

Results are presented from an investigation of the effect of remote ply orientation on the perceived mode I and mode II toughness of a laminated graphite/epoxy. Experimental and numerical studies were conducted on four commonly used stacking sequences. Each sequence contained a midplane delamination and was subjected to bending loads that produced either predominantly mode I or predominately mode II conditions. Of the four sequences, two contained delaminations at 30/30 interfaces, and two contained delamination at 30/-30 interfaces. For each interface type, one stacking sequence exhibited a relatively small transverse curvature under the applied loadings but a relatively large twisting curvature, whereas the other sequence exhibited a small twisting curvature but a large transverse curvature. For the two types of loadings (mode I and mode II), each of the four sequences was analyzed by three dimensional finite element analyses and tested in precracked and non-precracked configurations. Using appropriate nondimensional parameters, the results of this study are combined with the results of companion numerical investigations to make recommendations regarding the choice of stacking sequences for the determination of the toughness of arbitrary interfaces.