The role of delamination in strength, failure mechanism and hole size effect in open hole tensile tests on quasi-isotropic laminates

Results of several different series of open hole tension tests on quasi-isotropic IM7/8552 carbon fibre/epoxy laminates with the same stacking sequence but different ply block thicknesses and numbers of sublaminates are summarised. Specimens with single 0.125 mm thick plies failed by fibre fracture, with the strength decreasing with increasing hole size. Ones with 0.5 mm thick blocks of plies all delaminated, with the failure stress increasing with increasing hole diameter, the opposite to the usual hole size effect. Specimens with 0.25 mm thick ply blocks showed intermediate response, with small ones failing by delamination, and large ones by fibre failure, and constant strength over a range of hole sizes from 1.6 to 12.7 mm diameter. The crucial role of delamination in the strength and failure mechanism in open hole tension is examined in order to explain these results and show why conventional hole size corrections may not always be applicable.