The Effect of Thickness on Interlaminar Stresses and Delamination in Straight-Edged Laminates

The effect of ply and laminate thickness on the fracture of graphite/epoxy laminates induced by delamination was investigated. Five laminate types were manufactured from AS1/3501-6 graphite/epoxy and tested: [±15n]s, [±15n/0n]s, [0n/±15n]s, [±45n/0n]s, and [[0/±15]s]n. "Effective ply thickness" was varied by stacking several plies of the same angular orientation together. It was found that an increase in effective ply thickness caused a decrease in fracture stress resulting from delamination in all cases. Interlaminar stress calculations indicate that this results from the fact that the region of high interlaminar stresses increases as the effective ply thickness increases. These calculations also suggest that interlaminar shear stresses are at least as important in causing delamination as interlaminar normal stresses. "Back-to-back" [[0/±15]s]n laminates were found to have the same strength for values of n from one to three. This indicates that reductions in strength are not simply a laminate thickness phenomenon but are related to the effective ply thickness.