  Experimental investigations on the effect of thicknesses on mixed-mode I/II
fracture are performed with an aluminum alloy with thicknesses of 2, 4, 8 and
14 mm. It is found that under pure-mode I loading condition, the loading
capacity per unit thickness monotonically decreases with increasing thickness.
However, under mixed-mode loading condition, the dependence of loading capacity
on the thickness becomes very weak. Although the measured crack initiation
angles from the mid-plane of the fractured specimens coincide well with typical
existed criteria in the thin and thick specimens under all the mixed-mode
conditions except pure-mode II, exceptional low crack initiation angles are
found in the 8mm-thick specimens. Although ductile dimples are the dominated
microscopic features in all the specimens, the size of the final voids varies
significantly with specimen thickness and loading mode. The ductile dimples in
the crack initiation zone become larger and shallower as the specimen thickness
decreasing.
