Mixed mode Ⅰ/Ⅱ fracture erperiments of
LC4-CS aluminum alloy were conducted
by using tension--shear specimens with
thicknesses of 2, 4, 8 and 14mm. Fracture
mechanisms of thickness effect on
mixed mode Ⅰ/Ⅱ fracture were first examined from
fracture surface morphology to correlate with the macroscopic fracture behavior and
stress state. It is found that specimen thickness has a strong influence on mixed mode
fracture. As thickness varies from thin to thick the macroscopic fracture surfaces
appear the characteristics of plane stress state (2mm, 4mm--thick specimen), three--
dimensional stress state (8mm--thick specimens), and plane strain state (14mm--thick
specimens), respectively. The specimens of all kinds of thicknesses are typical of ten-
sile type failure under mode Ⅰ loading condition and shear type failure under mode Ⅱ
loading condition. Two distinct features coexist on the fracture surfaces under mixed
mode loading conditions, and the corresponding proportion varies with loading mix-
ity. Void--growth processes are the failure mechanism in both predominately tensile-
and shears--type fractures. The size and depth of dimples on the fracture surface vary
greatly with thickness. Therefore, it is extraordinary necessary to take into account
the thickness effect when a mixed mode fracture criterion is being established.