Fracture toughness tests were conducted on the SAEFDE Committee's round-robin A356-T6 cast aluminum alloy materials designated X, Y and Z. Compact type specimens with a thickness of 9.1 and 20.3 mm were tested. Valid K
lc values couid not be obtained for 9.1 mm thick specimens but were obtained for 20.3 mm thickness specimens. Due to larger castings, and hence slower cooling rates, a coarse secondary dendrite arm spacing, DAS, of 80 to 90 μm existed in the three materials. Similar K
lc values were 18, 16.7 and 17.3
for the A356-T6 materials X, Y and Z respectively. Final fracture surfaces were also similar with predominant cleavage fracture with some localized ductile dimples and secondary cracking.