Thin wall ductile cast iron is a promising material for reducing the weight of industrial products. To utilize this cast iron, mechanical properties have to be established. This report deals with the fracture toughness of thin castings with which CT test pieces cannot be obtained. We therefore focussed on the SP test which requires 0.5mm thickness test pieces. In the tests, a 2.4mm diameter bearing steel ball was indented into the test piece until it broke, and the load and displacement were measured. A drop in the load indicated the breaking of the test piece, and the equivalent fracture strain εqf was evaluated from the displacement δ* up to the point of breaking. There was a correlation between the fracture toughness JIC and equivalent fracture strain εqf in several materials, and the fracture toughness was estimated from the correlation. CT and SP tests were conducted on the same block of ductile cast iron, and the fracture toughness JIC and equivalent fracture strain εqf were measured and compared. In the SP test on ductile cast iron, the fracture point on the load-displacement curve was not certain. To determine the point on the curve, the fracture process was considered from the fracture appearance observation. The equivalent fracture strain εqf was evaluated from the fracture displacement δ*. The correlation between the fracture toughness JIC and equivalent fracture strain εqf was determined as JIC ＝ 400εqf2 ＋ 5 [kJ/m2]. It was established that the fracture toughness of thin wall castings can be measured by the SP test.
