The fracture toughness of ferritic, spheroidal graphite ductile cast iron has been measured by a single-specimen J-integral technique. The method used a servohydraulic test frame in the displacement control mode, with overall test control and data acquisition managed by a minicomputer. Digitized load and displacement values were combined to calculate incremental crack lengths based on measured crack length-compliance calibrations. Plots of J versus change in crack length were generated, and values of JIc (and equivalent KIc) were determined using ASTM Test for JIc, a Measure of Fracture Toughness (E 813). This technique thus provided a means of determining the plane-strain fracture toughness (KIc) using small samples (2.5 cm thick) at temperatures from −150 to +25°C. Previous attempts to measure KIc directly on similar materials had produced invalid results for all tests except those performed at very low temperatures (−60°C and below).
Two ductile cast irons were investigated that had similar volume fractions of graphite, graphite nodule sizes, and ferrite grain sizes, but one contained approximately 15% pearlite, while the other had less than 1% pearlite. The alloy with greater pearlite content exhibited higher strength and higher ductile-to-brittle transition temperature. Although cast irons are generally perceived as being low in toughness, the toughness values measured on fully ferritic nodular iron were quite high (KIc = 80 to 106 MPa√m [73 to 96 ksi√in.] on the upper shelf).