Fracture Toughness of Ductile Iron and Cast Steel

Abstract

The dynamic fracture behavior of ductile cast iron and cast steel has been studied to better explain why blunt notched Charpy bars indicate ductile iron has a quite inferior fracture toughness compared with cast steel with similar tensile properties (15 versus 75 J) while fracture mechanics tests indicate a much less significant difference in fracture toughness between the two materials. Using standard and fatigue precracked side-grooved Charpy specimens, it has been shown that the different predictions are due to a difference in constraint, the standard Charpy bar experiencing loss of constraint for the cast steel but remaining fully constrained for the ductile iron. For fatigue precracked and side-grooved Charpy specimens, initial crack extension occurred in both materials under conditions of full constraint, and the cast steel upper shelf fracture toughness was found to be only modestly better than the upper shelf fracture toughness of the ductile iron. Because of its lower ductile-to-brittle transition temperature, ductile iron has been found to have a superior fracture toughness to cast steel with similar tensile properties at temperatures below ambient.