Evaluation of static and dynamic fracture toughness in ductile cast iron
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I.
INTRODUCTION
DUCTILE cast irons have good strength and ductility compared with the other cast irons. Consequently, they are currently used widely as structural components in place of gray cast iron or cast steel, t~,2] Casting is a single-process manufacturing method that produces a complicated shape while requiring less energy than many alternative production routes. Furthermore, the matrix of ductile cast iron can be varied from high-ductility ferrite to high-strength pearlite by a control-casting process, heat treatment, and chemical compositions. Thus, the ductile cast irons satisfy a wide range of properties required of engineering materials and should become increasingly essential to the manufacturing industry. Ductile cast irons have been explored as a cask (container for spent nuclear fuel) material because of their low cost and good formability,t3'4~ The cask, which is a huge casting with 400-mm thickness and 100-Mg weight, envelops the nuclear material. Therefore, the fracture toughness of cask must be evaluated not only under the static loading condition but also under the dynamic loading condition to ensure its safety against an accident during the transport, fsl Additionally, because the application of ductile cast irons with various matrices under dynamic loading condition is increasing as their reliability increases, the evaluation of fracture toughness under dynamic loading conditions in ductile cast irons with various matrices is increasingly significant. One of the authors has already reported the dynamic fracture of ductile cast iron in the past decade. E6-~3j Fracture toughness of ductile cast irons should be evaluated on the basis of the elastic-plastic fracture mechanics (J integral, crack-tip opening displacement
TOSHIRO KOBAYASHI, Professor, and SHINYA YAMADA, Graduate Student, are with the Department of Production Systems Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan. Manuscript submitted November 15, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
(CTOD), etc.). The ductile fracture initiation point occurs at the end of the crack-tip blunting process in steels. By contrast, fracture toughness is not well evaluated, and the crack initiation point is still undefined in ductile cast irons, tl4] In this study, a series of experiments was carried out with heavy wall ferritic ductile cast iron; a cask material; and also with ferritic, ferritic-pearlitic, and pearlitic ductile cast irons of ordinary thickness in order to evaluate the fracture toughness of ductile cast irons. The effect of loading rate on crack growth behavior and fracture toughness was the focus of the work on the heavy wall ferritic ductile cast iron, while the effect of matrix microstructure on those under the dynamic loading condition was examined for the remaining cast irons.
II.
EXPERIMENTAL PROCEDURES
A. Materials and Test Specimen The microstructures of the materials used are shown in Figure 1. The materials studied included a heavy wall ferritic ductile cast iron in the as-c
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