Damage effect on the fracture toughness of nodular cast iron: Part-II. Damage zone characterization ahead of a crack tip
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INTRODUCTION
THE growing importance of nodular cast iron as an industrial material,[1,2,3] for use in the automobile industry, for example, requires a thorough knowledge of its tolerance to damage. A large amount of research has been conducted to determine the fracture toughness of cast iron.[4–7] By using linear elastic fracture mechanics (LEFM) approaches Bradley et al.[7] and Nanstad et al.[8] concluded that the conventional critical-stress intensity factor KQ gave an indication of crack tip plasticity development, but did not correspond to the fracture toughness. Unlike conventional materials, nodular cast iron exhibits an increasing apparent fracture toughness with the specimen thickness.[9,10,11] The fracture toughness should, thus, be measured by using the elasticplastic fracture mechanics (EPFM) approach. The critical stress intensity value KJ, deduced from the critical value of the J crack extension force, appears to be dependent on the percentage of unloading performed for the monitoring of crack extension.[12,13,14] Some interpretations are proposed and discussed as follows. (a) Influence of the stress state. The presence of graphite nodules preserves plane stress conditions throughout the whole thickness of the specimen, even for specimens having a large thickness.[15] The study of Nanstad et al.[8] also suggests that for nodular cast iron, the specimen is entirely subjected to plane stress conditions. Inversely, at low temperatures, cast iron remains essentially elastic in fracture toughness measurements. In this case, where LEFM applies and plane strain conditions can be fulfilled, the increase of fracture toughness with specimen thickness is not observed.[8] M.J. DONG, Researcher, and C. PRIOUL and D. FRANC¸OIS, Professors, are with the Laboratory MSS/MAT, CNRS, URA 850, Ecole Centrale de Paris, 92295 Chatenay Malabry Cedex, France. Manuscript submitted February 6, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
(b) Damage ahead of the crack tip. The decohesion of the interface between matrix and graphite nodules, which appears in the early stage of loading, leads to a large dissipation of energy. At the time when KQ is measured, nodule decohesion extends throughout the whole ligament. Consequently, a larger KQ is measured in specimens having a large thickness.[6] (c) Influence of the microstructure. The microstructure may vary along the specimen thickness, so that the influence of the microstructure on the fracture toughness can lead to an increment of the fracture toughness with specimen thickness.[16] Much research has been devoted to studying the influence of microstructure on fracture toughness,[17–20] and many parameters have been tested: among them, the size and distribution of graphite nodules, the size of the ferritic grain, and the chemical composition of the matrix.[21] To date, the role of graphite and its damage effect on the fracture toughness of cast iron remain unclear. As shown in Part I of this article, the initiation of voids at the graphite/matrix interface begins at an early
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