Microstructures controlling the ductile crack growth resistance of low carbon steels
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INTRODUCTION
IN the ductile-brittle fracture transition region of steels, blunting of the precrack and initiation and growth of a stable ductile crack generally precede the onset of unstable brittle fracture. The plastic work associated with such processes may play a substantial part in the observed fracture toughness, while plasticity also plays an important role on the brittle fracture nucleation stage. The fracture toughness of a steel is sensitive to its chemical composition and microstructure, as discussed in many review articles.[1–4] However, whether their functions originate in the ductile crack growth or brittle fracture initiation stages is still a subject to be elucidated. Grain size refinement, which is a major way of improving the fracture toughness of steels, hardly affects the ductile crack growth resistance.[5] On the other hand, Ca treatment of an A516-70 steel was revealed to increase the ductile crack growth resistance.[6] It has been well accepted that second-phase particles play a major role in the fracture strain of steels,[7] and the effect of Ca treatment can be through the volume fraction and morphology of second-phase particles. However, the ductile crack growth resistance is not determined solely by the nucleation process of voids. The plastic energy is also needed for their linking, which is controlled by other microstructural factors. In a macroscopic way, the role of ductile fracture on the entire fracture process is represented on the R curve, which correlates J-integral values with the crack extension length, in terms of the JIC, indicating the onset of the ductile crack, or by dJ/d Da, the resistance against the growth of the ductile crack length, Da. From a microscopic viewpoint, the morphology of the ductile fracture surface is an expected indication of the role of microstructures in toughness. Since the ductile fracture surface is generally characterized by the dimple pattern, correlation of the macroscopic toughness parameters with dimple morphology is a way to understand the effect of microstructure on the fracture toughness. Based on a concept that the dimple shape is related to local strain, Thompson and Ashby related JIC to a dimple shape HIROSHI YOSHIDA, Research Associate, and MICHIHIKO NAGUMO, Professor, are with the Department of Materials Science and Engineering, Waseda University, Tokyo 169, Japan. Manuscript submitted November 12, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
and a microstructural scale parameter.[8] Bray et al. examined the relationship between the crack tip opening displacement at fracture, dIC, and the extent of primary void growth, but no general scaling between them was obtained.[9] In such treatments of fractographic analysis, care must be paid to the inhomogeneity on the fracture surface. The stress state at the extending crack front will vary with the crack length, thus affecting the dimple shape. It is also noted that microstructure itself, including the second-phase particle spacing, is inhomogeneous. When compared with the brittle
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