Correlation of crack-tip opening angle for stable crack propagation with charpy and drop-weight tear test properties in
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I. INTRODUCTION
CURRENT models for characterizing arrest toughness of dynamic ductile fracture in gas-transmission pipelines have used upper shelf energy (USE) of Charpy V-notch (CVN) impact test and USE of drop-weight tear test (DWTT) as fracture resistance values.[1] In general, the arrest toughness of pipeline steels is predicted by a simplified semiempirical formula obtained from the correlation of fracture speed of pipelines with crack propagation speed, which depends on gas decompression behavior. According to the API RP 5L3 specification,[2] the CVN USE shows a linear relation with the USE of the pressed-notch (PN) DWTT or the chevron-notch (CN) DWTT. In the case of the currently produced high-toughness pipeline steels, however, the correlation of the CVN USE with fundamental fracture process or crack speed is less obvious. This is because a considerable amount of initiation energy is involved, which is unrelated to actual material resistance to fracture propagation as plastic deformation significantly increases at the crack tip.[3,4,5] Also, the impact testing methods are limited by specimen geometries, and the reliability of the correlation between CVN USE and DWTT USE decreases. The evaluation of the absorbed energy has provided reliable standards for low-toughness pipeline steels (CVN USE 100 J), but it is more or less inconsistent with full-scale fracture behavior in currently produced high-toughness pipeline steels. BYOUNGCHUL HWANG, Postdoctoral Research Associate, and SANG YONG SHIN, Research Assistant, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea. SUNGHAK LEE and NACK J. KIM, Professors, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, are jointly appointed with the Materials Science and Engineering Department, Pohang University of Science and Technology. Contact e-mail: [email protected] SANGHO KIM, Principal Researcher, and KI BONG KANG, Group Leader, are with the Plate Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Korea. Manuscript submitted December 9, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
Since the 1980s, many investigators have persistently endeavored to specify material resistance to fracture propagation using fracture mechanics variables such as crack-tip stress or strain, crack-tip opening displacement (CTOD) or crack-tip opening angle (CTOA), crack-tip force, energy release rate, J-integral, and tearing modulus.[6–9] Among them, the CTOA at a specified distance from a crack tip, i.e., the CTOA for stable crack propagation (CTOAsc), has been shown to be the most appropriate variable for modeling the stable crack growth and instability during the fracture process of pipeline steels. It has been used in aerospace industrial fields to predict the initiation of stable crack growth and was reported to remain constant irrespective of crack extension occurring during the steady-state fracture process.[10] The application of the CTOA to pipeline
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