Effects of Dynamic Strain Hardening Exponent on Abnormal Cleavage Fracture Occurring During Drop Weight Tear Test of API
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INTRODUCTION
RECENTLY, linepipe steels used to transport crude oil or natural gas over a long distance have become thicker and larger, and require high strength and toughness simultaneously to improve transportation efficiency under a high-pressure condition.[1–3] Oil-drilling operation in very cold areas, e.g., Alaska and Siberia, has been actively conducted because of worldwide resource depletion, and thus linepipe steels having excellent low-temperature toughness are highly demanded. Several laboratory testing methods for evaluating fracture properties of these steels have been developed to reasonably correlate with full-scale fracture behavior, and Charpy impact test and drop weight tear test (DWTT) are representative ones.[1,4–6] The absorbed energy obtained from the Charpy impact test is generally used as a standard to evaluate the resistance to unstable ductile fracture. The DWTT is an essential method to evaluate the fracture propagation transition MINJU KANG and HYUNMIN KIM, Research Assistants, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. SUNGHAK LEE, Professor, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, and also with the Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea. SANG YONG SHIN, Assistant Professor, is with the School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Korea. Contact e-mail: sshin@ ulsan.ac.kr Manuscript submitted June 26, 2012. Article published online October 16, 2013 682—VOLUME 45A, FEBRUARY 2014
temperature of linepipe steels,[4,7,8] and uses specimens having same thickness as actual linepipe steels to better explain the full-scale fracture behavior.[4–6] In recently developed high-toughness linepipe steels, however, an abnormal fracture appearance is sometimes observed on fracture surfaces of DWTT specimens, and acts as a problem in reliable evaluation of DWTT properties.[9–12] This abnormal fracture appearance occurs in the area impacted by a heavy hammer after the initiation of initial cleavage fracture at the notch.[12–14] In normal fractured DWTT specimens, initial cleavage fracture surfaces are observed near the notch tip and shear fracture surfaces are shown in the center of the DWTT specimens. In the abnormal fractured DWTT specimens, however, cleavage fracture surfaces are observed near the hammer-impacted regions as well as near the notch tip. The crack propagation direction of the abnormal cleavage fracture is from the center of the DWTT specimen to the hammer-impacted region.[4,9–12] It is well known that the abnormal cleavage fracture results from the reduced toughness of the hammerimpacted regions due to the work hardening by the strong and sudden hammer impact.[14,15] Large abnormal cleavage fracture surfaces are shown in the DWTT specimens of recently developed high-strength hightoughness API linepipe steels,[16] so that the steels do not satisfy the API t
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