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In view of the reduction in operational costs, container ships have become large-scaled so significantly that even larger ships of over 16,000 TEU have been designed and constructed in recent years. And these ships are applied heavy plates 70–90mm in thickness. The Kca ≧ 4,000–6,000 N/mm of arrestability in temperature-gradient ESSO tests (large scale crack-arrest test) is needed for arresting super-long brittle cracks (193 research committee of Japan Ship Research Association). Thus, in this paper, improvement in the arrestability of steel plates was achieved through investigating the effect of shear-lips. As a result, generating shear-lips at lower temperature and enhancing the propagating resistance of ductile cracks allowed us to improve arrestability. In addition, it is not practical for us to employ an ESSO test as a usual testing method for product inspection due to its large size. Accordingly, a Kca estimation method according to the combined use of a drop weight test together with a Charpy impact test was analyzed, and it was thus confirmed that said method had good accuracy regarding estimation. INTRODUCTION Figure1 shows ESSO specimen. The brittle crack forced to be initiated by wedging. Temperature gradient is 0.25–0.35°C/mm in standard ESSO test (ClassNK, Guidelines on Brittle Crack Arrest Design 2009) Figure 2 shows the relationship between 50%FATT (Fracture Appearance Transition Temperature:50% brittle, 50% ductile) of Chalpy impact test at 1/2t and at a Kca value obtained by ESSO test at 0°C for YP315 and YP355 N/mm E-grade steel plates. Chalpy is test for brittle crack initiation and ESSO is test for crack arrestability. However, there are mutual relation between Chalpy and ESSO test, because they strongly depend on grain size of steels. Therefore it is considered E-grade, best grade of ordinary ship steels in toughness, posses suitable or sufficient arrestability for prevention of brittle fracture.
- Materials > Metals & Mining > Steel (1.00)
- Transportation (0.93)
Evaluation Method For Crack Arrestability of Steel Plates Using Small-Scale Fracture Test Results
Ishikawa, Tadashi (Steel Research Laboratories, Nippon Steel Corporation) | Inoue, Takehiro (Steel Research Laboratories, Nippon Steel Corporation) | Funatsu, Yuuji (Plate Sales Division, Nippon Steel Corporation) | Otani, Jun (Oita works, Nippon Steel Corporation)
ABSTRACT The crack arrestability in a steel plate is determined by both its surface layer properties and its mid-section properties. Through investigation of the correlation between ESSO test results (as a large scale crack-arrest test) and NRL drop weight test results (as a small scale crackarrest test), it was found that crack arrestability can be estimated as a function of plate thickness using a proposed equation and the NDT temperatures of the NRL drop weight test obtained from both the surface layer section and the mid-section of a steel plate. Applying this concept to very thick EH47-class steel, it is shown that the crack arrestability can also be estimated using small scale test results, although the correlation factors should be determined independently for each steel plate manufacturing process. INTRODUCTION Large scale welded structures need to maintain their structural integrity against brittle fractures in order to avoid catastrophic failures and accidents. For large ships and low temperature storage tanks, the double integrity concept, which takes into account both the prevention of brittle fracture initiation and the arrest of brittle fractures in the case of an emergency, has been investigated (Machida, 1972) (Kanazawa, 1973). To realize the double integrity concept, high performance steel plates with improved crack arrestability have been developed, such as SUF (Surface layer with Ultra Fine grained microstructure) steel and EH47-class steel with sufficient crack arrestability. Such steel plates have already been used in the construction of important vessels, such as LPG carriers and mega-containerships. Recently, the significance of crack arrestability in steel plates for determining the structural integrity of mega-containerships has increased dramatically, because some ship classification societies are now considering and establishing new guide-lines or new standards concerning the crack arrestability of very thick, high-tensile steel plates.