Aihara, Shuji (The University of Tokyo) | Watabe, Yoshiyuki (The University of Tokyo) | Shibanuma, Kazuki (The University of Tokyo) | Inoue, Takehiro (Nippon Steel Corporation) | Koseki, Tadashi (Nippon Steel Technoresearch Corp.)
A new model for dynamic crack propagation and arrest in steel plates is proposed. The model is based on the local fracture stress criterion and incorporates the influence of plastic deformation on the tip of a dynamically propagating crack. The model explains anomalous data in temperature-gradient crack arrest test under high applied stress. The model also explains many aspects of crack propagation and arrest behaviors. The hypothesis that crack driving force is saturated at long crack propagation has been replaced by the present model. INTRODUCTION Prevention of brittle crack propagation as well as crack initiation is essentially important for ensuring double integrity in such steel structures as large containerships and offshore structures. Brittle crack initiation is prevented by controlling defects associated with welding practices and cracks associated with repeated loading during service and by selecting steel plates with high HAZ toughness and high toughness welding materials. On the other hand, brittle crack propagation must be prevented by adopting steel plates with high crack arrest toughness of base metal, together with proper crack arrest design. Nippon Kaiji Kyokai published the guidelines on Brittle Crack Arrest Design, (Nippon Kaiji Kyokai, 2009), in which fundamental procedures for preventing brittle crack propagation in containership hull structures. The guideline also addresses a testing method to measure brittle crack arrest toughness using temperature-gradient tests. Brittle crack propagation and arrest can be evaluated by fracture mechanics approach; a crack continues to propagate if crack driving force as expressed by stress intensity factor, K, exceeds crack arrest toughness, Kca, and vice versa. It has long been recognized, however, that a crack could be arrested in some of ultra-wide plate tests even if K exceeds Kca, (Machida and Aoki, 1972). Figure 1 shows their crack arrest test results of 30mm thick YP24 steel plates.
Kubo, Akifumi (Nippon Kaiji Kyokai) | Yajima, Hiroshi (Nagasaki Institute of Applied Science) | Aihara, Shuji (Tokyo University) | Yoshinari, Hitoshi (National Maritime Research Institute) | Hirota, Kazuhiro (Mitsubishi Heavy Industries, Ltd) | Toyoda, Masanobu (IHI Marine United Inc) | Kiyosue, Takaaki (Kawasaki Heavy Industries, Ltd) | Inoue, Takehiro (Nippon Steel Corporation) | Handa, Tsunehisa (JFE Steel Corporation) | Kawabata, Tomoya (SumitomoMetal Industries, Ltd) | Tani, Tokutaka (Kobe Steel, Ltd) | Yamaguchi, Yoshiya (Nippon Kaiji Kyokai)
Sugimoto, Kei (Nippon Kaiji Kyokai) | Hiroshi, Hiroshi (Nagasaki Institute of Applied Science) | Aihara, Shuji (Tokyo University) | Yoshinari, Hitoshi (National Maritime Research Institute) | Hirota, Kaszuhiro (Mitsubishi Heavy Industies, Ltd) | Toyoda, Masanobu (IHI Marine United Inc) | Kiyosue, Takaaki (Kawasaki Heavy Industries, Ltd) | Inoue, Takehiro (Nippon Steel Corporation) | Handa, Tsunehisa (JFE Steel Corporation) | Kawabata, Tomoya (SumitomoMetal Industries, Ltd) | Tani, Tokutaka (Kobe Steel, Ltd) | Usami, Akio (Nippon Kaiji Kyokai)
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/mm1.5 of arrestability in temperature-gradient ESSO tests (large scale crack-arrest test) is needed for arresting super-long brittle cracks (193rd 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/mm2 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.
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)