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Sakimoto, Takahiro (Steel Research Laboratory, JFE Steel Corporation) | Igi, Satoshi (Steel Research Laboratory, JFE Steel Corporation) | Endo, Shigeru (Steel Research Laboratory, JFE Steel Corporation) | Suzuki, Shinichi (JFE Steel Corporation) | Østby, Erling (SINTEF)
Alvaro, Antonio (SINTEF Materials and Chemistry) | Akselsen, Odd M. (SINTEF Materials and Chemistry, The Norwegian University of Science and Technology (NTNU)) | Ren, Xiaobo (SINTEF Materials and Chemistry) | Perillo, Giovanni (SINTEF Materials and Chemistry) | Nyhus, Bård (SINTEF Materials and Chemistry)
This paper focuses on the investigation, assessment and comparison of a 420 MPa structural steel Charpy (CVN and pre-cracked) and fatigue crack growth rate test at different temperature spanning from room temperature to −120°C. Since weldments constitute the most probable location for fatigue-related failures, the material have been weld simulated in order to isolate and represent its Coarse Grained Heat Affected Zone. Results are analyzed and compared and an attempt to relate the Fatigue Ductile to Brittle transition (FDBT) and the static Ductile to brittle transition (DBT) temperatures is attempted in order to exploit the possibility to avoid or limit the most expensive and time consuming crack growth rate testing.
In the last years, a great push for oil and gas explorations in the Arctic regions (Gautier, Bird, Charpentier, Grantz, Houseknecht, Klett, Moore, Pitman, Schenk and Schuenemeyer, 2009) together with the increase possibility of an alternative and more direct Asia-North Europe connection kept the interest of oil and gas and maritime industry high. The development of oil and gas fields in the arctic brings to the table several challenges due to the cold and harsh climate; when it comes to the use of structural ferritic steels, particular concerns relate to their low-temperature properties. More precisely, when it comes to structural integrity of offshore structures built with ferritic steels, Ductile to Brittle Transition (DBT) and Fatigue Ductile to Brittle Transition (FDBT) needs to be carefully assessed in order to avoid unexpected catastrophic failures.
It is long known that, as ferritic steels operates at lower temperatures, they undergo a transition from a ductile shear-dominated to a brittle cleavage dominated fracture mode. This phenomenon is known as Ductile to Brittle Transition (DBT) and it is commonly quantified through the typical fracture mechanics parameters, i.e. CTOD (Crack Tip Opening Displacement), Charpy impact energy Cv, KIc or J-integral values. A schematic is presented in Fig 1.
Brandt, Kristin (Dept. Material Sci. Eng., Norwegian University of Science and Technology) | Solberg, Jan Ketil (Dept. Material Sci. Eng., Norwegian University of Science and Technology) | Akselsen, Odd Magne (SINTEF Materials and Chemistry, SINTEF) | Østby, Erling (SINTEF Materials and Chemistry, SINTEF)
This paper is devoted to micro-scale fracture testing of Arctic steel, by use of focused ion beam machined notched cantilevers. Bainite packets of a weld-simulated course grained heat-affected zone (CGHAZ) was the targeted microstructural aspect, with reference tests performed in pure iron. Micro-scale fracture testing has been developing in the last decade. The main objectives of micro-scale fracture tests are to obtain relevant toughness values for materials used at this scale, and to evaluate the fracture toughness of local microstructural aspects. The latter is the focus of this paper. Several models, including multiple barrier models, require specific material property inputs that are not obtainable through traditional testing at larger scale. Hence, micro-mechanical fracture has been applied to quantify these properties. Linear-elastic and elastic-plastic fracture mechanics parameters are presented and compared, with respect to testing material and temperature. Additionally, a new analytical tool is utilized to determine the criticality of a growing crack in terms of determining the energy required for further crack growth following initiation of stable crack growth.
The industrial activity in the Arctic is rapidly increasing, where accidents may cause severe ecological ramifications. Rough climate conditions and temperatures as low as -60°C require materials with specialized mechanical properties. The materials must display sufficient fracture and wear resistance at low temperatures, while avoiding excessive maintenance and maintaining lifetime integrity. In order to overcome these challenges, small-scale fracture mechanisms and properties must be understood.
BCC structures typically exhibit a rapid transition from ductile to brittle fracture, due to reduced mobility of screw dislocations and a reduced number of available slip systems, as the temperature is lowered (Brinckmann et al., 2008, Schreijäg et al., 2015). Full understanding of this transition requires a defined transition criterion. The change in fracture mode from ductile to brittle occurs over a temperature range that is closely interconnected with the change in deformation energy. Inside this temperature range, the metal exhibits fracture characteristics from both modes. There will be some ductile fracture near the notch, which changes to cleavage as the crack propagates. This is due to increased hydrostatic stresses as the propagation speed increases (Petch, 1958), implying that fracture will switch from ductile to brittle when the stress ahead of the fracture tip becomes capable of Griffith propagation. Fracture mechanics have gained increased interest due to several incidents where structures fail within the designed region of operation, initiating extensive research on fracture mechanisms, fracture initiation, propagation and arrest, as well as the temperature dependence of these mechanisms. In an attempt to enhance the understanding of the fracture mechanisms small-scale testing has been used to localize testing and to reduce the number of variables tested in each experiment.