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.
This state-of-the-art paper is devoted to testing and evaluation of microstructural crack arrest. Testing and analysis of crack arrest have developed in the last decades, enhancing our understanding of the mechanisms behind crack arrest in a continuum mechanics perspective. Understanding crack arrest is important when operations are moving towards Arctic regions as low temperatures are detrimental to most steel’s fracture toughness. Large-scale testing is expensive and unpractical, and current methods fail to reflect the microstructural and micromechanical features of the fracture process. In order to increase the effectiveness of characterizing crack arrest properties, small-scale tests, as well as numerical methods, have been developed. The mechanical basis and mechanisms behind crack arrest are presented. Global and micro-arrest is considered. Key methods for understanding, evaluating and obtaining arrest parameters are presented: (i) statistical treatment of experimental results, (ii) barrier models for separating fracture and arrest sequences, and (iii) numerical tools for determining arrest behaviour. Brief presentations of the main mechanisms of crack arrest are presented with focus on the micromechanisms of arrest. The effect of grain boundaries, lattice orientation and second-phase particles upon propagation controlled cleavage are discussed, as well as their role in the arrest mechanism. Developments in arrest testing and evaluation are presented. Experimentally and numerically obtained results are linked to relevant mechanisms and theory, exhibiting the predictability and importance of crack arrest properties, and the understanding of the governing mechanisms behind crack arrest. The potential for increased understanding of the brittle fracture arrest phenomenon associated with new methods for nanomechanical testing of the material properties inside individual grains, and over grain boundaries, as well as the rapidly improving capabilities of atomistic modelling of deformation and fracture, is presented to pave the way for the future research within this field. Areas where further research could enhance our knowledge of crack arrest are listed.
Crack arrest is considering running cracks that are halted due to increasing resistance to crack propagation and/or reduced crack driving force. The former may be due to microstructural barriers or thermal gradients in the material. The latter may occur under partly displacement controlled loading, where the crack extension may increase the compliance of the structure and reduce the local crack driving force, or as a result of dynamic effects caused by impact loading or stress oscillations in the structure. This paper is mainly concerned with aspects related to the material’s resistance to crack propagation, i.e. the arrest toughness. Further, crack propagation is assumed to be dominated by cleavage fracture, i.e. ductile fracture and fatigue are not considered. The relative importance of these factors depends on the scale of which the arrest is considered. Further, the arrest can also be considered for different scenarios ranging from arrest of single grain sized microcracks up to arrest of macroscopic cracks on in the centimeter to meter range. In the first group the arrest happens locally, probably highly influenced by local microstructural features like grain boundary orientation, and would rather be categorized as avoidance of cleavage initiation on the macroscopic scale. In the latter group the problem is more of a conventional engineering fracture mechanics issue, ideally assessed through knowledge or measurements of the macroscopic arrest toughness, Kia. Ultimately, the two groups are part of the same problem, and there is a research aim to establish quantitative relations at different scales in orderto arrive at a general treatment of the problem.
Østby, Erling (SINTEF Materials and Chemistry, Trondheim, Norway) | Nyhus, Bård (SINTEF Materials and Chemistry, Trondheim, Norway) | Hauge, Mons (StatoilHydro ASA, Trondheim, Norway) | Levold, Erik (StatoilHydro ASA, Trondheim, Norway) | Sandvik, Andreas (StatoilHydro ASA, Trondheim, Norway) | Thaulow, Christian (Norwegian University of Science and Technology, Trondheim, Norway)
Østby, Erling (SINTEF Materials and Chemistry) | Nyhus, Bård (SINTEF Materials and Chemistry) | Sandvik, Andreas (StatoilHydro ASA) | Levold, Erik (StatoilHydro ASA) | Thaulow, Christian (Norwegian University of Science and Technology)
In this paper the results from SENT testing of two different welding procedures using an X65 base material is presented. The first welding procedure yields close to evenmatch conditions, whereas the second welding procedure gives 10-15% overmatch compared to the base material. Both defects lying in the weld metal and on the fusion line are investigated. It is observed that the ductile tearing resistances in both weld metals are significantly lower than for the base material. The resistance curves measured for the fusion line defects are more similar to the base material curve, however, slightly different crack growth is obtained depending on which side of the defect the measurements are performed. The crack driving force and strain capacity are on average higher in the overmatch specimens. However, a significant scatter is observed, especially for the weld metal defects. For the fusion line defects the scatter is smaller. For the material systems investigated the strain capacity will on average not depend strongly on the crack position.
Defects may limit the tensile strain capacity of pipelines. Such defects are mainly found in relation to girth welds. Mismatching in weld metal (WM) stress-strain properties compared to the base material will lead to a modification of the crack driving force as a function of the applied strain. It is common practice to specify overmatch conditions in the weld metal in order to shield or reduce the deformation in this region. However, overmatch can be difficult to obtain in some cases (e.g. for very high strength steels). Another aspect is related to the larger scatter in material properties usually found in weld metals. Also, the ductile crack growth resistance will in many cases differ between the weld metal and the base material. Although not without exceptions, the metallurgical conditions in the weld metal will usually lead to a reduced crack growth resistance compared to the base material of the pipe.
Haugen, Veronica (NTNU (Norwegian University of Science and Technology)) | Rogne, Bjørn Rune Søraas (NTNU (Norwegian University of Science and Technology)) | Akselsen, Odd M. (NTNU (Norwegian University of Science and Technology)) | Thaulow, Christian (NTNU (Norwegian University of Science and Technology) ,and SINTEF) | Østby, Erling (SINTEF)
Østby, Erling (SINTEF Materials and Chemistry) | Kolstad, Gaute T. (NTNU (Norwegian University of Science and Technology)) | Thaulow, Christian (NTNU (Norwegian University of Science and Technology)) | Akselsen, Odd M. (SINTEF Materials and Chemistry, and NTNU (Norwegian University of Science and Technology)) | Hauge, Mons (NTNU (Norwegian University of Science and Technology), and Statoil ASA)
Olsø, Erlend (SINTEF Materials and Chemistry) | Nyhus, Bård (SINTEF Materials and Chemistry) | Østby, Erling (SINTEF Materials and Chemistry) | Berg, Espen (FMC Technologies) | Thaulow, Christian (Norwegian University of Science and Technology)
Sørås Rogne, Bjørn Rune (Department of Engineering Design and Materials, Norwegian University of Science and Technology) | Thaulow, Christian (Department of Engineering Design and Materials, Norwegian University of Science and Technology)
Thaulow, Christian (Norwegian University of Science and Technology, Dept Engineering Design and Materials) | Østby, Erling (SINTEF Materials and Chemistry) | Akselsen, Odd M. (SINTEF Materials and Chemistry) | Lange, Hans Iver (SINTEF Materials and Chemistry) | Åldstedt, Synnøve (SINTEF Materials and Chemistry)