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.
Hauge, Mons (Statoil) | Maier, Mark (Shell Global Solutions International BV) | Walters, Carey L. (Structural Dynamics, TNO) | Østby, Erling (Det Norske Vertias, AS) | Kordonets, Sergei M. (Hull department, Head Office of Russian Maritime Register of Shipping) | Zanfir, Christian (Office of Public Safety, CWB) | Osvoll, Harald (FORCE Technology Norway AS)
An ISO subcommittee was set up in 2011 to improve the existing standards and norms with respect to arctic offshore operations for the petroleum, petrochemical, and natural gas industries. Within this subcommittee, a specific working group was established to address the application of materials in the environment of the arctic and cold regions. The work is focusing on a number of specific aspects related to the application of ferritic steels. The current paper summarizes the efforts of this working group to date.
Instrumented Charpy testing on 420 MPa grade construction steel was done. Two weld thermal simulated microstructures and two different initial crack geometries were studied over a wide range of temperatures in ductile-brittle-transition temperature (DBTT) region. Simulated microstructures included the coarse grained (CGHAZ) and inter-critically reheated coarse grained heat affected zones (ICCGHAZ). The base metal DBTT was around -130 and -80°C for V-notch and pre-cracked samples, respectively. For pre-cracked CGHAZ and ICCGHAZ microstructures, the DBTT is in the range -10 to 10°C, with ICCGHAZ showing a higher DBTT than CGHAZ. The arrest DBTT temperature (TKIa) for the base material, estimated from an empirical correlation with initiation reference temperature T0, was found to be around -50°C. Based on the suggested 4kN force drop value of the V-notch Charpy tests, TKIa for the ICCGHAZ (-17°C) was lower than for the CGHAZ (1°C).
This paper discusses the role played by FEA (Finite Element Analysis) in the development of basic understanding and procedures for the use in association with strain-based fracture assessments of pipelines. The basic fracture mechanics parameters and their assumptions are briefly presented and special challenges and possible limitations with respect to applications in large plastic strain scenarios are identified. The motivation for use of FEA in solving some of these challenges is discussed from different perspectives. A series of examples, although not claimed to be exhaustive, of the use of FEA in relation to strain-based facture assessment of pipelines is included to give a representative picture of the state of the art. The relation to general codes is also discussed, and the current lack of clear guidance on how to carry out such analyses is highlighted. The paper concludes with some perspectives regarding further development of the field, and some possible general steps are proposed in this respect.
In the North Sea oil and gas installations, steel castings have been used for many decades. Here, high strength steel castings offer the chance to manufacture complex heavy-lift and fatigue-critical components for larger offshore structures without increasing the weight of the components or platforms. However, when the activities are moving north to colder climates, current existing castings may fail to meet the toughness requirements, and there is very limited information available on behaviour of weldments of castings under such extreme conditions. Therefore, the present investigation was carried out addressing the low temperature toughness of high nickel (~1.5% Ni) steel casting with 460 MPa yield strength. Preliminary welding trials were performed with flux-cored arc welding (FCAW) with an overmatch in weld metal strength. Both Charpy V notch impact and CTOD fracture mechanical testing were included at ?60°C. The results show that the Charpy V notch toughness is excellent at -60°C (> 100 J). The fusion line CTOD fracture toughness showed low values for the SENB05 samples, while SENB02 gave higher values. For both geometries, the lowest values were connected with pop-in events. The weld metal fracture toughness was satisfactory with the lowest value of 0.28 mm.
The influence of low temperature during installation of rigid pipelines by the reeling method has been investigated. More specifically the resistance towards ductile tearing has been assessed for pipeline girth welds subjected to large scale simulated reeling at ambient and low temperature.
Test joints with girth welds were fabricated and subjected to simulated reeling (full scale bending rig) at ambient and “Arctic” temperature of +13°C and -17°C, respectively. A double installation cycle was applied and subsequently samples were extracted both from the material ending its deformation cycle in plastic tension or compression.
Small scale, i.e. Single Edge Notch Tension (SENT) fracture mechanical testing was performed at -30°C. Fracture resistance was assessed in terms of load resistance (SENT CTOD-R and J-R) curves.
The main conclusions from the performed study are:
1) The pre-deformation does not significantly modify the ductile tearing resistance.
2) Low pre-deformation temperatures do not lead to any significantly drop of the tearing resistance.
Acoustic emission (AE) is used to monitor cleavage microcracking activity in weld thermal simulated HAZ microstructures of a 420 MPa rolled plate. Fracture mechanics testing at different temperatures is carried out for three different simulated HAZ microstructures: ICCGHAZ Δt8/5=15 s, CGHAZ Δt8/5=5 s, and ICCGHAZ Δt8/5=5 s. Two parameters are extracted from the AE measurements: the rate of microcrack nucleation and the distribution of arrested cleavage microcrack sizes. The latter is obtained based on a first-order relationship between microcrack sizes and AE signal amplitude, earlier established by the authors. The results are discussed in terms of the effects of temperature and microstructure. It is shown that on average the arrested microcrack sizes are smaller in the microstructures with faster cooling rate, i.e. with smaller prior austenite grain size. It is also demonstrated that the effect of temperature on the microcrack nucleation rate depends on the microstructure. Further, it is shown that a rapid increase in fracture toughness with temperature is usually associated with a significant reduction in the microcrack nucleation rate. The results are interesting both in terms of understanding the temperature effects on fracture toughness and also as input to development of micromechanical models for cleavage fracture.
The previous paper studied the effect of the internal stress distribution in specimen thickness on CTOD. The critical CTOD tends to decrease in consequence of the internal stress distribution in specimen thickness in case that secondary stress is tension mode toward the crack. In estimating the critical CTOD to brittle fracture in structural component with large residual stress distribution, it is important to take account of that secondary stress effect.
This paper presents the effect of weld residual stress on critical CTOD to brittle fracture at lower shelf temperature of standard fracture toughness test. Using SENB and SENT specimens with / without residual stress, the fracture toughness tests have been conducted at -60°C. The effect of weld residual stress on critical CTOD to brittle fracture depends on critical CTOD level. The critical CTOD with residual stress is calculated by the superposition of δ or stress intensity factor based on Dugdale model. Moreover, the δ on secondary stress derived from residual stress field scarcely affects constraint loss correction because the stress field from residual stress falls within the small scale yield condition.
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.