The cleavage resistance of two microalloyed steels (steels A and B) was studied using several tests, including the instrumented precracked Charpy and Charpy V-notch (CVN) techniques. Ductile-tobrittle transition temperatures were measured for the base-metal and simulated heat-affected zone (HAZ) microstructures. Steel B showed inferior cleavage resistance to steel A, and this could not be explained by differences in gross microstructure. Scanning electron fractography revealed that TiN inclusions were responsible for cleavage initiation in steel B. These inclusions were well bonded to the ferritic matrix. It is believed that a strong inclusion-matrix bond is a key factor in why TiN inclusions are potent cleavage initiators in steel. Strong bonding allows high stresses in a crack/notchtip plastic zone to act on the inclusions without debonding the interface. Once an inclusion cleaves, the strong bond allows for transfer of the TiN crack into the ferritic matrix. It was estimated that only 0.0016 wt pct Ti was tied up in the offending inclusions in steel B. This indicates that extended times at high temperatures during the casting of such steels could produce TiN-related toughness deterioration at even modest Ti contents.

Strain-based design (SBD) pipelines are being considered to develop hydrocarbon resources in severe environments. As part of a research program to develop a SBD methodology, work was conducted to develop a suitable fracture mechanics test that can be used as part of a strain capacity prediction technique. The single edge notched tensile (SENT) specimen geometry has been chosen due to the similarity in crack-tip constraint conditions with that of defects in pipeline girth welds. This paper describes a single-specimen compliance method suitable for measuring ductile fracture resistance in terms of crack tip opening displacement resistance (CTOD-R) curves. The development work included investigation of the following items: specimen geometry, crack geometry and orientation (including crack depth effects), direct measurement of CTOD. The results demonstrate that toughness measurements obtained using a B = W configuration (B = specimen thickness, W = specimen width) with side grooves are similar to those using a B = 2W configuration without side grooves; however, specimens with side grooves and B = W geometry facilitates even crack growth. Studies of crack depth have shown that ductile fracture resistance decreases with increasing ratio of the initial crack depth to specimen width, a0/W. Studies of notch location and orientation (outer diameter (OD) and inner diameter (ID) surface notches and through-thickness notches) have shown an effect of this variable on the CTOD-R curves. This has been partly attributed to crack progression (tearing direction) with respect to weld geometry and this effect is consistent with damage modeling predictions. However the experimentally observed difference of CTOD-R curves between ID and OD notches is believed to be primarily due to the material variability through the pipe thickness.

In Part I of this study, cuboidally shaped inclusions were found to be responsible for cleavage initiation in a low-carbon, microalloyed steel. In Part II, electron microdiffraction was used to identify these inclusions as the fcc phase (NaCl prototype) in the titanium-nitrogen system. A model for cleavage as induced by these inclusions is proposed. A microcrack begins on one side of the TiN inclusion, propagates to the other side, and then transfers into the matrix. Initiation at a particular location in the particle is believed to be caused by dislocation pileup impingement and stress concentrations such as crystal defects and surface irregularities within the TiN. Dislocations in the TiN inclusions were imaged by transmission electron microscopy (TEM). After the TiN microcrack transfers into the matrix, propagation spreads radially. From the area of crack transfer, two simultaneous propagation paths reverse directions and rotate around the particle. The particle separates these cracks for a short distance, they travel on different cleavage planes, and upon rejoining, a ridge of torn matrix is created. The location of this ridge can be used to infer where cleavage began in the TiN and where the microcrack transferred into the matrix. Tessellated residual stresses arising from differential thermal contraction between the TiN and the matrix are suggested to increase the cleavage-initiating potency of TiN inclusions.

Various industry efforts are underway to improve or develop new methods to address the design of pipelines in harsh arctic or seismically active regions. Reliable characterization of tensile strain capacity of welded pipelines is a key issue in development of strain-based design methodologies. Recently, improved FEA-based approaches for prediction of tensile strain capacity have been developed. However, these FEA-based approaches require complex, computationally intensive modeling and analyses. Parametric studies can provide an approach towards developing practical, efficient methods for strain capacity prediction. This paper presents closed-form, simplified strain capacity equations developed through a large-scale 3D FEA-based parametric study for welded pipelines. A non-dimensional parameter is presented to relate the influence of flaw and pipe geometry parameters to tensile strain capacity. The required input parameters, their limits of applicability and simplified equations for tensile strain capacity are presented. The equations are validated through a comprehensive full-scale test program to measure the strain capacity of pressurized pipelines spanning a range of pipe grades, thickness, weld overmatch and misalignment levels. It is shown that the current simplified equations can be used for appropriate specification of weld and pipe materials properties, design concept selection and the design of full-scale tests for strain-based design qualification. The equations can also provide the basis for codified strain-based design engineering critical assessment procedures for welded pipelines.

Strain-based design (SBD) is used to complement conventional allowable stress design for pipelines operated in environments with potentially large ground movements such as those found in permafrost and seismically active regions. Reliable and accurate prediction of tensile strain capacity (TSC) plays a critical role in strain-based design. As reported previously over the past 6+ years, a comprehensive experimental and numerical program was undertaken to characterize the TSC of welded pipelines, develop a finite element analysis (FEA) approach and equations capable of predicting TSC, and establish a strain-based engineering critical assessment (SBECA) methodology. The previous FEA model and TSC equations were validated against about 50 full-scale pipe strain capacity tests and are accurate within the validated variable ranges. In the current paper, enhancements of the previous model and equations are described. The enhancements include incorporation of advanced damage mechanics modeling into TSC prediction, development of a new TSC equation, expansion of variable ranges and functionality upgrades. The new model and equation are applicable over larger ranges of material properties and flaw sizes. The new FEA model is also used to establish surface flaw interaction rules for SBD. The new FEA model is validated against more than 40 full-scale non-pressurized and pressurized tests and underpins the development of the new TSC equation. The equation is validated against a total of 93 full-scale tests (FST). In addition to the enhancements, sample applications of the TSC model and equation are presented in the paper, for example, an investigation of the effects on strain capacity of Lüders strain and ductile tearing. Challenges in predicting TSC are also addressed.