This work was conducted for the assessment of fracture resistance of lower toughness vintage line-pipe steel welds during a PRCI project to address DOT/PHMSA MegaRule toughness requirements. In a review of vintage line-pipe databases of Charpy data, high-constraint fracture toughness data, and full-scale data, a procedure was developed to determine the lowest toughness for ductile fracture initiation of vintage line-pipe base metal and welds. In a few of the weld cases, the procedure predicted that the ductile fracture initiation transition temperature was warmer than the minimum operating temperature. Hence a procedure like the ASME Reference Toughness curve (or Master Curve) was needed to determine the toughness in the cleavage fracture region. However, the Charpy upper-shelf toughness can be so low that the reference temperature (RTNDT) or the Master Curve T-0 value could not be determined. The full-size Charpy plateau (upper-shelf) energy (CVP) values ranged from 4 to 25 ft-lb. Therefore, a new Material-Specific Reference Toughness Curve needed to be established, with a different way of defining a Reference Temperature (T-ref). The shape of this new Material-Specific Reference Toughness Curve was consistent with the past ASME and prior toughness data and bounded them as well as C(T) data on these lower CVP steels. The key concept used was that the equation for the Reference Toughness curve was the boundary between cleavage fracture initiation and ductile tearing, which is similar to Kirk's T-us concept for the Master Curve for nuclear vessel applications. The prediction of the lowest temperature for ductile initiation was previously developed by use of hundreds of pipe fracture tests and thousands of specimen tests and validated against many additional full-scale pipe fracture tests afterwards. This Master-Curves of Fracture Transition Temperatures procedure allowed the use of Charpy transition temperature data to be correlated to the transition temperature of through-wall-cracked or surface-cracked pipes of different thicknesses. The through-wall-cracked transition temperature also correlated well to high-constraint fracture mechanics test specimens [i.e., C(T) or SEN(B)]. The Charpy transition temperature correlation predicted the high-constraint C(T) specimen fracture toughness transition temperature as well, allowing for the determination of a new T-ref, which is a link from the traditional Master Curve to predicting through-wall-cracked and surface-cracked-pipe transition-temperature differences. Together the modified Master Curve and the Master-Curves of Fracture Transition Temperatures procedure created a powerful pragmatic tool. With the fact that the high-constraint C(T) specimen initiation toughness corresponds to the surface crack having an a/t of 0.7 and knowing that on the upper-shelf the surface-cracked-pipe toughness changes with a/t allows for constraint adjustments to be made to the new Material Specific Reference Toughness Curve. Shallower surface cracks have higher upper-shelf toughness values, so that adds a shift to the Material-Specific Reference Toughness Curve created. Additionally surface cracks have a lower transition temperature than high-constraint specimens, so that is another transition temperature shift.

This project addresses pragmatic application of the new DOT/PHMSA MegaRule RIN 1 - 192.712 for recommended toughness values for determining if an axial flaw needs repair or can instead be observed for any future growth. The MegaRule toughness is expressed as the Charpy energy value, which came from Charpy test data at 50F. However, the fracture toughness of a surface-cracked pipe is quite different than Charpy impact toughness values, especially if the Charpy data is in the transition region. In this project, it was shown from past axial surface-cracked pipe tests that the lowest temperature for ductile initiation is significantly lower than the Charpy test transition temperature. In cases with axial surface cracks in vintage base metal pipe tests, the surface-crack ductile transition temperature was greater than 200F lower than the Charpy transition temperature. The procedures were extended to welds, and databases from member companies were examined to establish what Charpy energy values should be used to reflect the tough-ness of a surface crack in the pipe at the operating temperatures, and still be consistent with the MegaRule Charpy information and safety desires.

As standards discussions progress covering hydrogen transmission by pipeline, and new test data become available, it is important there exists a common understanding of the key terms and nomenclature associated with the various forms of hydrogen damage and the mechanisms of hydrogen embrittlement. A glossary of terms and descriptions of typical test methods and nomenclature are provided to enhance communication between individuals involved in discussions of the effects of hydrogen on pipeline steels. Differences will always exist in definitions of terms, particularly between various industries. However, the information provided here tries to provide the broadest possible usage, with particular emphasis on the pipeline community.

CO2 pipelines are being proposed for CCS of anthropogenic CO2. CO2 can be transported by pipelines in gaseous or liquid (dense-phase) states. In the dense-phase state, the traditional fracture control design procedure frequently requires such a high toughness for axial ductile fracture arrest in the pipe body that crack arrestors are needed. Even if material could be procured to meet the regulatory arrest criterion, such design conditions might result in an undesirable double-ended full-bore for hazard analysis. The full-bore opening from a pipe rupture can involve significant safety risk to the population along the right-of-way, as was experienced in the Denbury CO2 pipeline failure. An alternative design of a dense-phase CO2 pipeline that can prevent large openings/ruptures from happening was explored in this paper. With this design criterion, a new CO2 pipeline can become "Rupture-Proof", and evaluations to date have shown that the hazard zone could be reduced by a factor of similar to 100. The "Rupture-Proof" design for a CO2 line involves two modifications to the traditional pipeline design. The first design modification is to ensure that there is a "Quick Crack Arrest" of any potential axial flaw. With the appropriate thickness and material toughness specification, the crack would only propagate a few inches, ergo the "Quick Crack Arrest." The material aspects for the quick axial crack arrest require understanding of the material developments over the decades, and as shown there are four different types of line pipe behavior when conducting ductile fracture control analyses. An assessment of more recent CO2 full-scale burst tests in the UK is presented to show a more precise way of determining the arrest toughness for axial ductile fracture control. Using the best of the current material types, the methodology for a "quick crack arrest design" procedure was found viable with sample calculations. The second modification is to eliminate the circumferential pipe break at a girth weld, like what occurred at Denbury. From work done in the nuclear piping industry for repair of circumferential flaws in girth welds, a modest change to the weld cross-sectional geometry can make it so that even with large unintentional girth weld defects or undermatched weld metal, the girth weld would not fracture, and the rest of the pipe body could take the longitudinal strain.

This project addresses pragmatic application of the new DOT/PHMSA MegaRule RIN 1 - 192.712 for recommended toughness values for determining if an ILI detected axial flaw needs repair or can instead be observed for any future growth. The MegaRule toughness is expressed as the Charpy energy value, which came from Charpy test data at 50�F. However, the fracture toughness of a surface-cracked pipe is quite different than Charpy impact toughness values, especially if the Charpy data is in the brittle-to-ductile transition region. The surface-cracked pipe burst-pressure transition temperature can be greatly lower than the Charpy impact specimen transition temperature due to loading rates and constraint effects (bending versus tension loading). In cases with axial surface cracks in vintage base-metal pipe tests, the surface-cracked pipe burst pressure transition temperature was greater than 200�F lower than the Charpy transition temperature. The procedures were extended to welds, and in the Level 1 report, databases from member companies were examined to establish what Charpy energy values should be used to reflect the toughness of a surface crack in the pipe at the operating temperatures, and still be consistent with the MegaRule Charpy information and safety desires.

In the frame of the licensing process of the AtuchaII PHWR (pressurized heavy water reactor) the BEPU (Best-Estimate Plus Uncertainty) approach has been selected for issuing Chapter 15 of FSAR (Final Safety Analysis Report) dealing with accident analysis. In case of a LOCA (Loss of Coolant Accident) the assumption of instantaneous opening time is too conservative, especially for this kind of reactor with a positive coolant void reactivity coefficient.Analysis was performed to assess the ability of Atucha II nuclear power plant’s fast boron-injection system to prevent overheating of the fuel elements in the event of a Beyond Design Basis Accident (BDBA). The goal of the Analysis was to develop a Dynamic Break Opening which was used as the input for the LOCA analyses. The results show that the fast boron injection can effectively shut down the reactor, thereby preventing the overheating of the fuel elements.

As part of DOT/PHMSA project efforts being conducted at Emc(2), a number of sensitivity studies are being conducted on the effect of hydrogen on the integrity of pragmatic pipeline integrity challenges. These sensitivity studies will provide the basis for recommendations provided to DOT/PHMSA on what hydrogen challenges are more important for additional future experimental/analytical efforts. Three different types of analysis evaluations were conducted, and some are still in progress. The first set of sensitivity analyses are for evaluating the hydrogen concentrations that will locally develop at different integrity locations of concern with plastic-strain history effects and elastic residual stresses. The second type of analyses involves simple assessments of critical flaw size changes with existing methods. The third set of analyses involved unique and advanced numerical modelling for flaws for burst pressure predictions, for instance a crack in a pipeline hardspot. A fourth type of analysis underway involves predictions of hydrogen cracking by phase field/crack growth finite element (FE) analyses, and will be the subject of a future paper. Hydrogen concentration evaluations for pipeline-integrity challenge assessments involved the following cases. (a) The effects of the plastic straining on hydrogen concentration at an axial surface crack were examined with and without a prior hydrotest before going into hydrogen service. (b) The hydrogen concentration at weld residual stress locations such as the fillet weld of a repair sleeve is examined. The FE models in this case included welding simulations to develop the plastic strain history and the residual stresses. (c) The hydrogen concentration at a corroded local thin area in a pipe was also explored where there was significant plastic straining during hydrotesting, although the plastic strains and elastic residual stresses vary through the thickness from tension to compression. For evaluations using existing fracture mechanics models, evaluations of hydrogen effects on the burst pressure of axial surface flaws in pipes examined two cases. (a) The first case involves an axial surface crack in the base metal of a pipe with and without hydrogen using existing J-estimation analyses and constraint adjustments to the C(T) specimen J-R curve of the vintage linepipe steel. (b) The second involves evaluation of axial surface cracks in a hardspot where the strength and toughness changes with different hardness levels. The crack-driving force in these evaluations comes only from the pressure, and the analyses assume homogeneous strength properties in the pipe. Evaluations using more advanced FE modelling are undertaken for two cases as well. (a) For a crack in a hardspot, advanced FE modelling includes the pipe fabrication stresses from plate to pipe, the pressure-induced bending stresses through the thickness due to the flat region of the hardspot included in the FE mesh, as well as the pressure loading. The thermal plastic/phase transformation residual stresses are being added in additional future analyses, which is the fourth contribution to the crack-driving force. The simple burst pressure models only account for the pressure and assume the material is homogeneous and the pipe is perfectly circular. (b) For an axial surface crack in a harder electric resistance weld (ERW) seam weld, rigorous FE modelling was performed. In this FE modelling, the hard region of an ERW weld is included with the axial surface crack in the center of that ERW weld region, while the rest of the pipe has base metal strength. A variety of crack lengths and depths are evaluated to systematically see the difference due to including the hardness of the weld metal. The toughness difference for an inert environment (air or methane) and in hydrogen on the weld and base metals is included in the critical crack evaluations.

Traditional Leak-Before-Break (LBB) evaluations are generally easy to meet the acceptance criteria for large-diameter primary pipe loops but become more difficult to satisfy as the pipe diameter decreases. An advanced LBB method, the Engineering Mechanics Corporation of Columbus (Emc2) Robust LBB procedure, was implemented for representative 3-inch and 4-inch nominal pipe size pipe systems that are employed at a U.S. pressurized water reactor (PWR) and boiling water reactor (BWR), respectively. The Robust LBB procedure uses a finite element model of a piping system which accounts for plasticity from the applied forces and moments. In the Robust LBB procedure, it is necessary to input the seismic loading time-history inputs due to the plasticity making the traditional design elastic response-spectrum analyses unusable. The inertial and seismic anchor motion (SAM) contribution can be based on the maximum allowable elastic stress limits for reaching Service Level D loading. The SAM stresses in a dynamic displacement-time stress analysis are dependent on the difference between the applied and natural frequencies of the pipe system, i.e., if there is an exciting frequency right at the natural frequency to reach the SL-D inertial stress limit, the displacement amplitudes are small and consequently the SAM stresses are small. The analysis procedures were: first, the displacement-time input at the anchor points with different frequencies around the first natural frequency of the pipe system was done with elastic uncracked analyses to reach the moment corresponding to 3Sm inertial stresses. Second, the uncracked pipe analysis is performed with the nonlinear stress-strain curve to calculate the reduction of the applied moments compared to the design elastic limits with the same forcing function. A circumferential through-wall crack (TWC) of a small size was then inserted at the high stress location using a cracked-pipe-element (CPE) methodology. The crack size was increased until it was at least 75% around the circumference or pipe severance/rupture was reached. The findings to date for the two pipe-system geometries involving 3-inch and 4-inch diameter A106B and TP304 stainless steel pipes, show that when doing the FE time-dependent analyses at the maximum SL-D inertial stress loading, the circumferential cracks were stable for TWC lengths greater than 75-percent of the circumference. Simply introducing plasticity into the piping system has a significant impact on the peak applied moment for the same displacement-time history. As the crack size is increased in the peak applied moment continues to decline due to flexibility changes in the pipe system. The decrease in moment as a function of crack size shows that the piping system under seismic inertial loading is acting more like it is under displacement-controlled loading rather than load-controlled loading.

This project details the methodologies used to collect and review crack failures, near misses and false positives, and how the available technologies are used in consideration of the CM-SRP pillars of crack (1) susceptibility, (2) inspection, (3) assessment and remediation, and (4) management. This project outcome enhances crack management by providing an independent review and understanding of the research gaps as they relate to cracking in pipelines, based on historic incidents, operator interviews, and subject matter expert opinions. The scope of work consisted of developing future research guideline suggestions through the following tasks: * Task 1 - Collection and review of publicly available reports, * Task 2 - Collection and review of PRCI member incident reports and operator interviews, * Task 3 - Compilation of root causes for historic crack-related pipeline incidents, * Task 4 - Categorization of root causes within the CM-SRP, and * Task 5 - Identification of research gaps in the CM-SRP. This report provides more details about the research suggestions, and cross-references them with the core priorities as outlined in CM-SRP report "Pathway to Achieving Efficient and Effective Crack Management," as well as the future research project ideas that have been submitted to PRCI four CM-SRP pillars. This report has a related Webinar.

In flaw evaluation criteria, the design limits for the secondary stresses are frequently different than the primary stresses. The evaluation procedure for primary stresses is load-controlled based which is independent of pipe deformation. The evaluation procedures for secondary stresses are displacement controlled which are dependent on pipe deformation. Certain stress components such as thermal expansion, thermal stripping, welding residual stress, misalignment/cold-springing, dynamic anchor motion are historically considered secondary stresses and their design limits are based on elastic stress analysis. In reality, there can be much more rotation/displacement of the pipe with nonlinear fracture behavior due to nonlinear material behavior and plasticity at the crack plane providing extra margins on the elastically calculated rotation values that come from uncracked-pipe design analyses. In assessing secondary stress margin, a secondary stress reduction factor is defined as the ratio of elastic-plastic moment to elastic moment. This is equivalent to another concept using a plastic reduction factor (PRF) as well as the inverse of structural (or safety) factor (SF) in ASME Section XI flaw evaluation criteria for various service levels. In this work, the secondary stress reduction factor was determined for a representative pipe system with multiple crack sizes, crack locations, and loading conditions. Nonlinear finite element (FE) analyses of a whole uncracked-pipe system were performed using ABAQUS (R) under various loading combinations to determine the critical locations for cracks in the pipe system. Next, FE analyses of the cracked-pipe system were carried out using cracked-pipe element-a methodology developed by the authors. Cracked-pipe system analyses were conducted for two loading conditions-one producing contained plasticity or single-hinge system and the other producing larger plasticity in the pipe system. Several analyses were conducted for each loading condition with a combination of two crack sizes at two key locations. Secondary stress reduction factors were then calculated for both loading conditions in the pipe system. Finally, the margin in secondary stress was assessed for the pipe system by comparing the secondary stress reduction factors with that for straight pipe sections (determined for experimental bend tests) as well as with the recommended equivalent PRF and the equivalent ASME secondary stress correction factors.

The area of complex cracking in piping components and its effects on the fracture behavior and leak-before-beak evaluations has been highly researched. Several researchers have conducted experiments to quantify the behavior through complex cracked piping experiments starting from the 1980s and also more recently in dissimilar metal welds (DMWs). The area has also seen several contributions on the modeling aspects to characterize the crack initiation as well as the ductile crack growth behavior. In this work, the crack growth in complex-crack geometries is revisited through a novel laboratory specimen model, developed by modifying a Single-Edge Notch Tension SEN(T) specimen that is routinely used to obtain the fracture toughness values for both crack initiation as well as crack growth/tearing behavior. Details on the cell size used in the finite element analysis (FEA), and the effects on the predictability of the experimental observations are highlighted. The effects of constraint based on the relative levels of complex-cracking (aspect ratios) are discussed. While the results are precursors to the understanding of the correlations of constraints and fracture for these complex-cracked geometries, they provide guidelines for path forward towards development of methodologies to treat these when making reliable comparisons between material fracture resistance and crack driving forces that are routinely employed in fracture-based leak-before-break assessments for piping and piping components.

Cold-working of elbows and other fittings results in higher strength of the material and has traditionally been considered to be beneficial for piping applications. Although it is known that cold-working leads to spatiotemporal variation in the grain size and the concomitant material property and fracture toughness variations they are not routinely investigated. Hence, this paper reports the findings from elbow fracture experiments conducted at 550°F and internal pressurized conditions (2,250 psi) on TP304 cold-worked elbows. The objectives for conducting the tests were to evaluate the effects of cold-working on the applicability of existing techniques such as an “Original” Net-Section-Collapse (NSC) and recently developed “Apparent NSC” equations for pipes with different inner diameter (ID) surface crack (SC) depths and lengths. This is to determine the failure moments and the plastic reduction factor (PRF) obtained to translate these to piping system evaluations. Preliminary comparisons of the experimental findings with the maximum stress predictions existing for straight pipes and elbow fracture prediction methods developed in the International Piping Integrity Research Group (IPIRG)-2 program were revisited, along with the verification of the applicability of the American Society of Mechanical Engineers (ASME) B2 stress indices and flaw acceptance criteria.

Significant efforts were undertaken to develop experimental data to assess the effect of sustained load crack growth during a simulated hydrotest and the effect on fatigue crack growth during subsequent pressure cycling service operation. The pipe used was a 1960ERW 16-inch diameter by 0.250-inch thick X46 pipe. There were a variety of laboratory specimen tests including: standard and isochronous tensile tests at various rates, Charpy transition curves, C(T) J-R curve tests, and SENT J-R curve tests. There were axially surface-cracked pipe tests with monotonically increasing pressure to failure, as well as step-hold pressure tests. Cracks were put in the base metal as well as the ERW seam weld. Some key results to be shown are: 1. The fracture toughness of the base metal and ERW HAZ varies with a/t of the flaw in the SENT testing. Larger a/t specimens have lower toughness. It is believed that this behavior is a key aspect in leak-before-break behavior under pressure loading. 2. Slow strain rate tensile test showed that the yield point decreases with slower loading rates, but the strain-hardening exponent remained relatively constant. The drop in the stress-strain curve causes an increase in the crack-driving force, which is why crack growth can occur with sustained loading. 3. Step-hold pressurized pipe tests with axial surface cracks were conducted with a great amount of data and refined crack growth measurements. In one pipe test, there were four identical flaws. One flaw failed by leakage during a long hold-time. It could be patched, and the other three “identical” flaws all had some small (but different) amounts of ductile tearing. On subsequent pressure cycling, the crack growth rate was incredibly lower than standard fatigue crack growth rates. This was due to the crack retardation effect from the small ductile tear and crack-tip plasticity, with the reverse crack-tip compressive plasticity from unloading after the simulated hydrotest. The fatigue crack growth rates were about 2 to 3 orders of magnitude slower. The crack retardation benefit easily overcame the small loss in the ligament from ductile tearing. Instead of failure being about 11,300 cycles it took 267,400 cycles. This is a huge benefit for liquid lines. A subsequent follow-on program to this is being undertaken by DOT/PHMSA to better characterize the crack retardation effect and also develop a methodology to predict the “Rupture Free Operational Time” after hydrotest.

In typical leak-before-break (LBB) analyses in the nuclear industry, the uncracked piping normal operating forces and moments are applied in a cracked-pipe analytical procedure to determine normal leakage, and the combined forces and moments under normal operating condition and safe shutdown earthquake seismic loading are used in a fracture analysis to predict margins on "failure." The International Piping Integrity Research Program (1PIRG) performed in 1990-1998 provided some insights to typical LBB behaviors where pipe system tests were conducted with simulated seismic loadings. The test results showed a large margin on LBB, which was also recognized in 2011 when the Argentinian Atucha II plant was analyzed using a robust full FE model. It was found that when circumferential through-wall cracks were put in the highest stressed locations, the applied moment dropped for both normal operating and N + safe shutdown earthquake (SSE) loading as the crack length increased. The through-wall crack size for causing a double-ended guillotine break (DEGB) was greater than 90% of the circumference. Similar results were also found for a petrochemical pipe system where thermal expansion stresses are much higher than the primary stresses. Even with very low toughness materials, the critical crack size leading to DEGB was greater than 80% of the circumference. The implication of this work is that pragmatically there is much higher margin for DEGB failure in nuclear plant operation, and efforts would be better focused on the potential for a small-break loss-of-coolant accident (SB-LOCA).

During an in-depth review of the MAT-8 method, it was noted that there were assumptions for determining the FAD-curve shape by matching three points along FAD curves from FE analyses to get the plastic contribution of the crack-driving force (Jpl). Because the FAD-curve shapes from the FE analyses for different flaw sizes (depths and lengths) of interest for liquid line operation were quite different from both the MAT-8 assumed FAD-curve shapes and the FAD-curve fitting function for the FE-based FAD curves, an alternative procedure was pursued. For the cases of deeper and longer flaws, FAD-curve shapes cannot be found easily due to the highly nonlinear plastic behavior. To develop a new FE-based crack-driving force equation, a modification of the GE/EPRI basic formulation with contributions for elastic and plastic crack-driving force relationships was implemented. Several hundred FE analyses for axial surface cracks in pipes were performed. The new procedure was developed first by separating the elastic contribution from the total Japplied, then using the plastic part of Japplied versus pressure (Jpl-P) curve to establish an analytical solution. A nonlinear-regression analysis of the FE results from the entire Jpl-P curve was used, not just a few selected points. The new Jpl-P analytical solution showed good agreement with the carefully conducted FE-based solution from short, shallow flaws to deep, long flaws. The MAT-8 based solutions were good for short flaws, but deviated from the FE-based solution with both larger flaw depths and lengths. In addition to the new procedure, we included the use of the J-R curve for surface cracks which has been found to be a function of the surface crack a/t in pipe and SEN(T) tests, i.e., toughness is not a constant for all surface crack geometries which is discussed in other papers. Predicted burst pressures from the various methods including the new procedure were compared to pipe tests are presented.

The last several decades have seen growth in elastic-plastic fracture mechanics and the modeling of the behavior of structural steels employed in the nuclear, oil and gas, and other construction industries. Among these are a particular class of problems that provide challenges in modeling the physical behavior of structural steels using finite element modeling (FEM) approach that are based on microstructural damage and using parameters that depict the strain and stress states in the material region ahead of an existing crack. In this work, a recently experimented and investigated pipeline steel X80 material was modeled through two different fracture specimen geometries, namely single-edge-notch-tension, SEN(T) and compact-tension, C(T) to compare and contrast the predictions from two material damage models (microstructure and continuum based). The predictions from both these damage models that predict the ductile crack growth have been compared to the experimental findings of the crack growth (obtained using a d-c Electric Potential measurement technique), the corresponding load levels, and crack opening displacements (CODs). The points of similarity between the experimental measurements and the fracture surface observations of crack growth and the predictions from the FEM approach have been discussed. The same X80 material properties and damage model parameters were employed to predict the ductile crack growth in the two different fracture specimen geometries, SEN(T) and C(T) with a subtle change of one of the parameter values. This sheds light on the predictability of the crack initiation event and the subsequent ductile crack growth until failure using these damage models. The findings provide credence to the applicability of either model (after they are carefully tuned to arrive at optimized parameters) for piping materials while providing a framework for flaw evaluation methodologies. The investigation also opens the doors for regions where mesh regularization methods and modeling approaches along with mathematical relations can be developed to form a more efficient framework for modeling specimens with diverse constraints efficiently and develop material fracture resistance curves.

Abstract Single-edge-notch-tension, SEN(T), specimens have been found to provide a good similitude for surface cracks in pipes, where a surface-cracked structure has lower constraint condition than single-edge notch bend, SEN(B) (bend-bars), and compact-tension, C(T) specimens. This lower constraint condition gives higher upper-shelf toughness values, and also a lower brittle-to-ductile transition temperature. Additionally, the SEN(T) specimen eliminates concern of material anisotropy since the crack growth direction in the SEN(T) specimen is the same as in a surface-cracked pipe. While the existing recommended and industrial practices for SEN(T) have been developed based on assumption of homogenous or monomaterial across the crack, their applicability for the evaluation of fracture toughness of the heat-affected zone (HAZ) were evaluated in this investigation. When conducting experiments on SEN(T) specimens with a prescribed notch/crack in the HAZ, the asymmetric deformation around the crack causes the occurrence of a combination of Mode-I (crack opening) and mode-II (crack in-plane shearing) behavior. The extent of this mode mixity is dependent on the relative difference between the material properties of the adjacent girth weld and pipe base metals, as well as the amount of crack growth in the experiment. This mode mixity affects the measurement of the crack-tip-opening-displacement (CTOD) and evaluation of fracture mechanics parameter, J. The CTOD-R curve depicts the change in toughness with crack growth, in a manner similar to the J–R curve methodology. Observations also show a mismatch in the length of the crack growth that is measured on the fracture surface, attributable to the material deformation differences across the two adjacent materials (weld and base metals). This paper discusses the experimental observations of Mode I and Mode II behavior seen in experiments conducted on SEN(T) specimens with a notch/crack in the HAZ and as the crack propagates through the weld/HAZ thickness. The paper addresses the issues related to and the changes needed to account for such behavior toward the development of recommended practices or standards for SEN(T) experiments of weld/HAZ. The effects of mode mixity in HAZ SEN(T) experiments is critical to the development of crack growth resistance, CTOD-R and J–R curves employed in engineering critical assessment (ECA) of pipelines.

Cracking due to high temperature hydrogen attack (HTHA) has been observed in non-PWHT'd carbon steel process equipment at conditions of temperature and hydrogen partial pressure below the original design limits recommended in API RP 941, necessitating changes to that standard. Consequently, flaw assessment procedures are needed to manage defects detected during inspection, or to establish appropriate inspection frequency. The latter typically involving estimation of the time for a detected or postulated crack to reach a critical size. This type of evaluation has been difficult to perform owing to the scarcity of data of fracture toughness as well as crack growth rate for steels in high temperature hydrogen. To address this gap, an experimental program was undertaken to help describe the ductile tearing characteristics of steel removed from service with various levels of HTHA damage. Near full thickness single edge notched tension SEN(T) specimens were machined from field samples and tested using existing "natural" cracks as the starter-crack. This provided insight into the behavior of real flaws subject to constraint conditions closely matching circumferential flaws in piping. Tests of undamaged steel were also performed in hydrogen at conditions designed to produce HTHA and compared with tests run in nitrogen. Crack growth tests obtained from the literature have been used to develop an empirical crack growth law for use in fitness for service assessments. The C* integral was also explored as a parameter for describing crack growth rate due to the strong similarity of HTHA damage to creep. The key results show substantial reduction in tearing resistance resulting from HTHA damage. A crack growth law similar to the Nikbin-Smith-Webster (NSW) model using the C* integral was found to show promise in describing the combined effects of creep and HTHA on crack propagation, although additional testing is needed to validate the correlation.

Single-Edge-Notch Tension, SEN(T) specimens have been used for fracture toughness determinations of base, weld, and heat-affected-zone (HAZ) materials. While the experimental results and computational finite element analysis (FEA) of SENT specimens with cracks in base and weld metals are relatively straight-forward, the analysis of SENT specimens with cracks in the HAZ poses several complexities that are highlighted through computational FEA in this work. SENT specimens have been found to provide good similitude for surface cracks in pipes, where a surface-cracked structure has lower constraint condition than bend-bars, SEN(B) and compact tension, C(T), specimens. The lower constraint condition gives higher upper-shelf toughness values, and also a lower brittle-to-ductile transition temperature. Also, the SEN(T) specimen eliminates concern of material anisotropy since the crack growth direction in the SEN(T) specimen is the same as in a surface-cracked pipe. While traditional methods of evaluation allow for a direct interpretation and application of elastic-plastic fracture mechanics approaches such as those that have evolved over the last five decades, for SEN(T) specimens with a flaw/crack in base materials and to some extent in weld materials, the presence of cracking in the HAZ complicates such routine approaches. This work initially highlights findings from an earlier published SEN(T) experiment that was investigated and reported in support of the development of recommended practices and worldwide standards. From that experiment the presence of mode-mixity in HAZ fracture experiments was identified, and their characterization and determination of how to account for them in the recommended practices are under development. This work supplements the earlier experimental findings through computational FEA that shows the effects of mode-mixity and also highlights the differences when the initial crack is located in different regions of the HAZ and is of different lengths so as to keep the initial crack and the crack growth within the HAZ region. Of particular interest is the extent of crack growth that can be used for J-R and CTOD-R curves depicting fracture resistance curves for HAZ material cracks. Towards such an end, this work starts with an experimental discussion, followed by computational FEA to provide initial guidelines and a framework for future discussions. The goal being to create a better procedural analysis and treatment of the elastic-plastic crack growth in HAZ materials which will benefit the industry. The effects of mode-mixity in HAZ testing is critical to the development of crack growth resistance, CTOD-R and J-R curves employed in Engineering Critical Assessment (ECA) of pipelines.

Current flaw evaluation procedures for piping systems use elastic design analysis for secondary stresses (load-controlled), but fracture analysis is used for elastic-plastic loading behavior. The actual rotations/displacements of the pipe system that occur due to non-linear fracture behavior are much higher than the elastically calculated rotations/displacements. These actual non-linear rotations provide margins on the elastically calculated rotation values that come from uncracked pipe-design analysis. To account for this inconsistency, a Secondary Stress Weighting Factor (SSWF) was established which is the ratio of elastic-plastic moment to the elastic moment calculated through an elastic stress analysis. As long as the remote uncracked pipe stresses are below yield, the SSWF is 1.0, and if the uncracked pipe plastic stresses are above the yield stress, the SSWF reaches a limit which is called the Plastic Reduction Factor (PRF). A PRF was developed in an earlier investigation for outer diameter (OD) surface-cracked pipe tests at room temperature (RT). In this study, a PRF factor was developed for internal diameter (ID) surface-cracked pipes and elbows at an elevated temperature from recently conducted experiments. This methodology can be used on any pipe size, material, and pipe system geometry. To validate this methodology further, SSWF and PRF values were determined for ID surface-cracked pipes and elbows of two different materials at elevated temperature of 550 degrees F with internal pressure of 2,250 psi. The PRF values at RT (70 degrees F) and 550 degrees F were compared. The two materials used in this study were TP304 stainless steel and Alloy600. Four-point bend pipe tests and elbow tests were conducted on pipes and elbows with varying circumferential surface crack sizes. The actual PRF value for a cracked pipe/elbow has a lower bound, which occurs when the test section of interest is at uniform stress. The center region of pipe in a four-point bend test is at uniform stress. A lower-bound limiting PRF value can also be calculated from stress-strain curves of pipe/elbow materials. Unlike tensile specimens, actual pipe systems are not uniformly loaded. Hence the reduction in stresses from the plasticity developed in the pipe system can vary depending on 1) loading conditions which determine if the pipe system experiences more elastic stresses or plastic stresses, 2) location of crack; if the crack is in a high strength/low toughness pipe or weld, plastic reduction is less, and 3) pipe size; if the pipe is large enough elastic-plastic conditions occur even for high toughness materials. Prior to developing SSWF or equivalent PRF, it is important to recognize the relative nonlinear-rotation/displacement contributions from the crack and the uncracked pipe adjacent to the crack. This nonlinear rotation/displacement is key to translating fracture mechanics predictions (that include large plastic behavior) back to elastic pipe-system design stresses. Examination of the pipe test data for different materials and under different operating conditions showed that with smaller flaws, the nonlinear rotation of the uncracked pipe was much larger than the nonlinear rotation from the crack; however, for large deep flaws (with failure stresses closer to potential operating/transient stresses) the rotation/displacement due to the crack was larger than from the uncracked pipe. Hence, both nonlinear rotation/displacement contributions are needed in the SSWF development to avoid excessive conservatisms.