Timothy S. Weeks

Direct Comparison of Single-Specimen Clamped SE(T) Test Methods on X100 Line Pipe Steel

The clamped single edge-notched tension (SE(T)) specimen has been widely used in a single-specimen testing scheme to generate fracture resistance curves for high strength line-pipe steels. The SE(T) specimen with appropriate notch geometry is a low-constraint specimen designed to reduce conservatism in the measurement of fracture toughness. The crack driving force is taken as either the J-integral or crack tip opening displacement (CTOD); it is generally accepted that the two parameters are interchangeable and equivalent using a simple closed form solution. However, the assumption that they are interchangeable, and to what extent, hasn’t been previously investigated experimentally on the same SE(T) specimen. This paper presents multiple test methods that were simultaneously employed on the same SE(T) specimens. The instrumentation includes: clip-gauges to measure surface crack mouth opening displacements (CMOD) and CTOD by the double-clip-gauge method; strain-gage arrays for direct J-integral measurements; and direct-current potential-drop (DCPD) instrumentation for supplementary crack size measurement. A direct comparison of ductile crack-growth resistance curves generated using J-integral and CTOD is presented here where each represents a different experimental and analytical approach. The two methods are in reasonable agreement over a narrow range of crack growth, differing slightly at initiation and diverging with increasing crack growth. Analysis of the supplementary instrumentation (i.e., strain gages, extensometers and DCPD) will be provided in a future publication.© 2014 ASME

Fracture Toughness Characterization of High-pressure Pipe Girth Welds Using Single-edge Notched Tension [SE(T)] Specimens

The safety and reliability of large-diameter pipelines for the transport of fluid hydrocarbons is being improved by the development of high-strength steels, advanced weld technologies, and strain-based design (SBD) methodologies. In SBD, a limit is imposed on the applied strains rather than the applied stresses. For high-pressure pipelines, SBD requires an assured strength overmatch for the weld metal as compared to the base material, in order to avoid strain localization in the weldment during service. Achieving the required level of strength overmatch, as well as acceptable ductility and low-temperature fracture toughness, is a challenge as the pipe strength increases. Published studies show that low constraint geometries such as single-edge tension [SE(T)] or shallow-notched single-edge bend [SE(B)] specimens represent a better match to the constraint conditions of surface-breaking circumferential cracks in large-diameter pipelines during service (Shen, G., Bouchard, R., Gianetto, J. A., and Tyson, W. R., “Fracture Toughness Evaluation of High Strength Steel Pipe,” Proceedings of PVP2008, ASME Pressure Vessel and Piping Division Conference, Chicago, IL, July 27–31, ASME, New York, 2008). However, the SE(T) geometry is not included in any of the most widely used elastic-plastic fracture mechanics (EPFM) test standards. A procedure has been developed for performing and analyzing SE(T) toughness tests using a single-specimen technique that includes formulas for calculating the J-integral and crack-tip opening displacement, as well as for estimating crack size using rotation-corrected elastic unloading compliance. Here, crack-resistance curves and critical toughness values obtained from shallow-crack SE(T) specimens (a0/W ≈ 0.25) are compared to shallow-crack (a0/W ≈ 0.25) SE(B) specimens. We believe that the SE(T) methodology is mature enough to be considered for inclusion in future revisions of EPFM standards such as ASTM E1820 and ISO 12135, although additional work is needed to establish validity limits for SE(T) specimens.

Broad Perspectives of Girth Weld Tensile Strain Response

Tensile strain capacity (TSC) is a critical component of the strain-based design of pipelines. TSC is affected by a number of material parameters, such as the strain hardening rate, weld strength mismatch, and toughness. Girth weld high-low misalignment, internal pressure, and flaw size are additional influential parameters. The impact of those parameters can be rationalized by fracture mechanics principles and is supported by an increasingly large library of experimental test data. A number of predictive TSC models are under development. One of the most significant challenges in the development of these models is the scatter of experimental test data. As more test data are collected with specially arranged precision instrumentation, it is become apparent that the scatter of test data is a matter of true material response. It is, therefore, critical to see beyond the scatter and understand the overall material behavior in the development and validation of TSC models. This paper highlights the material behavior observed in a large number of large-scale experimental tests. The material response is then classified into different categories to assist the understanding of the experimental data scatter and rationalize the trends expected from test data.Copyright

CURVED-WIDE PLATE TESTING OF X100 GIRTH WELDS

Curved-wide plate (CWP) tests are frequently used for assessing the quality of pipeline girth welds. Despite a large number of CWP tests having been conducted at great expense over many decades, an industry consensus standard remains unavailable. Considerable effort at several research institutions is focused on the standardization of test protocols. It is widely recognized that comparing results from CWP tests from different institutions is difficult without accounting for all the possible parametric differences.This paper presents the procedural details recently used in testing X100 girth welds. The protocols cover (1) specimen design and dimensions, (2) instrumentation plan and data acquisition, (3) specimen fabrication and preparation, (4) preparing and executing the tests, (5) processing of raw test data and (6) post-test metallurgical examination.The evaluation of specimen deformation, flaw growth, and comparison of test data with model predictions will be presented in a future paper. Selected CWP test data from this program were evaluated and compared to tensile strain models of the girth welded pipe in a recent paper [1].Copyright

Fatigue Pre-Cracking Curved Wide Plates in Bending

Curved wide plate (CWP) testing in tension, on API 5L X100 pipes of 36-inch (916-mm) diameter and 0.75-inch (19-mm) wall thickness, has been initiated in support of strain-based design using high strength steel for oil and gas pipeline applications. The CWP tests are being used to optimize and validate welding procedures and to determine the defect tolerance within the girth welds. A traditional pre-requisite for fracture mechanics testing is a final extension of a crack via fatigue pre-cracking to produce a representative flaw. A method of fatigue pre-cracking CWP specimens for final notch preparation in bending was developed to meet ASTM guidelines for fracture mechanics testing. Fatigue pre-cracking for the present specimen geometry was possible in bending due to lower requisite force capacity equipment which allowed for greater cyclic loading frequencies. In order to achieve sufficient stress levels for fatigue crack growth in the curved plate, a stress field analysis was performed to optimize the loading support configuration in four-point bending. In addition to the stress field analysis, a 3-D finite element model of the CWP specimen was generated to analyze the notched CWP specimen in four-point bending. Finite element analysis (FEA) results and experimental data were used to confirm the hypothesis that, under the proposed loading arrangement, the closed-form solutions for stress-intensity (K) of flat plates in bending can be used to approximate the K for CWP specimens in bending. Validation of a solution for stress-intensity factor subsequently allowed the determination of force amplitude levels for fatigue crack growth. Force and crack mouth opening displacement (CMOD) data were analyzed to correlate compliance with crack length measurements. From experimental results, a method was developed that enable the repeatable and well characterized extension of surface flaws by fatigue pre-cracking in curved wide plate specimens in bending.© 2010 ASME