There has been a trend recently to use specimen geometries for toughness measurement that are more representative of actual flaw geometries in service. A prominent example is the use of single-edge-notched tension specimens for assessment of surface flaws in pipelines. To obtain a resistance (R) curve, i.e. J-integral or CTOD as a function of crack growth, it is necessary to monitor the crack size as a function of J or CTOD. To facilitate obtaining these data from a single specimen, the elastic CMOD unloading compliance C has been used in several testing programs to estimate crack size. C is a function of several variables in addition to crack sizenotably, specimen constraint (plane stress or plane strain). In this paper, the dependence of C on these variables will be discussed.

Fracture propagation toughness is important to pipeline steels. In this study, the effect of non-singular T-stress (a measure of constraint) on crack growth resistance curves (R-curves) and crack tip opening angle (CTOA) was investigated using modified boundary layer (MBL) models of pipeline steels. Two sets of steel types: 1) TH (a typical high strength steel) and 2) C4 (X100 steel) were used in this work. Surface-based cohesive zone models with four sets of bilinear traction-separation (TS) laws were used for TH steel. The models of C4 steel were computed using element-based cohesive zone modeling with one bilinear TS law. All finite element simulations were conducted using the finite element (FE) program ABAQUS. It was assumed in these simulations that there was no effect of T-stress on the TS laws per se. With this assumption, it was found that the T-stress does not have a significant effect on the CTOA for the two materials studied.

This paper presents recent results of numerical studies on stable crack extension of high toughness steels typical of those in modern gas pipelines using cohesive zone modeling. Two sets of materials are modeled. The first material set models a typical structural steel, with variable toughness described by four traction-separation (TS) laws. The second set models an X70 pipe steel, with three different TS laws. For each TS law, there are three defining parameters: the maximum cohesive strength, the final separation and the work of separation. The specimens analyzed include a crack in an infinite plate (small-scale yielding, SSY) and a standard drop-weight tear test (DWTT). Fracture propagation characteristics and values of crack-tip opening angle (CTOA) are obtained from these two types of specimens. It is shown that cohesive zone models can be successfully used to simulate ductile crack propagation and to numerically measure CTOAs. The ductile crack propagation characteristics and CTOAs obtained from SSY and DWTT specimens are compared for each set of steels. In addition, the CTOA results from numerical cohesive zone modeling of DWTT specimens of X70 steel are compared with those from laboratory tests.

Pipelines in certain regions are expected to survive high longitudinal strains induced by seismic activities, slope instability, frost heave, and mine subsidence. Material properties, of both pipes and girth welds, are critical contributing factors to a pipeline’s strain capacity. These factors are examined in this paper with particular focus on the modern high strength pipes (grade X70 and above) usually made from microalloyed control-rolled TMCP steels. The examination of the tensile properties of pipes includes some of the most basic parameters such as yield strength, strength variation within a pipe, and newly emerging issues of strength and strain hardening dependence on temperature. The girth weld tensile properties, particularly yield strength, are shown to be dependent on the location of the test specimen. There are strong indications from the tested welds that strain hardening of the welds is dependent on test temperature. The effects of strain aging on pipe and girth weld properties are reviewed. This line of reasoning is extended to possible strain aging effects during field construction, although experimental evidence is lacking at this moment. The paper concludes with considerations of practical implementation of the findings presented in the early part of the paper. Recommendations are made to effectively deal with some of the challenging issues related to the specification and measurement of tensile properties for strain-based design.

Toughness is a critical parameter in all phases of a pipeline service life. There are a variety of toughness testing forms, ranging from relatively inexpensive small-scale laboratory specimens to highly sophisticated full-scale tests. The correlation of physically significant parameters, such as transition temperature and upper shelf energy/toughness, from those test forms is examined. The ultimate objective is to understand of the similitude of various specimen forms. The similitude allows the prediction of large-scale behavior from small-scale laboratory test specimens. The paper presents data developed from various current and past research programs. The trends exhibited by the various forms of specimens are examined. The implications of those trends in the strain-based design of pipelines are summarized at the end of the paper.