Crack Initiation and Propagation in Static Loaded Fracture Mechanics Tests in Steels Containing Atomic Hydrogen

Hydrogen is well known to have a detrimental influence on the ductility of low alloy steels, reducing the fracture toughness. Standard test methods to characterize fracture toughness of steels in terms of ductile tearing resistance curves have not been developed to account for any hydrogen-driven contribution to the crack extension, Δa. Simply plotting J or CTOD against Δa is not necessarily appropriate for defining the initiation fracture toughness for tests performed in a hydrogen-charging environment. This paper explores a method to further analyse experimental data collected during fracture toughness tests, which allows the contribution of plasticity (i.e. when blunting precedes ductile tearing) to be considered separately from the initiation of crack extension (which could be by stable tearing and/or by hydrogen-driven crack extension). The principle is based on the assumption that a crack growing by a hydrogen-driven mechanism in a quasi-static fracture mechanics test performed in environment may not be associated with significant ductility in the plastic zone (which would accompany crack growth by stable tearing). The analytical method presented in this paper compares the different points of deviation from linear behavior of the components of J, to isolate the effects of ductility within the plastic zone from pure crack extension. In this way, the point of crack initiation can be defined in order to determine the relevant initiation fracture toughness; whether by blunting and stable tearing, or by hydrogen-driven crack growth. This approach offers a screening method which is illustrated using examples of fracture mechanics specimens tested in environments of varying severity (air, seawater with cathodic protection, and sour service). This method can be used to identify the relevant definition of initiation fracture toughness while allowing for a combination of ductile tearing, hydrogen-driven crack extension, or both, to be present during the test.

Modeling Stress-Activated Creep at Axial Cracks in Pipelines

The failure of a pipeline that had just passed a proof-pressure test as it was being re-pressurized for its return to service (a so-called pressure reversal) reflects the stable growth due to stress-activated creep of a near-critical anomaly that had remained stable as the proof test ended. In the same way that stable growth of a near-critical anomaly can lead to a pressure reversal, stable tearing (cracking) can occur and remain stable at the pressure first imposed upon the pipeline’s return to service, and so pose concern for in-service failure. Ductile failures that are absent evidence of time-dependent degradation mechanisms, like corrosion, and show the traits of stable tearing have been termed time-delayed failures. As time passed, the reasons for time-delayed failures became clear, and criteria to prevent such failures through a pressure reduction were established. The advent of much tougher steels opened to the potential for crack initiation and stable tearing at service pressures under circumstances that differed from that for the early line pipe steels. The 2004 incident at Ghislenghien involving a modern high-toughness X70 pipeline raised the need to better understand how to manage time-delayed failures in such steels. This paper develops a model to quantify stable tearing and possible instability at axial part-through-wall defects as a function of the steel, the length and depth of the defect, and the operating pressure. The theoretical basis for this nonlinear fracture mechanics (NLFM) model is outlined first. Case-specific finite-element analysis were used to benchmark NLFM Handbook results, which extended the use of predictive technology developed previously for lower toughness steels. As before, this solution is recast for time-marching analysis that is coupled with isochronous stress-strain response and NLFM resistance curves. Finally, the model is used to make blind predictions of cracking and instability in step-load and hold testing, and found to be viable in that context. Companion papers at this conference present the details of related work.

SENT Stable Tearing Crack Path Deviation and its Influence on J

Crack path deviation in Single Edge Notch Tension (SENT) specimens, and its influence on the determination of J, has been investigated as part of the development of a new British Standard for SENT testing, BS 8571 [1]. Crack path deviation by angles up to 50° have been observed during stable tearing in parent material SENT specimens. This paper investigates the effect of crack path deviation on the measured fracture toughness, and offers a correction formula when crack path deviation invalidates the default standard J equations. Mixed mode effects in crack path deviation are also investigated. A parametric study using finite element analysis has been carried out to compare the value of J calculated using standard equations (which assume a straight crack propagation path) with the value of J calculated using the contour integral method for different levels of crack path deviation. Crack path deviation from the initial crack plane resulted in a non-conservative estimate of fracture toughness using the standard equations. This means that any SENT test exhibiting crack path deviation may need to be discarded, wasting valuable test specimens. Instead, a correction factor has been developed to adjust the calculated value of J if path deviation is observed.

Simplified Toughness Resistance Curve

This paper presents a method in order to plot a simplified toughness resistance curve (R-Curve) from three single conventional toughness tests. The simplified toughness resistance curve will be used to carry out a tearing assessment as part of Engineering Criticality Assessment (ECA) (ref. to Level 3B, [2]). This level of assessment takes into account of stable tearing in order to reduce over-conservatism, but it usually requires additional toughness tests to identify the toughness resistance curve. This present method allows to reach this assessment level without impacting the welding procedure qualification (WPQ).