The latest oil and natural gas discoveries in the pre-salt layer of the Brazilian coast represent a huge potential and put Brazil in a strategic position in face of the increasing energy demand in the world. In this context, the production and exploitation of such reserves imposes many technological challenges related to new design requirements due to more hostile environmental conditions. One of the key challenges facing the oil and gas industry is to maintain economic feasibility and maintain reliable and fail-safe operations of the infrastructure for production and transportation. Engineering Critical Assessment (ECA) adopts fracture mechanics for assessing the criticality of defects in welded structures, commonly applied to derive tolerable defect sizes during installation and operation of risers and pipelines by combining the most critical operating conditions and material properties. Fracture toughness test procedures producing overconservative fracture toughness values have been considered in the past to prevent brittle fracture. However, current standardization efforts now make extensive use of single edge notch tension SE(T) specimens, yielding apparent toughness values that are closer to the application. This allows to reduce the level of conservatism during the assessment, without compromising on safety. A great effort has been made to evaluate CTOD as a means to assess fracture toughness, especially for offshore structures. Moreover, international standards were published as guidelines to obtain the fracture toughness in terms of CTOD parameter. Among them, ASTM and BSI standards are the most popular ones; differences are observed between the CTOD values due to different assumptions for their formulations. The plastic hinge model and J-integral based measurement methods are the most popular techniques that have been used in standard procedures. The double clip gauge method (DCGM) is an alternative technique, which is based upon the same assumption used in the plastic hinge model that the specimen rotates as a rigid body about a hinge point. The introduction of a second extensometer eliminates the necessity of defining the hinge point. Experimental and numerical studies showed that the DCGM produces less conservative values for CTOD when compared to the CTOD based on J-integral. Different definitions might lead to a misconception about tolerable crack sizes. Moreover, a lot of fracture toughness data are defined on CTOD values by using the double clip gauge method. Likewise, it is clear that there is no wrong or correct definition for the CTOD parameter because there is no unique definition. The unique relationship between J and CTOD was proved by Shih, considering the dominance of the HHR fields at the crack tip area. Also, since the J-integral was originally defined by a complex analytical expression, many researchers have conferred a high status upon J as a fracture parameter. Thus, the CTOD converted from the J-integral can be considered as a valid fracture parameter as well. This study focus on the theoretical and numerical differences between the  CTOD obtained by the DCGM and the CTOD from J-conversion.

Therefore, some effort needs to be made to correlate the two CTOD definitions widely used in offshore pipelines, in consideration of the fracture toughness is usually obtained by J-Integral approach and the crack driving force is obtained by double clip gauge method. So, motivated by the earlier explanation, the goal of this paper is to present a new formulation to estimate the CTOD by the DCGM, which must be approximately equal to the CTOD converted from the J-integral. The triangulation of the opening displacement at the crack mouth (CMOD) and a point on the crack flank, near the crack tip, is proposed for the double clip gauge method. A proportional factor is identified to estimate the displacement close to the crack tip eliminating the needs for two physical clip gages during fracture toughness tests. The 3D numerical analyses were performed on clamped SE(T) specimens, varying their dimensions, such as the thickness, width, crack size, and mechanical properties (yield strength and ultimate tensile strength).


A New Proposal to Obtain the Ctod Fracture Parameter for Offshore Pipeline Steels by Using Only One Clip Gage

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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