Fracture assessment procedures for pressurized components play a crucial role in design, fabrication, and fitness-for-service (FFS) methodologies (such as repair decisions and life-extension programs) for pressure vessels, piping systems, and storage tanks. 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 over-conservative fracture toughness values have been considered in the past to prevent brittle fracture. However, current standardization efforts now make extensive use of constraint designed fracture specimens, yielding apparent toughness values that are closer to the application. This allows reducing the level of conservatism during the assessment, without compromising on safety.

This work proposes a methodology to characterize the fracture toughness values for either brittle or ductile materials, such as steels of risers and pipelines used in the oil and gas industry using non-standard four-point bending specimens. It is shown that four-point bending (4PB) specimens are reliable configuration to characterize fracture toughness values. The methodology goes through obtaining compliance equations, stress intensity factors, the proportionality factors between the deformation energy and J-integral, known as ��, and the factor to account for ductile crack growth effects on the J-Integral, known as �� factor. This study evidences the impact of geometry variation over the constraint in the plastic deformation at the crack tip characterized. It is shown that the J-Q trajectories are similar between the 4PB geometry and circumferentially cracked pipes under bending. Laboratory tests were performed in four-point bending specimens. These experiments were compared with experimental data of standardized geometries SE(B) and SE(T). The results obtained from the preliminary experimental campaign validated the numerical analysis. Thus, the proposed equations can be used to obtain the fracture toughness values using four-point bending specimens.


Test Protocol Definition to Measure Fracture Toughness Parameters Using Non-Standard Four Points Bending Specimens

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


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

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