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    Fracture Mechanics Analysis of Pipeline Girthwelds

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    Size limits for surface flaws in pipeline girthwelds are calculated on the basis of fracture mechanics analysis. Parameters for the analysis were selected from data on a 1.22-m-diameter (48 in.), 12-mm-thick (0.46 in.) pipe welded by the shielded metal-arc process. The minimum fracture toughness of the welds as determined by the crack opening displacement (COD) method was 0.1 and 0.18 mm (0.004 and 0.007 in.), depending on the flaw location. The yield strength of the welds was 413 MPa (60 ksi). Because the toughness to yield strength ratio was high, elastic-plastic fracture mechanics analysis methods were required to determine critical flaw sizes. Four approaches were employed: (1) a critical COD method based on the ligament-closure-force model of Irwin; (2) the COD procedure of the Draft British Standard Rules for Derivation of Acceptance Levels for Defects in Fusion Welded Joints; (3) a plastic instability method based on a critical net ligament strain developed by Irwin; and (4) a semi-empirical method that uses plastic instability as the fracture criterion developed by Kiefner on the basis of full-scale pipe rupture tests. Allowable flaw sizes determined by the Draft British Standard method are compared with the critical flaw sizes calculated using critical-COD and plastic instability as the respective fracture criteria. The results for both axial- and circumferential-aligned flaws vary significantly depending on the analysis model chosen. Thus, experimental work is needed to verify which model most accurately predicts girthweld behavior.


    carbon-manganese steel, fracture mechanics, fracture toughness, pipe-line, radiographic inspection, weld flaws, weldments, crack propogation

    Author Information:

    McHenry, HI
    Metallurgist and physicist, National Bureau of Standards, Boulder, Colo.

    Read, DT
    Metallurgist and physicist, National Bureau of Standards, Boulder, Colo.

    Begley, JA
    Associate professor, Ohio State University, Columbus, Ohio

    Committee/Subcommittee: E08.08

    DOI: 10.1520/STP35851S