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TitleEffect of crack orientation on toughness of contemporary EW pipe seam welds
AuthorXu, SORCID logo; Yang, L; Xue, J; Liang, J; Tyson, W RORCID logo
SourceProceedings of the Biennial International Pipeline Conference, IPC; 2022 p. 1-10,
Alt SeriesNatural Resources Canada, Contribution Series 20230026
PublisherThe American Society of Mechanical Engineers
MeetingInternational Pipeline Conference; Calgary; CA; September 26-30, 2022
Mediapaper; digital; on-line
File formatpdf
Subjectsengineering geology; pipes
ProgramCanmetMATERIALS Joining and Structural Performance
Released2022 12 19
AbstractToughness values of pipe steel and welds are key mechanical properties for material specification and integrity assurance. Toughness is influenced by crack size (deep vs. shallow), loading mode (bending vs. tension), crack orientation and sampled microstructure (notch location in weldments). To the best of the authors' knowledge, no data exist in the open literature for crack orientation effects on Electric Welded (EW) welds. Charpy notch toughness and single-edge-notched bend (SE(B)) fracture toughness tests using surface-notched specimens of three modern (EW) pipe seam welds were performed at ?20°C and ?45°C, and the results are compared to those for through-thickness-notched (TTN) specimens. Charpy results indicated that the tests were in lower-transition or lower-shelf regions. Charpy impact V-notch absorbed energies (CVNs) of EW welds displayed large scatter; the effect of crack orientation was small. The initiation J-integral values of SE(B) surface-notched specimens were higher than those of TTN specimens, although when brittle fracture initiation occurred the effect of orientation on toughness was negligible. Note that the specimen geometry likely affects the results; surfaced-notched specimens were B×B and through-thickness-notched specimens were B×2B, although they are all standard SE(B) specimens in ASTM E1820. A limited study of geometry effects on crack-tip stress was done in the present work by Finite Element Analysis (FEA). It was found that these geometries all have similar maximum crack-tip opening stresses at the same J value although the stress gradient through the ligament and hence the constraint parameter Q (measured at a distance 2J/?y from the crack tip) and the J-Q curves differ. Thus, differences in fracture toughness values for brittle fracture would be expected to be small because cleavage is stress controlled, although ductile fracture toughness may reflect the differences in J-Q curves. There was considerable scatter in both notch toughness and fracture toughness, and much of this may be attributed to slight variations in notch or crack location with respect to the weld bond line because the toughness can vary markedly across an EW weld. The variation in location is superimposed on the variation in toughness inherent in each weld microstructure. The present data shows that the conventional use of TTN specimens is appropriate for EW weld toughness specification because toughness in this orientation is either equivalent to or lower than the toughness of surface-notched specimens (i.e. it is conservative).
Summary(Plain Language Summary, not published)
This report pertains to the CanmetMATERIALS Fracture Control of Pipelines project in alignment with NRCan's Materials for Energy under the Energy Innovation Program. Pipeline standards normally specify Charpy absorbed energy (CVN) values of through-thickness-notched (TTN) specimens for which the crack propagation direction is axial. However, in service it is most commonly axial surface cracks that lead to fracture of seam welds. For such cracks the propagation direction is through-surface rather than axial. The intent of this work is to assess whether CVN values of TTN specimens provide conservative values for cracks propagating in the through-thickness direction, i.e. to assess the influence of orientation on toughness. To the best knowledge of the authors, there is no data on crack orientation effects for Electric Welded (EW) welds in the open literature. In the current investigation, the effect of crack orientation on Charpy toughness was found to be small. Additionally, fracture toughnesses of surface-notched Single-Edge-Notched Bend (SE(B)) specimens were higher than those of through-thickness-notched SE(B) specimens, although this result must be used with caution because the specimen geometries were different (B×B vs. B×2B where B is specimen thickness, the former being used for surface-notched and the latter for TTN specimens). To assess the effect of specimen geometry, FEA was applied to SE(B) specimen types. It was found that these geometries all have similar maximum opening stresses although the stress gradients through the ligaments differ; this implies that there should be little effect of geometry on brittle fracture, although ductile initiation toughness may be affected. Size requirements of ASTM standards for initiation toughness were broadly met, supporting specimen geometry independence. Hence, the conclusion regarding orientation is that the inherent fracture toughness of surface cracks is higher than that of through-thickness cracks. The new data shows that the use of through-thickness-notched Charpy specimens in Canadian pipe standards is appropriate for EW weld toughness specification. This is because the CVN specifications have been derived from correlation with through-thickness-notched fracture toughness specimens so as to provide adequate toughness for surface-notched flaws, and the present work has shown that this is conservative with respect to surface-notched fracture toughness specimens.

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