PRCI PR-140-512


Small and Full Scale Fracture of Thick Section Girth WeldmentsA. G. GloverPipeline Research Council International / 01-Mar-1987 / 79 pages

Welding Institute of Canada
Need: The use of fracture mechanics analysis, to evaluate the significance of weld imperfections with respect to fracture initiation, has become an accepted practice in a number of industries. At present many pipeline codes allow the use of a fracture mechanics analysis to determine the need for repair of weld imperfections, which exceed the maximum size allowed by the workmanship standards. Many of these analyses have been based on small scale and full-scale tests in relatively thin wall material. Increasingly thicker wall pipe materials are being used for both onshore and offshore pipeline applications. It has also been realized from the start that, in the elastic-plastic regime, toughness is thickness-dependent. The effects of constraint, in terms of cleavage initiation, are therefore even more important. If fracture toughness tests are to be carried out to ensure that the material will be operating above its transition temperature it becomes important to correlate the small scale thickness effects to full scale behavior.
Result: The results of the test program are consistent with present assessment methodologies and show that the constraint in thicker wall pipes is high enough to be adequately addressed by normal SENB test specimens. The fracture program has assessed the behavior of small and full-scale fracture tests of girth welds in 30 inch by 0.75 inch, and 28 inch by 1.00 inch line pipe. Within the transition region considerable variation in toughness occurred and small variations in specimen configurations lead to changes from cleavage to microvoid coalescence with considerable increases in toughness. It is evident that when going to thicker wall materials that the constraint in the structure will be higher and that its fracture behavior can be conservatively analyzed using highly constrained small-scale specimens. The level of conservativism is not excessive varying from 2 to 3, when failure is not by a collapse mode. In carrying out the assessment normally two calculations should be performed; one to determine the maximum defect size permissible to prevent brittle fracture and a second to calculate the maximum imperfection length to avoid plastic collapse. The present conservative analyses have been carried out without the inclusion of residual strain, and it is clear that a crude assumption of yield strain levels would make the whole approach extremely conservative and future research should be addressed to assess the strain level at crack tips.
Benefit: The objective of this two year program was to evaluate the behavior of geometry effects of small scale CTOD specimens and subsequent full scale behavior for thick section girth weldments. This research program evaluated the behavior of geometry effects of small scale crack tip opening displacement (CTOD) specimens for thick section girth weldments. The geometry of the small specimens has varied the width to thickness ratio and the crack depth to thickness ratio for single edge notched bend specimens. The small scale CTOD results have been correlated with full scale fracture experiments. The fracture program has been carried out using 30 inch diameter by 0.75 inch wall and 28 inch diameter by 1.00 inch wall grade X65 line pipe material. Fracture tests have been carried out primarily in the transition regime (elastic-plastic), where the behaviour of the material is either controlled by a cleavage or microvoid coalescence mechanism. The cleavage resistance of the materials was shown to be affected by restraint, and the resistance to ductile tearing was geometry independent.

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