This paper presents an investigation concerning the ultra-low-cycle fatigue (ULCF) characterization of large-scale elbows produced from line pipes subjected to hot bending process. Two distinct pipes were used in this process: a 16” (w.t. 9.5 mm) X60 and an 8 5/8” (w.t. 5.59 mm) X65 pipes that were bent to 45 and 90° elbows (8 tests). Cyclic external loading was applied to the elbows, combined with internal pressure, until failure was observed. The failure was preceded by a local plastic instability (bucking) and resulted due to intense cyclic plastic deformation. In general, the number of cycles to failure was lower than 100 cycles which typifies this failure mechanism as ultra-low-cycle fatigue. Besides the full-size tests, the plain material was investigated under ULCF conditions using both smooth and notched specimens. The thermal process used in the hot bending manufacturing process was also accounted for in the material testing in order to understand the effect of this process on pipe material. Non-linear finite element models of the elbows were constructed to simulate the cyclic behaviour of the elbows using the actual loading histories applied to the elbows. Damage models (e.g. Coffin-Manson, Xue) identified using material test data are applied to simulate the failure cycles of the tested elbows. Besides the use of damage models available in the literature and identified with generated materials experimental data, current ASME VIII Div.2 procedures are also used to compute the failure cycles of the elbows to allow an assessment of these existing procedures.

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