As a further extension to the structural stress based master S-N curve method adopted by ASME Div 2 since 2007, this paper presents an analytical-based structural strain method for girth-welded piping components. Here, structural strain is defined as outer and inner fiber strains calculated corresponding to a deformation condition in which a pipe section plane before deformation remains as a plane after deformation. The analytical formation takes into account all possible plastic deformation conditions a pipe section subjected to a combined remote cyclic bending and axial tension. A simple numerical procedure is used for solving both outer fiber and inner fiber strains, as well as the corresponding elastic core size. For fatigue evaluation purpose, the outer fiber strain can be used to calculate the corresponding pseudo elastic structural stress range so that the structural stress based master S-N curve can be directly used. Under linear elastic deformation conditions, the structural strain definition becomes exactly the same as that calculated by the structural stress method which is the basis on the ASME Div 2 master S-N was developed.

A set of a recent full scale girth-welded pipe component test data in low-cycle regime was analyzed using the structural strain method. The results showed that all these new test data fall well within the ASME Div 2 master S-N curve scatter band defined by mean+-standard deviations. In addition to its demonstrated effectiveness, the key advantage of this structural strain method is its simplicity for dealing with girth-welded pipe sections, since finite element stress analysis is no longer needed.

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