Inclusion of Torsion Loads in Section XI Flaw Evaluation Procedures for Pipes Containing Circumferential Planar Surface Crack-Like Flaws on the Basis of Limit Load Analysis

Abstract

Piping systems in power plant may experience combined bending-torsion loads in the presence of planar crack-like flaws. ASME Boiler and Pressure Vessel Code Section XI nonmandatory Appendix C provides flaw evaluation procedures for pipes with flaws. These are currently limited to straight pipes under pressure and bending loads and no provision is made for torsion loading. The working group on pipe flaw evaluation is developing guidance for including the torsion load within the existing solutions provided in the Appendix C for bending loading on a straight pipe under fully plastic fracture regime. This paper reports on the finite element limit load analyses performed on the straight pipe containing a circumferential planar crack-like flaw. Pipe diameters were ranging from 4 in. (100 mm) to 24 in. (600 mm) nominal diameter (OD) and R/t ranging from 6 to 40. For the purpose of nonmandatory Appendix C flaw evaluation, it is concluded that the torsion loads can be combined with bending loads using the root of the sum of the squares (RSS) method of Section III of the ASME Boiler Code, without any additional weighting on torsion.

Collapse load analysis for circumferentially cracked cylinder subjected to combined torsion and bending moments

“…Bezensek [5] also found that when the torsion stress acting on the crack section was less than 20% of flow stress � / � < 0.2�, the values of �� were almost constant. This implies that �� can be used to predict the collapse plastic failure for the case of small amount of torsion.…”

“…Figure 6 illustrates that the values of �� is almost constant when the value of � �� ⁄ are less than 0.25. This information corresponds to the works presented in literatures [5] which indicates that �� can be used to predict the plastic collapse failure for the case of small amount of torsion. Figure 7 illustrates the relationship between � �� ⁄ and � �� ⁄ for the case of � ⁄ = 10.…”

“…For the case of cracked cylinder subjected to combined bending and torsion moments, Bezensek et al [5] showed that the collapse moment can be described as equivalent moments �� defined by;…”

“…At present, several methods for predicting the failure load of cracked cylinder under combined torsion and bending moments have been developed. These methods are based on the equivalent collapse moment and biaxial failure parameter [5,6]. However, these methods still have some limitations when they were used to estimate the failure load.…”