Conservatism in the B2 and B2 ' Index

This paper presents the reasoning and justification for using the B₂’ = (2/3)*B₂ in Paragraphs NB-3656(b) and NC/ND-3655(b) of ASME Section III.  Concern has been expressed that since the tests that form part of the basis for setting B₂’ = (2/3)*B₂ were conducted at ambient temperature, the effect of dynamic strain aging of carbon steels could reduce the seismic margins at temperatures in excess of 300˚F (150˚C).  In response to this concern, the authors prepared this paper as a team working under the Piping Seismic Task Group.  This paper demonstrates that:

• For typical carbon steel under seismic strain rate loading, at elevated temperature [(above 300˚F (150˚C)] and at stress levels permitted by the alternative Code equation [NB-3556(b)(2) and NB-3556(b)(3)], the margin to failure is at least 1.5, as recommended by Dr. R.P. Kennedy.  Thus, the use of B₂’ = (2/3)*B₂ results in a component with acceptable margin.

• The B₂ index itself is a conservative estimate of the stress in the component, both from the method of application in the Code (use for prediction of seismic stresses) and from the value itself.  Thus, the use of B₂’ = (2/3)*B₂ results in a component with acceptable margin.