This paper presents the technical reasoning and justification for using the B2′ = (2/3)*B2 in Paragraphs NB-3656(b) and NC/ND-3655(b) of Section III. ASME revised the rules for piping subjected to seismic and other building filtered loads in the 1994 addenda to the 1992 Code to provide an alternative to the existing rules. The purpose of the revision was to recognize the results of significant testing and experience that supported a decrease in the multiplier (B index) of the moment term, among other changes. The use of a B2′ index set equal to 2/3 of the current B2 index effectively raised the Level D allowable 50%. As part of its review in 10CFR50.55a, the NRC noted that use of the new rules was not permitted, due to disagreements in the approach. Since the 1994 addenda were published, the NRC and ASME have worked together to revise the changes. At this time, it is ASME’s understanding that the rules proposed for inclusion in the 2007 Code will be accepted by the NRC, with the exception of the use of a B2′ index equal to 2/3 of B2 for bends and tees. For those items, the NRC believes a multiplier of 3/4 is more appropriate for ferritic steels at temperatures above 300°F, due to dynamic strain aging. 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, ASME Code Committee. This paper demonstrates that: 1) In order to investigate this possibility, authors collected test data at room temperature, and then benchmarked its analytical work against both its tests and data from the EPRI test program since dynamic testing of components at elevated temperature and high stress levels can be quite difficult. From its analytical and test work on components, plus elevated temperature and strain rate work on small specimens, it was concluded that strain rate effects at typical seismic strain rates and amplitudes are not a concern. 2) 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(1). Thus, the use of B2′ = (2/3)*B2 results in a component with acceptable margin. This is a part of the paper prepared as a team working under the Piping Seismic Task Group under ASME Sec. III, Subgroup Design.
Characterization of localized plastic deformation behaviors associated with dynamic strain aging in pipeline steels using digital image correlation
