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This chapter discusses the structural limits in Section III NB and Section VIII Division 2 as related to specific structural failure modes considered applicable to pressure vessel and piping design. The rules and requirements for Section III NB and Section VIII Division 2 are based on the design-by-analysis philosophy addressing specific structural failure modes of structural collapse, rupture, instability, fatigue, and progressive deformation. The chapter begins with a discussion on the interactions between materials, environments, mechanical, and thermal loads as aided by other disciplines since this is critical for every part of every pressure vessel design. It provides a brief history of development of Code analytic tools and discusses the use of elastic 3D finite element analysis (FEA) results and the application of elastic-plastic (EP)-FEA. The chapter then discusses the intent of the primary stress limits in the Code. Consideration is given to the definitions in the Code and the discussion in the Criteria document. The chapter then addresses why the primary-plus-secondary stress intensity range is necessary and shows how to apply it. Two procedures for calculating stress intensity are presented, and the impact of thermal stresses is discussed for definition and application. The chapter also discusses six areas in which the Code presents fundamental procedures and expands on these fundamentals by presenting guidelines for a singular view of acceptable procedures that are consistent with the original intent for Section III and consistent with the current rules. Finally, it comments on the elastic-plastic FEA methods to protect against plastic collapse, to determine protection against local failure, and to assess fatigue and ratchet limits.
Tom Ahl was the original author of this chapter for the first edition. Marcus N. Bressler updated this chapter for the second and third editions. Chakrapani Basavaraju updated this chapter for the fourth, fifth and sixth editions. The current online edition has been updated by Chakrapani Basavaraju and Jason Lambin.