6 Subsection NB—Class 1 Components
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This chapter discusses the structural limits in Section III NB and Section VIII Div 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 Div 2 are based on design by analysis philosophy addressing specific structural failure modes of structural collapse, rupture, instability, fatigue, and progressive deformation. Beam, plate, and shell theory have been used in the past to develop design analysis methods. These methods are giving way to finite element analysis (FEA) methods that are commonplace in engineering practice today. This chapter discussed ways to use FEA to show compliance with the ASME Code structural design limits. The applicability of linear versus non-linear analysis assumptions are discussed relative to the failure mode being addressed. Examples are given with guidelines on use of current tools to address these failure modes. Comments on the direction for development of new tools are given. Also discussed is the development of fracture mechanics methods to address ductile and brittle fracture failure modes as required in Appendix G, in Section XI, and often in the context of Section III NB design requirements. Comments are provided on the limitations of deterministic assessments as made in the current Section III NB assessment with respect to new initiatives into reliable design methods based on probabilistic concepts.