ASME NTB-3-2020 pdf download.Gap Analysis for Addressing Adequacy or Optimization of ASME Section III, Division 5 Rules for Metallic Components.
1.1.1 Summary O’Donnell, Hull and Malik [1] summarized this issue in Item OG- 10 as follows: Appendix T in NH [7] provides three expressions for determining strain range [8] using elastic analysis and, if these rules cannot be satisfied, additional rules are provided, presumably less conservative, based on the results of inelastic analyses which require detailed constitutive models of material behavior under time varying loading conditions. For the CRBR, these behavioral models were a contractual provision based on RDT Standards. These applicable standards are no longer maintained and there have been numerous technical developments in this area since then [9]. Appendix T rules cover strain, deformation, creep and fatigue limits at elevated temperatures for 304SS/316 SS (816°C), Alloy 800H (760°C), 2.25Cr-1Mo (593°C), 9Cr-1Mo-V (649°C). Development of material models for materials not currently covered or for temperatures beyond their original range of verification will be a considerable effort. Modifications in Appendix T rules for higher temperatures and additional materials (e.g., Alloy 617, Hastelloy X/XR) may be needed.
1.1.2 General Assessment Appendix HBB-T of BPVC Section III, Division 5 provides procedures to evaluate strain limits and creep fatigue damage using elastic analysis. Alternatively, for some Class A materials, recent code cases provide procedures based on Elastic-Perfectly plastic (EPP) analysis. If these rules cannot be satisfied, additional rules are provided which are based on results of inelastic analyses. However, inelastic analysis requires detailed constitutive models of material behavior under time varying loading conditions. For the CRBR, these behavioral models were based on Nuclear Standard NE F9-5T.
1.2.2 General Assessment The current strain accumulation rules have geometric and service level transient design restrictions. The rules are also complicated to apply. The BPVC Section III, Division 5 elastic rules for strain limits evaluation are based on the decoupling of creep and plasticity. For temperatures above a certain cut off, the decoupling of creep and plasticity can no longer be justified and a unified viscoplastic model is necessary to describe the deformation behavior. The recently developed EPP methodology for strain limits evaluation of Type 304 and 316 stainless steels does not depend on the decoupling of creep and plasticity. It has been demonstrated by tests to be applicable to the full temperature range permitted code allowable stresses, including very high temperatures where creep and plasticity are no longer decoupled. Cut off temperatures for the Class A materials have recently been established [12].ASME NTB-3 pdf download.