ASME NTB-3 pdf download Gap Analysis for Addressing Adequacy or Optimization of ASME Section III, Division 5 Rules for Metallic Components
1.2 Issue I. 2 – Simplified Bounds for Creep Ratcheting
1.2.1 Summary From O’Donnell, Hull and Malik  Item OG- 5: The Draft Code Case for Alloy 617 imposes ratcheting strain limits that are similar to the limits given in Subsection NH, but is restricted to an upper temperature limit of 649 o C. Simplified ratcheting evaluation procedures require development for temperatures above 649 o C . New work has been proposed to ensure that recent technology developments are incorporated . Huddleston and Swindeman stated this issue in Item HS- 13 as “difficult, overly conservative ratcheting design rules” and noted that the rules have been improved since the CRBRP to include applicability to structural discontinuities and application to nonaxisymmetric geometries and nonlinear temperature gradients. It is considered a design basis economic issue and is not included as one of the top ten major issues.
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 .
1.2.3 Material Specific Remarks An Alloy 617 Code Case that includes the EPP methodologies which do not depend on the decoupling of creep and plasticity, and hence do not have the 649 o C upper temperature limit for Alloy 617 as in the current strain accumulation rules, is being balloted by ASME Code committees.
1.2.4 Action Required (1) Complete the ASME approval process for the Alloy 617 Code Case, and (2) complete the extension of the EPP methods to the remaining Class A materials.
1.2.5 Conclusion Tentatively categorized as optimization.
1.3 Issue I. 3 – Strain and Deformation Limits at Elevated-Temperature
1.3.1 Summary From O’Donnell, Hull and Malik Item OG- 12: Current NH rules address these issues; however, extrapolation of creep- fatigue data is an ongoing challenge particularly at the extremes of the creep regime. At the low temperature end the concern involves the definition of negligible creep and at the very high temperature end one of the issues is whether or not plasticity and creep can be separated. Although there can probably be improvement in the current NH approaches, the major issues identified with NH, particularly with respect to other international Codes, is that NH is too conservative by comparison . The extrapolation of time-dependent data where fatigue is present represents a very significant challenge to the design .
1.3.2 General Assessment The temperature cut off issue is the same as the simplified bounds discussed in Issue I. 2 above. The conservativism of the current BPVC Section III, Division 5 elastic rules for strain limits evaluation above the cut off temperature is currently being evaluated by ASME.
1.3.3 Material Specific Remarks
1.3.4 Action Required
Complete the extension of the EPP methods to the remaining Class A materials.
Categorized as Optimization.
1.4 Issue I. 4 – Creep-Rupture and Fatigue Damage
From O’Donnell, Hull and Malik Item OG- 4:
Creep is expected to be a problem for VHTR (hot vessel option) and Gen IV. Subsection NH design rules need extension to higher temperatures to account for creep rupture, excessive creep deformation, creep buckling, cyclic creep ratcheting, and creep-fatigue damage. Fatigue, creep, and creep-fatigue interactions are expected to be technical issues of concern , . Improved correlations for creep and creep-fatigue have been developed from research of the 1990s but are not yet included in the ASME Code . New work has been proposed to ensure that recent technology developments are incorporated .