DESIGN GUIDELINES FOR THE EFFECTS OF CREEP, FATIGUE & CREEP-FATIGUE INTERACTION WITH DESIGN-BY-ANALYSIS AND NONDESTRUCTIVE INSPECTION ACCEPTANCE CRITERIA
|Publication Date:||20 June 2014|
As part of the modernization of Section I - Rules for
Construction of Power Boilers of the ASME Boiler and Pressure
Vessel Code (Section I), a project was established to develop
design guidelines for the effects of creep-fatigue interaction and
flaw size acceptance criteria within the overall framework of
Design-By-Analysis (DBA). The existing methods within Section I,
which are based around Design-By- Rule/Formula and do not
explicitly consider creep, fatigue or their interaction. The
oversimplifications involved are such that the safety of boilers
operating at higher steam cycle conditions, and under cyclic
service, has been questioned. Design-By-Analysis (as an alternative
This report has been prepared to recommend design guidelines for
components in Section I - Power Boilers. As such, it does provide a
comprehensive review of issues. However, to properly introduce the
context of the recommendations, some background is provided to a
number of philosophical topics that surround Design-By-Analysis,
Design-By-Formula, Design-For-Safety and Design-For-Lifetime.
To that end, a Design-By-Analysis approach is recommended that is both relevant and technically consistent, and which considers the key modes of structural behavior and material response. Some features of the recommended approach are:
• It provides design checks for the structural failure modes relevant to modern Power Boiler, including cyclic service with checks on local creep and fatigue damage as well as the possibility of creep-fatigue interaction.
• It requires minimal material data (most of which is already available within Section II - Materials, Part D of the ASME Boiler and Pressure Vessel Code (Section II, Part D), or which is a logical extension of that based on Section III, Rules for Construction of Nuclear Facility Components Subsection NH of the ASME Boiler and Pressure Vessel Code (Section III, Subsection NH)).
• All design checks are accomplished with an elastic or
• It incorporates weld strength reduction factors where weldments operate in the time dependent (creep) regime.
The recommended approach is based on validated methods and the explanation of the methodology highlights where particular features are adopted or adapted from other Codes (most particularly from Section VIII, Rules for Construction of Pressure Vessels Division 2 of the ASME Boiler and Pressure Vessel Code (Section VIII, Div. 2), Section III, Subsection NH and from EN13445-3, Annex B). The approach includes all aspects of a Design-By-Analysis methodology because without considering the complete design procedure any proposals on specific aspects, such as creep-fatigue interaction, may not be technically consistent with the overall framework.
Flaw size criteria and previous work in this area are reviewed to provide context to the development of future methods. While flaw size criteria can be developed based on engineering mechanics considerations, this can invoke arbitrary assumptions which limit the generality and practical value of the results. Because of this it is highlighted that flaw size criteria should primarily be defined by workmanship quality standards; as is the approach adopted in many other Codes. It is also recognized that if flaw size criteria are to be established using engineering mechanics then this can be accomplished independently of the overall approach to Design-By-Analysis.