Design & Analysis of ASME Boiler and Pressure Vessel Components in the Creep Range
5 Analysis of Components: Strain- and Deformation-Controlled Limits
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Stress analysis of a component in the creep regime requires both load- and strain-controlled evaluation. Load-controlled limits were discussed in Chapter 4 and the strain-controlled limits are described in this chapter. The evaluation for the strain-controlled limits is made in accordance with one of three criteria detailed in III-NH. The first criterion, based on elastic analysis, is the easiest to use but is very conservative. When the stress calculations cannot satisfy the elastic limits then the component is redesigned or the analysis proceeds to the second criterion based on simplified inelastic analysis. This second criterion is costlier to perform than the elastic analysis and requires additional material data. However, the results obtained from the simplified inelastic analysis are more accurate than those obtained from an elastic analysis.
When the simplified inelastic stress analysis cannot satisfy the simplified inelastic stress limits, then the component is redesigned or the analysis proceeds to the third criterion based on inelastic analysis. To perform inelastic analysis, it is necessary to define the constitutive equations for the behavior of the material in the creep regime. These equations are not readily available for all materials. As a consequence, this third criterion is costly to perform but gives reasonably accurate results once performed.