ASME STP-NU-042-2013 pdf download
ASME STP-NU-042-2013 pdf download.NEW MATERIALS
FOR ASME SUBSECTION NH.
With the accumulated strain limit the membrane strain limit should be derived from the creep ductility but the creep-fatigue interaction ignored.
2.1.4 Creep Analysis Method: Creep Constitutive Equation and Related Hardening/Flow Rules, and a Creep Analysis-Based Method of Evaluating Creep Damage and Creep Strain
A creep analysis method needs to be developed for use in the following analyses of the structural integrity assessment of a VHTR plant component.
• Stress relaxation analysis to evaluate the creep damage discussed above
• Creep strain analysis. in particular structural discontinuities for use in analyzing the creep deformation localization induced by large differences in stiffness or strength that is well- known as elastic follow-up.
In developing such a creep analysis method, in addition to the creation of a creep constitutive equation and related hardening/flow rules, design factors that take into account the several effects causing variation in creep damage and creep strain in plant components will need to he derived, typically by using the variation in creep curves.
The present report discusses the creep constitutive equation and design factors.
2.1.4.1 Creep Constitutie Equation
The Garofalo type creep constitutive equation was used to lit the creep curves of Hastelloy XR because of the limited data on the primary creep regime. An example of the creep curve data is given in Figure 17. The secondary creep regime is clearly observed in many creep curves. The data fitting is good, but if the early stage of the primary creep regime (in Figure 17 less than 0.1%) is focused upon a small offset can be observed, which is known to cause poor estimations of stress relaxation behavior. As pointed out when developing the “Monju” elevated temperature design criteria [11] the following Blackburn type creep constitutive equation is more recommendable for use in analyzing creep in NGNP components.
As is well known, slower creep strain rates cause higher creep damage but lower creep strain. An averaged correlation of the creep curves therefore was used in creating a creep constitutive equation or nominal creep constitutive equation. Design factors that take into consideration the variations in creep damage and creep strain primarily caused by those variations in the creep curves when using the nominal creep constitutive equation. About twenty years ago, the author analyzed the variations in creep damage and creep strain caused by variations in the creep curves 1121. The results of analysis resulted in design factors of 20 for creep damage and 2 for creep strain being recommended, with, however, the classical strain hardening rule being used in the analysis.
2.1.5 Thermal Aging Effect on Low Temperature Strength
Long-term exposure to the very high temperatures of 850°C and above does not cause any significant reduction in mechanical strength at very high temperatures, as revealed in Figure 18, but does cause significant reductions in fatigue strength and toughness at low temperatures, around room temperature for example. Because of this the structural integrity of IHX tube bundles in a stand-by operation against intense earthquake loading (intense seismic loading) need to he evaluated. The gap between the bundles (in particular the tube support devices) and the shell (or liner attached to the internal thermal insulator) is at its maximum in stand-by operations. Intense earthquakes shake the IHX. and thereby also shaking the bundle. That shaking then results in impact loading both to the tube bundle and to the shell.
However, with the HTTR Il-IX this issue is not considered to be critical because of the following reasons.
l)The stress levels of the high temperature parts of the IHX in both normal operations and with abnormal events are low in comparison with the fatigue strength at room temperature.
2)The impact energy is low in comparison to the toughness (Charpy V-notch values).
This, then, resulted in the decision that there would be no need to specify any thermal aging requirements. However, as a generic design criterion for application with NGNP components a specific design rule was recommended.
Thermal aging effects on other mechanical properties are given for Inconel 617 in Figure 19 and Figure 20. As revealed by Figure 20. no unique behavior in the monotonic stress-strain relationship due to dynamic recrystallization, as discussed above, can be observed after aging at 950°C.
