Verification of the Prediction Methods of Strain Range in Notched Specimens Made of Mod.9Cr-1Mo

Abstract

Several innovative prediction methods of strain range have been developed in order to apply to the Generation IV plants. In a component design at elevated temperature, ‘strain range’ is used to calculate the fatigue and creep-fatigue damage. Therefore, prediction of ‘strain range’ is one of the most important issues to evaluate the components’ integrity during these lifetimes. To verify the strain prediction method of discontinues structures at evaluated temperature, low cycle fatigue tests were carried out with notched specimens. All the specimens were made of Mod.9Cr-1Mo, because it is a candidate material for a primary and secondary heat transports system components of JSFR (Japanese Sodium Fast Reactor). Deformation control fatigue tests and thermal fatigue tests were performed by ordinary uni-axial push-pull test machine and equipment generating the thermal gradient in the notched plate by induction heating. Stress concentration level was changed by varying the notch radius in the both kind of tests. Crack initiation and propagation process during the fatigue test were observed by the digital micro-scope and replica method. Elastic and inelastic FEAs were also carried out to estimate the ‘strain range’ for the prediction of fatigue life. Then the ranges of several strain predictions and estimations were compared with the test results. These predictions were based on the sophisticated technique to estimate the ‘strain range’ from elastic FEA. Stress reduction locus (SRL) method, simple elastic follow-up method, Neuber’s rule method and the methods supplied by elevated temperature design standards were applied. Through these results, the applicability and conservativeness of these strain prediction and estimation methods, which is the basis of the creep-fatigue life prediction, is discussed.

Stress relaxation and elastic follow-up using a stress range-dependent constitutive model

“…The simplified methods were compared to experimental studies and detailed elastic-plastic analyses of notch bar specimens with various notch sizes. Ando et al [27] concluded that, in terms of estimating the stressstrain trajectory, the SRL method gave good comparisons with the experimental results while Neuber's Rule and the JSME elastic follow-up method were quite conservative at higher strains, the latter perhaps being overly conservative. Like Seshadri's GLOSS method, by definition, the SRL method '.…”

“…Since only elastic analysis is required, the trajectory is independent of the constitutive relation in the GLOSS method. More recently, Ando et al [27] compared several simplified methods to predict strain range, used in hightemperature design to estimate fatigue and creep-fatigue damage, at localized (peak) stress concentrations. The methods used were Neuber's Rule [24], the JSME elastic follow-up approximation (q = 3), and the stress reduction locus (SRL), which is based on elastic analysis coupled with a uniaxial stress-strain curve.…”