Final report on improved creep-fatigue models on advanced materials for SFR applications.

TLDR: In this article, the authors present a Table of Contents and Table of Table of TABLES (Tables and figures) with a list of FIGURES and FIGURES.

Abstract: ............................................................................................................................... i TABLE OF CONTENTS .......................................................................................................... iii LIST OF TABLES .................................................................................................................... iv LIST OF FIGURES .................................................................................................................... v 1 Introduction ............................................................................................................................ 1 2 ASME Creep-Fatigue Design Rule for G91 Steel and Current Development ....................... 3 3 Creep-Fatigue Experiments .................................................................................................... 5 3.1 Experimental Procedure .................................................................................................. 5 3.2 Experimental Results ...................................................................................................... 8 3.2.1 Creep-Fatigue Data ............................................................................................... 8 3.2.2 Microstructure .................................................................................................... 13 4 Modeling Creep-Fatigue Interaction .................................................................................... 14 4.1 Cyclic Softening Model ................................................................................................ 14 4.2 Stress Relaxation Model ............................................................................................... 18 4.3 Improved Bilinear Creep-Fatigue Damage Model ....................................................... 22 4.4 Interactive Damage Rate Model ................................................................................... 29 5 Accelerated Creep-Fatigue Testing Methodology ............................................................... 31 6 Summary and Future Work .................................................................................................. 33 References ................................................................................................................................. 35

Figures

Figure 5-1. Accelerated creep-fatigue damage testing methodology ...........................................32

Figure 3-12. (a) Relaxed stress as a function of time during hold and (b) normalized relaxed stress as a function of time during hold for the G91 specimen tested at 20°C with total strain amplitude of 0.5%, and a hold time of 60 s applied at peak strains in tension and compression (TCH 60 s).

Figure 4-14. An example showing the creep damage rate and the accumulation of creep damage as a function of hold time per unit cycle for G91 steel.

Figure 3-14. Microstructure of the specimen tested at 20°C with total strain amplitude of 0.5%, and a tension and compression hold time of 60 s.

Figure 4-5. Comparison of the modeled cyclic softening curves with experimental data in (a) linear scale and (b) log scale, for G91 specimens tested at 550°C with total strain amplitude of 0.25%, 400°C with total strain amplitude of 0.5% and 20°C with total strain amplitude of 0.5%, respectively. All three tests were carried out with a hold time of 60 s applied at peak strains in tension and compression (TCH 60 s) in air.

Figure 4-9. Comparison of modeled stress relaxation curves with experimental data for cycles 1, 10, 100, and 1000 for G91 tested at

Full text

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ANLARC204
Final Report on Improved Creep-Fatigue Models on Advanced
Materials for SFR Applications
NuclearEngineeringDivision

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ANLARC204
FinalReportonImprovedCreepFatigueModelsonAdvanced
MaterialsforSFRApplications
MeimeiLi,WilliamK.Soppet,SaurinMajumdar,DavidRink,andKenNatesan
NuclearEngineeringDivision
ArgonneNationalLaboratory
September2011

FinalReportonImprovedCreepFatigueModelsonAdvancedMaterialsforSFRApplications
September2011
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FinalReportonImprovedCreepFatigueModelsonAdvancedMaterialsforSFRApplications
September2011
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  ANLARC204
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i
ABSTRACT
This report provides an update on the materials performance criteria and methodology
relevant to sodium-cooled fast reactors (SFRs). The report is the second deliverable (Level 2) in
FY11 (M2A11AN040303) under the work package A-11AN040303 “Materials Performance
Criteria and Methodology” as part of the Advanced Structural Materials Program for the
Advanced Reactor Concepts.
The overall objective of the Materials Performance Criteria and Methodology work project is
to evaluate the key requirements for the ASME Code qualification and the Nuclear Regulatory
Commission (NRC) approval of advanced structural materials in support of the design and
performance of sodium-cooled fast reactors. Advanced materials are a critical element in the
development of fast reactor technologies. Enhanced materials performance not only improves
safety margins and provides design flexibility, but also is essential for the economics of future
advanced fast reactors. Qualification and licensing of advanced materials are prominent needs
for the development and implementation of advanced fast reactor technologies. Nuclear
structural component designs in the U.S. comply with the ASME Boiler and Pressure Vessel
(B&PV) Code Section III (Rules for Construction of Nuclear Facility Components), and the
NRC grants licensing. As the SFRs will operate at higher temperatures than the current light
water reactors (LWRs), the design of elevated-temperature components must comply with
ASME Section III Subsection NH (Class 1 Components in Elevated Temperature Service). A
number of technical issues relevant to materials performance criteria and high temperature
design methodology in the SFR were identified and presented in earlier reports. A viable
approach to resolve these issues and the R&D priority were also recommended. The
development of mechanistically based creep-fatigue interaction models for life prediction and
reliable data extrapolation was chosen to be the central focus in near-term efforts.
Our current focus is on the creep-fatigue damage issue in high-strength ferritic/martensitic
steels such as mod.9Cr-1Mo (G91) and NF616 (G92) steels. The current ASME creep-fatigue
design rule puts severe limits of fatigue and creep loads for G91 steel, the lead structural material
for fast reactors. High-strength ferritic/martensitic steels behave fundamentally differently from
austenitic stainless steels, for which the current ASME creep-fatigue design rules were
developed. The unique deformation and damage characteristics in G91 steel, e.g. cyclic
softening, degradation of creep and rupture strength during cyclic service, demands a new creep-
fatigue design procedure that explicitly accounts for the material’s unique creep-fatigue
behavior. G92 steel, a variant of G91 steel, is the lead candidate in the Advanced Alloy
Development program. There is currently no creep-fatigue design rule available for this
advanced alloy.
To support the predictive model development and resolve the over-conservative issue with
the ASME design rule for G91 steel, we recovered stress relaxation data from thirteen creep-
fatigue tests conducted in the late 1980s and early 1990s at Oak Ridge National Laboratory, and
conducted extensive data analysis in FY10. Based on this database and available literature data,
we have developed a Cyclic Softening Model and a Stress Relaxation Model specifically applied
to G91 steel. The Cyclic Softening Model describes the cyclic stress variation as a function of
cycle number during creep-fatigue loading, and well captures the cyclic softening effect in G91
steel. The Stress Relaxation Model predicts the stress relaxation curve during the hold time of
cyclic loading, and its dependence on the cyclic softening effect of G91 steel.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Final report on improved creep-fatigue models on advanced materials for sfr applications" ?

In this paper, the ASME Creep-Fatigue design rule for G91 steel and current development is discussed. 

Q2. What is the important factor in determining the extent of creep damage?

Among various factors of different heats, strain range level, hold type, and hold time, the hold type, i.e. either hold in tension or in compression, seems to play the most important role in determining the extent of creep damage. 

Q3. What was the unit creep damage for the first cycle?

Because only the stress relaxation data of the first (or second) cycle, 10th cycle, 100th cycle, and the half-life were available in digital form, the unit creep damage was calculated for these cycles only. 

Q4. What is the need for the model to be validated and improved?

The models must be validated and further improved by a large number of well-designed creep-fatigue experiments that cover a wide range of variables such as temperature, strain amplitude, waveform, strain rate, hold time, etc. 

Q5. How much creep damage can be calculated using the unit creep damage at the half-life?

When the stress relaxation curves at the 10th cycle were used in the evaluation of creep-fatigue damage, the calculated creep damage can be one order of magnitude higher than that calculated using the unit creep damage at the half-life. 

Q6. What is the stress-rupture life curve at 538°C?

At 538°C, the stress-rupture life curve can be described by a single power-law relation, while the stress-rupture life curve at 593°C shows a deflection at the stress level of ~140 MPa, below which the rupture time decreases more rapidly with increasing stress. 

Q7. What is the purpose of the creep-fatigue testing method?

Previous creep-fatigue experiments focused on providing testing data for qualification of the material, which often have insufficient experimental details that are required for a deeper understanding of deformation and damage mechanisms, optimization of key materials parameters, and validation of models. 

Q8. What is the optimum hold time for unit creep damage?

While a number of variables including heat variations, strain range, hold type, and hold time are involved in the tests given in Fig. 4-12, the length of the hold time is apparently a critical factor in determining the amount of unit creep damage. 

Q9. What is the tensile stress-strain curve of G91 steel?

For a given temperature, T and a total strain range, Δεt, the value of stress constant, σ0, can be obtained from the tensile stress-strain curve of G91 steel. 

Q10. What was used to remove surface scratches and oxide layer?

Gauge sections of the creep-fatigue specimens were polished longitudinally with 1-µm diamond paste to remove surface scratches and oxide layer, if any, before testing. 

Q11. What is the need for an accelerated testing approach to assess the performance of nuclear reactor components?

The extended service life of nuclear reactor components also calls for an accelerated testing approach to properly assess the performance of reactor materials and components in real nuclear reactor environments. 

Q12. How many cycles did the creep damage at half-life show?

When the unit creep damage at half-life was used in the calculations, a majority of the creep-fatigue tests show creep damage below 0.1. 

Q13. What materials have been successfully applied to predict the creep-fatigue behavior?

Type 304 austenitic stainless steel and 2.25Cr-1Mo ferrtic steel, to predict the creep-fatigue behavior under various loading conditions [Majumdar and Maiya 1978, 1979, 1980, 1981].