Experimental Investigation of Strain Concentration Evaluation Based on the Stress Redistribution Locus Method

Transcription

1 Experimental Investigation of Strain Concentration Evaluation Based on the Stress Redistribution Locus Method Nobuhiro ISOBE Nubuchika KAWASAKI Masanori ANDO Masayuki SUKEKAWA Hitachi JAEA JAEA Hitachi-GE Nuclear Energy

2 Contents 1. Background 2. Strain concentration evaluation methods 3. Fatigue tests 4. Results and discussion 5. Summary

3 1. Background Inelastic strain should be considered in the design of fast reactors because of high temperature operation. Evaluation methods based on elastic analysis are used in the current codes and they include relatively large conservatisms. To improve and rationalize high temperature design rule of fast reactors. Elastic Prediction of plastic strain Elastic-plastic Creep behavior Strain 2

4 2-1. Strain concentration evaluation methods Neuber s rule K σ K ε = K 2 t the locus of stress redistribution from elastic to plastic state is a hyperbolic curve Elastic follow-up Inelastic strain concentration by the difference of stiffness between components is considered. The locus is obtained by using the factor q as follows; e ( ε ε )E q= + 1 e σ σ In the Japanese code, DDS; Neuber s rule - local strain, Elastic follow-up - nominal strain 3

5 2-2. Stress redistribution Locus (SRL) concept The locus of stress redistribution for a structure can be obtained from elastic-creep analysis. The effect of creep law and loading magnitude on loci is small and the locus mainly depends on the structure. A master curve for this method has been proposed; ~ 1 1 ε = + ~ κ σ~ ( κ 1) σ K ~ ε ε = K t K ~ σ σ = K t The master curve coincide with the Neuber s rule when κ=1. κ is usually

6 2-3. Comparison of methods Stress (MPa) Elastic 316FR steel, 550 o C Elastic follow-up (q=3) Cyclic stress-strain curve Monotonic stress-strain curve 100 SRL(κ=1.6) Neuber s rule Strain (%) The SRL method gives smaller strain than conventional methods. The SRL method improves high temperature design methods if its validity has been verified. 5

7 3-1. Fatigue tests Material:316FR steel C Si Mn P S Ni Cr Mo N φ16 M φ16 φ16 R1 K t =1.61 K t =3.39 R4 Circumferentially notched specimen 1 6

8 3-2. Experimental conditions Fatigue test condition Temp.( ) Kt Nominal strain range(%) Room temp % % % % 16 Gauge length of nominal strain Local strain measurement Capacitance type high temperature gage: Strain Pecker 7

9 Experimental results 550 o C, K t =3.39, ε n =0.26%, N=1000 Load (kn) Measured local strain range by the gauge Maximum strain Gauge length (1mm) Strain (%) Nominal strain Local strain The maximum strain at the notch root is larger than the gauge output: K t = , K t = Measured local strains were compared with prediction. The following methods are discussed; 1. The SRL master curve, 2. Stress redistribution locus by elastic-creep analysis, 3. Elastic-plastic analysis, 4. The Neuber s rule, 5. Strain range corresponding to failure life of each specimen. 8

10 3-2. Evaluation results of strain concentration o C, K t = o C, K t = Neuber's rule SRL Master Curve (κ=1.6) Elastic-plastic analysis Experiment Elastic-creep analysis Strain (%) Strain (%) All the methods discussed in this study gives conservative prediction. The SRL master curve, elastic-creep and elastic-plastic analysis give similar estimations except for the condition that a large plastic strain arises. 9

11 3-3. Evaluation results of strain concentration Prediction method SRL(k=1.6) Elastic-creep analysis Elastic-plastic analysis Neuber s rule Stress-strain Strain range (%) curve 550 RT, K t =1.61 RT, K t =3.39 o C, 550 K t =1.61 o C, K t =3.39 Monotonic > Cyclic Monotonic Cyclic Monotonic Cyclic Monotonic > 2 > 2 > 2 > 2 Cyclic Estimation from fatigue curve Measured value 1st cycle Middle life Strain range corresponding to failure life is closed to measured strain range. Method based on the SRL concept and inelastic analysis gives percent smaller strain than the Neuber s rule. 10

12 3-4. Comparison of predicted failure lives Factor of 4 Factor of 2 Neuber's rule SRL master curve Elastic-creep analysis Elastic-plastic analysis Measured strain Predicted life The SRL method has a reasonable conservatisms about fatigue life. 11

13 3-5. Creep-fatigue tests Creep-fatigue tests had been carried out to discuss local strain behaviors during dwell. Test condition Temp. ( ) 650 Kt ε n (%) % % Strain hold inserted at 1-5, 10, 20, 50, 100, 200, 500 cycles - not every cycle Local strain Nominal strain K t =3.39 N= Strain (%) dwell 12

14 3-6. Comparison of relaxation behaviors K t = Kt=1.61, N=500 Kt=3.39, N= Time (s) Straining behaviors during dwell The experimental locus is almost coincide with the locus by elasticcreep analysis. The SRL method improves the accuracy of inelastic strain concentration in comparison with current. Stress (MPa) Stress (MPa) Elastic-creep analysis K t =3.39 N=1 N=200 Strain (%) Cyclic stress-strain curve Monotonic stress-strain curve Cyclic stress-strain curve SRL master curve (κ=1.6) Monotonic stress-strain curve N=1 N= Strain (%) SRL master curve (κ=1.6) Elastic-creep analysis 13

15 4. Summary The stress redistribution locus (SRL) method was investigated experimentally in order to improve and rationalize creep-fatigue damage evaluation methods in the high temperature design rule of fast reactors. (1) High temperature fatigue tests of circumferential notched specimens were conducted accompanying with local strain measurement. The SRL method is able to rationalize the evaluation of inelastic strain concentration with keeping a reasonable conservatisms. (2) Creep-fatigue tests are also conducted and local strain behaviors at the notch root during dwell is observed. Stress-strain loci estimated from experimental results shows a good agreement with prediction by the SRL concept. 14

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