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The effect of hold-times on the fatigue life of 20% cold-worked Type 316L stainless steel under deuteron irradiation
jou lof fluciear ELSEVIER
'
Journal of Nuclear Materials 224 (1995) 311-313
Letter to the Editors
The effect of hold-times on the fatigue life of 20% cold-worked Type 316L stainless steel under deuteron irradiation R. Scholz CEC, Joint Research Centre, 21020 Ispra (Va), Italy
Received 13 December 1994; accepted 29 May 1995
Abstract
Strain-controlled fatigue tests have been performed in torsion on 20% cold-worked Type 316L stainless steel specimens during irradiation with 19 MeV deuterons. A hold-time was imposed at the minimum strain value in the loading cycle. The irradiation creep induced stress relaxation led to the buildup of a mean stress. The number of cycles to failure may be significantly reduced in comparison to analogous continuous cycling tests under thermal conditions.
The next generation tokamak reactor for controlled thermonuclear fusion will have a cyclic operation at low frequency. A plasma burn time of about 1000 s will be followed by a plasma off burn time of about 10 s. So, the plasma facing first wall will be subjected to a fatigue cycle with periods of irradiation creep such that an interaction of both phenomena, irradiation creep and fatigue, will occur [1]. The interaction between irradiation creep and fatigue can be studied during an irradiation with light ions. Light ion irradiations require the use of mini-specimens such that the push-pull loading cycle and stress reversal can be accomplished only applying sophisticated experimental techniques. The torsional stress mode used for the present tests, offers the possibility to impose a stress or strain controlled loading cycle including stress reversal with the aid of a rather simple experimental equipment: hourglass type specimens having a minimum diameter of 140-200 izm are stressed in torsion by an electromagnetic system. The stress of the specimen is maintained by controlling an electric current. Stress reversal is obtained simply by inverting the current direction. Thermally induced stresses can be avoided by mounting the specimens in a way that they are free to elongate in axial direction [2].
Strain-controlled fatigue tests have been performed at 400°C under thermal condition in continuous cycling and during irradiation with 19 MeV deuterons by imposing a hold-time at the minimum shear strain in the loading cycle. In addition, two control tests under thermal conditions at 400°C were performed. In these tests a hold-time of 20 s was applied at the minimum strain value such that the in-beam tests with hold-times can be directly compared to out-of-beam tests with equal loading conditions imposed. The experimental parameters for the irradiation tests are gathered in Table 1: tests No. 1 and 2 have been conducted entirely under irradiation conditions, test No. 3 was started under irradiation conditions by imposing a loading cycle with hold-time and completed under thermal conditions in continuous cycling whilst maintaining the same specimen temperature and strain interval. The effect of the irradiation on the stress-strain hysteresis curves for the low strain tests is illustrated in Fig. 1. The stress-strain curves of test No. 2 are plotted for the first cycle ( ) and after 2000 cycles ( . . . . . ) in comparison to the hysteresis loop taken after 2000 cycles of the thermal control test ( - • - ) conducted for the same loading conditions. The hysteresis loop for the first cycles of both tests were equal
0022-3115/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3115(95)00099-2
312
R. Scholz /Journal of Nuclear Materials 224 (1995) 311-313 10
Table 1 Parameters imposed for the irradiation tests, Ay stands for the total shear strain range No.
Ay (%)
Hold-time
dpa/s
Dose (dpa)
Ny
1 2 3
5.5 1.3 1.13
20 s 20s 50 s
1X 10 - 6 6×10 -6 6 × 10 - 6
0.15 0.9 0.8
6700 7200 26400
Z
< tr z
+1
<
G G
tr
~9 tr
,,<, within the error limits. The plot illustrates the change in the stress-strain relationship due to the irradiation creep process: (i) The irradiation creep relaxation leads to an increase in the minimum shear stress Zmi. in the loading cycle, at which the hold-time is imposed. The maximum shear stress Zmax in the cycle has to be increased in order to keep the total strain interval constant. As a result of the growth in both values, Zmi. and ~'max, the hysteresis loop is shifted on the stress axis in the direction of rm~x such that a mean stress is built up which amounts to 62 MPa after 2000 cycles. The shifting of the hysteresis loop causes the loading parameters to be changed in a way that a steady-state situation is reached. (ii) The hysteresis loop of the control test under thermal condition remains nearly symmetric in stress indicating that thermal creep effects are small. For this test, the absolute value of both, the minimum and the maximum shear stress in the loading cycle, decrease with the number of cycles. This cyclic softening of the
500 ~ 0-
'
250 03 03 LU r~ k~O nr
<
uJ -r
....."
I
-0.008 -0.008
i ..~o. - 0 ~ /
.~;'//7 /
I ;~,/
I
0.004
0.008
03
<
" -250
-500
SHEAR STRAIN Fig. 1. Shear stress-shear strain diagram of a specimen during 19 MeV deuteron irradiation at 400°C, for the first cycle ( ) and after 2000 cycles ( . . . . . ). A hold-time of 20 s was imposed at the minimum strain. In comparison, the 2000th cycle of a control test (- • -) conducted for the same temperature and loading conditions without irradiation.
-r ~o
+2
•
D
~3 1
. . . . . . . .
1.00E+03
'
. . . . . . . .
1.00E+04
'
. . . . . . . .
1.00£+05
1.00E+06
NUMBER OF CYCLES TO FAILURE Fig. 2. Fatigue of Type 316L stainless steel under thermal conditions in continuous cycling ([3)with a hold-time (11), and under irradiation including a hold-time (+). material has been observed for all tests conducted under thermal conditions at 20 and 400°C regardless whether a hold-time was imposed or not. (iii) The plastic strain component of the 2000th cycle under irradiation is smaller than that of the analogous cycle under thermal conditions due to the irradiation induced increase in strength of the material. If these results are applied to tension-compression fatigue tests on austenitic stainless steel samples subjected to analogous loading and irradiation conditions, the buildup of a tensile m e a n stress is expected, its magnitude depending mainly on the amount of irradiation creep induced stress relaxation in compression. In Fig. 2, the total strain range is plotted versus the number of cycles to failure for tests under thermal and irradiation conditions. The test numbers according to Table 1 are indicated for the irradiation tests. The fatigue life observed for the low strain irradiation tests are reduced in comparison to corresponding tests under thermal conditions. The reduction in fatigue life can be ascribed, at least partly, to the buildup of a mean stress. A reduced fatigue life observed on 20% cold-worked 316L stainless steel specimens after irradiation with neutrons was attributed mainly to the radiation induced loss in ductility since ductility is a life determining factor for low cycle fatigue testing [3]. A similar interpretation is possible for the present low strain tests since the plastic strain component which is small in the first cycle, grows with on-going cycling to values as they are encountered in low cycle fatigue tests. Fig. 2 shows that the irradiation has only little or no effect on the fatigue life when a shear strain range of 5.5% is imposed. A t these high strain ranges, the irradiation creep induced stress relaxation is small in
R. Scholz /Journal of Nuclear Materials 224 (1995) 311-313 comparison to the loading stress and the symmetry of the torque twist curves seemed to be unaffected by the irradiation. The involved irradiation doses are too small for embrittlement effects to play a major role. In conclusion, irradiation creep may change significantly the stress distribution and therefore, the fatigue life of a fusion reactor first wall element under service conditions.
313
References [1] B. van der Schaaf, J. Nucl. Mater. 155-157 (1988) 156. [2] E.K. Opperman, J.L. Straalsund, G.L. Wire and R.H. Howell, Nucl. Technol. 42 (1979) 71. [3] M.L. Grossbeck and K.C. Liu, Nucl. Technol. 58 (1982) 538.
'
Journal of Nuclear Materials 224 (1995) 311-313
Letter to the Editors
The effect of hold-times on the fatigue life of 20% cold-worked Type 316L stainless steel under deuteron irradiation R. Scholz CEC, Joint Research Centre, 21020 Ispra (Va), Italy
Received 13 December 1994; accepted 29 May 1995
Abstract
Strain-controlled fatigue tests have been performed in torsion on 20% cold-worked Type 316L stainless steel specimens during irradiation with 19 MeV deuterons. A hold-time was imposed at the minimum strain value in the loading cycle. The irradiation creep induced stress relaxation led to the buildup of a mean stress. The number of cycles to failure may be significantly reduced in comparison to analogous continuous cycling tests under thermal conditions.
The next generation tokamak reactor for controlled thermonuclear fusion will have a cyclic operation at low frequency. A plasma burn time of about 1000 s will be followed by a plasma off burn time of about 10 s. So, the plasma facing first wall will be subjected to a fatigue cycle with periods of irradiation creep such that an interaction of both phenomena, irradiation creep and fatigue, will occur [1]. The interaction between irradiation creep and fatigue can be studied during an irradiation with light ions. Light ion irradiations require the use of mini-specimens such that the push-pull loading cycle and stress reversal can be accomplished only applying sophisticated experimental techniques. The torsional stress mode used for the present tests, offers the possibility to impose a stress or strain controlled loading cycle including stress reversal with the aid of a rather simple experimental equipment: hourglass type specimens having a minimum diameter of 140-200 izm are stressed in torsion by an electromagnetic system. The stress of the specimen is maintained by controlling an electric current. Stress reversal is obtained simply by inverting the current direction. Thermally induced stresses can be avoided by mounting the specimens in a way that they are free to elongate in axial direction [2].
Strain-controlled fatigue tests have been performed at 400°C under thermal condition in continuous cycling and during irradiation with 19 MeV deuterons by imposing a hold-time at the minimum shear strain in the loading cycle. In addition, two control tests under thermal conditions at 400°C were performed. In these tests a hold-time of 20 s was applied at the minimum strain value such that the in-beam tests with hold-times can be directly compared to out-of-beam tests with equal loading conditions imposed. The experimental parameters for the irradiation tests are gathered in Table 1: tests No. 1 and 2 have been conducted entirely under irradiation conditions, test No. 3 was started under irradiation conditions by imposing a loading cycle with hold-time and completed under thermal conditions in continuous cycling whilst maintaining the same specimen temperature and strain interval. The effect of the irradiation on the stress-strain hysteresis curves for the low strain tests is illustrated in Fig. 1. The stress-strain curves of test No. 2 are plotted for the first cycle ( ) and after 2000 cycles ( . . . . . ) in comparison to the hysteresis loop taken after 2000 cycles of the thermal control test ( - • - ) conducted for the same loading conditions. The hysteresis loop for the first cycles of both tests were equal
0022-3115/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3115(95)00099-2
312
R. Scholz /Journal of Nuclear Materials 224 (1995) 311-313 10
Table 1 Parameters imposed for the irradiation tests, Ay stands for the total shear strain range No.
Ay (%)
Hold-time
dpa/s
Dose (dpa)
Ny
1 2 3
5.5 1.3 1.13
20 s 20s 50 s
1X 10 - 6 6×10 -6 6 × 10 - 6
0.15 0.9 0.8
6700 7200 26400
Z
< tr z
+1
<
G G
tr
~9 tr
,,<, within the error limits. The plot illustrates the change in the stress-strain relationship due to the irradiation creep process: (i) The irradiation creep relaxation leads to an increase in the minimum shear stress Zmi. in the loading cycle, at which the hold-time is imposed. The maximum shear stress Zmax in the cycle has to be increased in order to keep the total strain interval constant. As a result of the growth in both values, Zmi. and ~'max, the hysteresis loop is shifted on the stress axis in the direction of rm~x such that a mean stress is built up which amounts to 62 MPa after 2000 cycles. The shifting of the hysteresis loop causes the loading parameters to be changed in a way that a steady-state situation is reached. (ii) The hysteresis loop of the control test under thermal condition remains nearly symmetric in stress indicating that thermal creep effects are small. For this test, the absolute value of both, the minimum and the maximum shear stress in the loading cycle, decrease with the number of cycles. This cyclic softening of the
500 ~ 0-
'
250 03 03 LU r~ k~O nr
<
uJ -r
....."
I
-0.008 -0.008
i ..~o. - 0 ~ /
.~;'//7 /
I ;~,/
I
0.004
0.008
03
<
" -250
-500
SHEAR STRAIN Fig. 1. Shear stress-shear strain diagram of a specimen during 19 MeV deuteron irradiation at 400°C, for the first cycle ( ) and after 2000 cycles ( . . . . . ). A hold-time of 20 s was imposed at the minimum strain. In comparison, the 2000th cycle of a control test (- • -) conducted for the same temperature and loading conditions without irradiation.
-r ~o
+2
•
D
~3 1
. . . . . . . .
1.00E+03
'
. . . . . . . .
1.00E+04
'
. . . . . . . .
1.00£+05
1.00E+06
NUMBER OF CYCLES TO FAILURE Fig. 2. Fatigue of Type 316L stainless steel under thermal conditions in continuous cycling ([3)with a hold-time (11), and under irradiation including a hold-time (+). material has been observed for all tests conducted under thermal conditions at 20 and 400°C regardless whether a hold-time was imposed or not. (iii) The plastic strain component of the 2000th cycle under irradiation is smaller than that of the analogous cycle under thermal conditions due to the irradiation induced increase in strength of the material. If these results are applied to tension-compression fatigue tests on austenitic stainless steel samples subjected to analogous loading and irradiation conditions, the buildup of a tensile m e a n stress is expected, its magnitude depending mainly on the amount of irradiation creep induced stress relaxation in compression. In Fig. 2, the total strain range is plotted versus the number of cycles to failure for tests under thermal and irradiation conditions. The test numbers according to Table 1 are indicated for the irradiation tests. The fatigue life observed for the low strain irradiation tests are reduced in comparison to corresponding tests under thermal conditions. The reduction in fatigue life can be ascribed, at least partly, to the buildup of a mean stress. A reduced fatigue life observed on 20% cold-worked 316L stainless steel specimens after irradiation with neutrons was attributed mainly to the radiation induced loss in ductility since ductility is a life determining factor for low cycle fatigue testing [3]. A similar interpretation is possible for the present low strain tests since the plastic strain component which is small in the first cycle, grows with on-going cycling to values as they are encountered in low cycle fatigue tests. Fig. 2 shows that the irradiation has only little or no effect on the fatigue life when a shear strain range of 5.5% is imposed. A t these high strain ranges, the irradiation creep induced stress relaxation is small in
R. Scholz /Journal of Nuclear Materials 224 (1995) 311-313 comparison to the loading stress and the symmetry of the torque twist curves seemed to be unaffected by the irradiation. The involved irradiation doses are too small for embrittlement effects to play a major role. In conclusion, irradiation creep may change significantly the stress distribution and therefore, the fatigue life of a fusion reactor first wall element under service conditions.
313
References [1] B. van der Schaaf, J. Nucl. Mater. 155-157 (1988) 156. [2] E.K. Opperman, J.L. Straalsund, G.L. Wire and R.H. Howell, Nucl. Technol. 42 (1979) 71. [3] M.L. Grossbeck and K.C. Liu, Nucl. Technol. 58 (1982) 538.