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Influence of light ion irradiation on fatigue crack propagation in austenitic stainless steel
Journal
of Nuclear
North-Holland,
Materials
155-157
(1988) 963-967
963
Amsterdam
INFLUENCE OF LIGHT ION IRRADIATION ON FATIGUE CRACK PROPAGATION IN AUSTENITIC STAINLESS STEEL P. FENICI Commission of the European Communities, 21020 Ispra (Va), Italy
Joint Research
Centre, Ispra Establishment,
Materials
Science Diuision,
Fatigue crack growth is considered to be a limiting factor for the life time of the first wall of a Tokamak type controlled thermonuclear reactor. As a result of the high energy neutron spectrum in the D-T fusion reaction not only defects but also large quantities of helium due to nuclear transmutation are produced in the structural materials. One way of simulating the
damage of high energy neutrons is to use light ions by means of a charged particle accelerator. In the present work Type 316 stainless steel specimens were irradiated in a suitable irradiation chamber using a variable energy cyclotron. Fatigue crack growth under cyclic tensile stress was measured under simultaneous 18 MeV proton irradiation producing a displacement damage rate of the order of 1OK’ dpa s -‘. The same type of specimens were implanted with 38 MeV a-particles and tested after bombardement. It was found that light ions irradiation has only a slight influence on fatigue crack growth
at 500
’ C in type 316 stainless steel.
1. Introduction
Studies of reactor design concepts have shown that fatigue crack growth initiating from pre-existing mechanical flaws is one of the major parameters to be considered when evaluating the endurance life of the first wall of tokamak reactors [l] and the reaction chamber of inertial confinement systems [2]. The understanding of the interaction of fatigue crack growth and radiation damage is then necessary for an accurate estimation of first wall operating lifetimes. Moreover, to assess the maximum allowable crack length in a structure, reliable fatigue crack growth data under appropriate reactor operation conditions are needed. The anticipated operating conditions for a fusion reactor first wall vary according to reactor design. In general neutron fluxes of - 10’7-10’s n mm2 s-’ of 14 MeV are to be expected. The corresponding displacement damage rates for austenitic stainless steels are estimated to be 10~7-10-6 dpa s-l. Adequate simulation of neutron irradiation is obtained by light ions produced by a particle accelerator. Previous studies have shown that protons and deuterons can generate comparable recoil energy spectra together with realistic ratios of helium production to displacement damage [3-51. The effect of helium produced by transmutation reaction is studied by a-particle irradiation experiments. The relatively short range of these particles in metals and alloys and the difficulties of extracting the thermal energy deposited in the specimen, require the use of thin specimens (= 200 pm). The uncertainties associated with post-irradiation experiments, in particular the fact that tests may not always adequately represent the synergistic effects associated with irradiation under cyclic stressing, have lead to attempts to perform experiments directly under irradiation [6-111. No data have as yet been published on fatigue crack growth under light ion irradiation. 0022-3115/88/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
In the present work we will discuss the influence of light ions irradiation on fatigue crack propagation in Type 316 stainless steel. Thin miniaturized specimens were irradiated in a suitable irradiation chamber [ll] using a variable energy cyclotron. Fatigue crack growth under cyclic tensile stress was measured under simultaneous 18 MeV proton irradiation producing a displacement damage rate of the order of lo-’ dpa s-l. The same type of specimens were implanted with 38 MeV a-particles to obtain up to 800 appm helium. Tests after implantation were conducted under an inert atmosphere using a conventional mechanical testing machine [12]. Thin miniaturized specimens developed for these experiments, have been extensively studied, demonstrating the validtiy and the reproducibility of the data obtained with this specimen geometry on unirradiated material at room temperature [13,14] and at elevated temperature [12]. 2. Experimental The specimens used for irradiation studies (fig. 1) were of the single edge notch (SEN) type. Specimens were milled with longitudinal axes parallel to the rolling direction from solution annealed sheets of Type 316 stainless steel with the following nominal composition (wt%): 0.08 C, 0.006 S, 0.6 Si, 12.0 Ni, 17.0 Cr, 0.2 Cu, 1.5 Mn, 2.5 MO, balance Fe. The fatigue crack notches
Fig. 1. Specimen
geometry
(dimensions
in mm).
964
P. Fenicr / Influence of lighr ion irradiatron on fatigue truck propagarion
were formed by spark erosion, and were 0.1 mm wide, with end radii of 0.05 mm, and either 1 or 2 mm in length. The sheets were 0.15 mm thick, with an average grain diameter of - 0.01 mm. In-beam testing was carried out using a specially designed irradiation facility [ll]. Specimen temperature was monitored by an infrared pyrometer, with a spot diameter of 0.8 mm. Crack growth was measured by means of a remotely controlled long focal length optical microscope, with a magnification of 40 X , equipped with a video camera and recorder. Measurements of crack length were made on a high resolution television screen during experiments, using a videomicrometer at intervals of 0.20-0.25 mm with a precision of kO.02 mm. The specimen was at an angle of 15” to the beam axis in order to allow direct observation with the pyrometer and microscope through quarz windows. The loading waveform was approximately triangular, with a frequency generally of 60 cpm. During irradiation both specimen surfaces were continuously cooled by two opposing high velocity helium jets. The helium, generally at atmospheric pressure, was supplied by a closed purification system giving < 0.1 ppm oxygen and < 1 ppm each of the residual impurities up to a maximum of < 3 ppm total. The irradiation chamber used for a-particle implantation was much simpler. A small aluminium vacuum chamber in which specimens are mounted on a water cooled copper sample holder was utilized. In order to produce a homogeneous implantation through the specimen thickness, a rotating beam energy degrader was used. Implantations were carried out at temperatures Q 50 o C. Subsequent fatigue testing was performed at elevated temperature using a Schenck-Trebel mechanical testing machine operated under closed loop conditions in a load controlled tension-tension mode. A triangular loading waveform was used with a frequency of 60 cpm. Crack growth was measured using the equipment for in-beam tests already described. A small environmental cell was constructed, enclosing the specimen and grips, to allow testing in an inert helium atmosphere to avoid undesirable effects due to crack tip reactions. Irradiations were performed at the Cyclotron Laboratory of Ispra using a Scanditronix MC-40 Variable Energy Cyclotron.
where u is the applied stress, a is the average crack length between two successive measurements and M’ is the specimen width. The average crack growth per cycle da/dN has been plotted against the stress intensity factor range AK, following a power law of the type [17]: da/dN
=
I”,
(2)
where c and n are constants. factor range, given by: AK=K,,,(~
AK is the stress intensity
-R).
where R is the load ratio. Crack growth rates at 500°C in 316 stainless steel specimens, pre-implanted with 400 appm and 800 appm of helium by 38 MeV a-particles, are presented in fig. 2. All tests have been carried out in inert helium atmosphere. Results of these tests, published elsewhere [12], have demonstrated that crack growth below about 10-s mm/cycle was generally slightly lower than that for unimplanted specimens tested under similar conditions, indicated by the solid line. With further increasing growth rates a gradual deviation from this line of the data for pre-implanted specimens has been measured. In fact, the pre-implanted specimens were more resistant to fatigue crack growth in the final stage, as shown in fig. 2. Up to now no other fatigue crack growth data have been reported on specimens implanted with a-particles.
10
-,
3. Results Crack growth rates have been calculated using the secant method recommended in ASTM Test Method E 647-83 [15]. The appropriate stress intensity factor K was obtained from [16]:
n
Arr= 180 MPa T = 500 “C
10 -5
102
10
AK (MN
m 3’2]
Fig. 2. Fatigue crack growth in 316 steel implanted with 400 appm helium (O), 800 appm (m) helium and in unimplanted material (solid line).
P. Fenici / Injiuence
of light ion irradiation on fatigue
However, some data do exist for specimens containing helium produced in reactor irradiation, with a maximum helium concentration of - 300 appm [18]. Tests were performed at 430 and 590” C and no helium bubble formation occurred in both cases. At 430 o C a small reduction in fatigue crack growth rates was measured, whilst at 590 o C an increase was observed. It was concluded that in this temperature range, the fatigue behaviour is controlled by the influence on dislocation motion of helium atoms in solution. The data reported in our work suggest that also at 5OO’C the effect of helium is to slightly reduce the fatigue crack growth rate. This is particularly evident at higher values of AK, as can be seen in fig. 2. The final failure of implanted specimens is delayed with respect to unimplanted specimens, presumably as a result of an increased ultimate tensile stress due to the presence of helium in solution. Moreover, low cycle fatigue experiments performed on 20% cold-worked 316 steel after irradiation in the HFIR reactor up to 15 dpa, producing up to 800 appm helium, have shown little effect on fatigue life at 550 a C [19]. Reverse bending fatigue tests in solution annealed Type 316 steel with up to 3000 appm of cyclotron implanted helium have demonstrated that, below 600 o C, only a small reduction in fatigue life occurs [8]. At higher temperatures a severe reduction in fatigue life has been observed as a result of helium bubble growth at grain boundaries. In the test under 18 MeV proton irradiation, the average irradiation temperature was kept at c 500’ C whilst the damage rate was of the order of - 6 x lo-’ The beam current homodpa s-t for all experiments. geneity in the horizontal axis was 55% on the entire width of the specimen. The load ratio R and the applied stress range Au were changed for the different experiments. The frequency was generally 60 cpm. Crack growth rates are generally lower than those for unirradiated specimens tested under similar conditions, indicated always by the solid line in the figure. In fig. 3 a typical result is presented: crack growth propagations below about 1O-3 mm/cycle are generally slightly lower than those obtained for unirradiated material. With further increasing growth rates, a gradual deviation of the data has been observed for the irradiated specimen. Increasing the applied stress range Au to more than 200 MPa has only a slight effect: crack growth rates were still lower than those measured for unirradiated material (fig. 3). Decreasing the applied stress range Au to 150 MPa resulted in a further decrease in crack growth rates. The effect of varying the load ratio R on crack growth rates has been studied in unirradiated material [13] and it was difficult to discern any significant influence. This was true also in the present case under proton irradiation. An effect is noticeable when varying the applied stress range Au, at comparable values of irradiation temperature and damage rate. It is worth noticing that in specimens irradiated with 18 MeV
crack propagation
965
AK [MN m “2] Fig. 3. Influence of irradiation on crack growth rate in type 316 steel at 490 o C, with Au = 184 MPa, R = 0.06 (0) and with Au = 202 MPa, R = 0.09 (A). protons at 500 o C and at a damage rate of 6.4 x lo-’ dpa s-’ with an applied stress range of 140 MPa, after 6 days of continuous irradiation no crack initiation has occurred. The data reported in the present work suggest that at 500° C the irradiation hardening has a slight influence on fatigue crack propagation already at low fluences ( < 1 dpa). The test have demonstrated the ability of the fatigue cyclic loading apparatus and of the optical crack length measurements system to function under irradiation. No data have as yet been published of fatigue crack growth under light ion irradiation. Recently a fatigue crack growth experiment carried out under irradiation in the Oak Ridge Research Reactor has been described [6]. Preliminary results for low fluences have indicated no effect of dynamic irradiation on crack growth rate in 20% cold-worked 316 stainless steel at 460°C. Results of post-irradiation tests are equivocal as to the effect on fatigue behaviour. Work on austenitic stainless steels [20-231 has shown that the effect of irradiation on fatigue crack propagation is dependent on test temperature, fluence, energy spectrum and stress intensity factor range. In general, higher temperature and fluence tended to promote increased crack growth rates whilst lower temperature and fluence tended to results in a decrease. The data reported here on in-beam tests seem to suggest the same behaviour. Insufficient crack initiation data exist to draw any firm conclusions, but the tendency was for rather slower initiation with irradiated and implanted specimens [12]. Evidence of this effect may be seen in fig. 4 in the crack
P. Fenicr / Influence of light ion irradration on fatigue crack propagation
966
have shown a very strong, detrimental influence of helium on fatigue life. The difference in behaviour is due to the lack of helium bubble formation at grain boundaries below 600 o C. Relatively high damage rates of the order of 10 ’ dpa s-r, with ratios of displacement damage to helium production rate realistic for fusion reactor materials, have been achieved by irradiation with 18 MeV protons, In-beam proton irradiation has a small effect on resistance to crack growth of Type 316 stainless steel at 500 o C. The present results tend to suggest that irradiation hardening slightly influences the fatigue crack growth already at low fluences (< 1 dpa). The proton irradiation data are in agreement also with the preliminary results obtained with low fluence neutron irradiations.
Impian tee
; I I I Irradiated : : : I1 I
Unirfadiated
The assistance of Mr. J. de Greeff and of the cyclotron laboratory staff is gratefully acknowledged. 1
0
20
1
I
I
60
80
References
Fig. 4. Fatigue crack growth curves of 316 stainless steel at - 500 ’ C showing slower growth and longer life due to irradiation.
Ul B.A. Cramer and J.W. Davis, Nucl. Engrg. Des. 58 (1980)
growth curves which show that fatigue life was shorter for the unirradiated 316 steel. It appears that about 20 or 30% of the crack extension occurred in the first 80% of the total cycles. On examining the corresponding crack growth rate data, shown in figs. 2 and 3, the effect on fatigue life is not readily evident.
Technology, Varese, Italy, 1984 (Pergamon Press, New York, 1984) p. 77. [51 P. Jung, Chr. Swaiger and H. Ullmaier, J. Nucl. Mater. 85 & 86 (1979) 867. [61 A.M. Ermi and B.A. Chin, J. Nucl. Mater. 103 & 104 (1981) 1505. [71 W.F. Sommer, D.S. Phillips, W.V. Green, L.W. Hobbs and C.A. Wert, J. Nucl. Mater. 114 (1983) 267. VI K. Sonnenberg, G. Antesberger and B. Brown, J. Nucl. Mater. 102 (1981) 333. [91 D. Kaletta, J. Nucl. Mater. 133 & 134 (1985) 878. VOI H. Mizubayashi, S. Okuda, K. Nakagome, H. Shibuki and S. Seki, Trans. Jpn Inst. Met. 25 (1984) 257. [Ill D.G. Rickerby and P. Fenici, J. Nucl. Mater. 103 & 104 (1981) 1577. WI P. Fenici and D.G. Kickerby, in: Proc. Int. Conf. and Exposition on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Analysis, Salt Lake City, USA (1985) Ed. V.S. GoeI (ASM, 1986) Vol. on Fatigue Life, p. 371. [I31 D.G. Rickerby and P. Fenici, Eng. Fract. Mech. 19 (1984) 585. 1141 D.G. Rickerby, P. Fenici, P. Jung, G. Piatti and P. Schiller, The Use of Small-Scale Specimens for Testing Irradiated Materials, STP 888, Eds. W.R. Corwin and G.E. Lucas (ASTM, Philadelphia, 1986) p. 220. [I51 Annual Book of ASTM Standards, Vol. 03.01 (American Society for Testing and Materials, Philadelphia, 1983) p. 710. [I61 G.C. Sib, Handbook of Stress Intensity Factors, Vol. 1 (Lehigh University, Bethlehem, PA, 1975) p. 1.3.2-1. U71 P. Paris and F. Erdogan, Trans. ASME J. Basic Eng. 85 (1963) 528.
4. Conclusions Fatigue crack growth appears to be a limiting factor for the life-time of the first wall of a tokamak type fusion reactor. Since post-irradiation experiments do not adequately simulate possible interaction effects between fatigue and radiation damage, a considerable advantage is obtained if fatigue tests are conducted directly under irradiation. A suitable instrumentation has been developed to perform fatigue crack growth measurements in beam and after a-particle implantation using a variable energy cyclotron. This equipment, capable of fully remote operation for experiments made under irradiation, has demonstrated the ability of the whole system to function under irradiation. To study the effect of helium on mechanical properties in the presence of relatively low displacement damage, u-particle implantation experiments have been carried out in thin miniaturized specimens. Using this technique it was found that helium concentrations of up to 800 appm had only a slight influence on fatigue crack growth at 500°C in Type 316 stainless steel. This is in contrast with experiments at higher temperatures which
267. 3 (1983) 149. VI J.H. Pitts, Nucl. Technol./Fusion 131 H. Ullmaier, Trans. Indian Inst. Met. 34 (1981) 324. [41 H. Ullmaier, in: Proc. of the 13th Symp. on Fusion
P. Fenici / Influence of light ion irradiation on fatigue crack propagation
[18] D.J. Michel, J. Nucl. Mater. 120 (1984) 113. [19] M.L. Grossbeck and K.C. Lih, J. Nucl. Mater. 103 8r 104 (1981) 853. [20] P. Shahinian, H.E. Watson and H.H. Smith, in: Effects of Radiation in Substructure and Mechanical Properties of Metals and Alloys, STP 529 (American Society for Testing and Materials, Philadelphia, 1973) p. 493.
961
[21] P. Shahinian, in: Properties of Reactor Structural Alloys after Neutron or Particle Irradiation, SPT 570 (American Society for Testing and Materials, Philadelphia, 1975) p. 191. (221 G.J. Lloyd, J. Nucl. Mater. 110 (1982) 20. [23] G.J. Lloyd, J.D. Walls and J. Gravenor, J. Nucl. Mater. 110 (1982) 115.
of Nuclear
North-Holland,
Materials
155-157
(1988) 963-967
963
Amsterdam
INFLUENCE OF LIGHT ION IRRADIATION ON FATIGUE CRACK PROPAGATION IN AUSTENITIC STAINLESS STEEL P. FENICI Commission of the European Communities, 21020 Ispra (Va), Italy
Joint Research
Centre, Ispra Establishment,
Materials
Science Diuision,
Fatigue crack growth is considered to be a limiting factor for the life time of the first wall of a Tokamak type controlled thermonuclear reactor. As a result of the high energy neutron spectrum in the D-T fusion reaction not only defects but also large quantities of helium due to nuclear transmutation are produced in the structural materials. One way of simulating the
damage of high energy neutrons is to use light ions by means of a charged particle accelerator. In the present work Type 316 stainless steel specimens were irradiated in a suitable irradiation chamber using a variable energy cyclotron. Fatigue crack growth under cyclic tensile stress was measured under simultaneous 18 MeV proton irradiation producing a displacement damage rate of the order of 1OK’ dpa s -‘. The same type of specimens were implanted with 38 MeV a-particles and tested after bombardement. It was found that light ions irradiation has only a slight influence on fatigue crack growth
at 500
’ C in type 316 stainless steel.
1. Introduction
Studies of reactor design concepts have shown that fatigue crack growth initiating from pre-existing mechanical flaws is one of the major parameters to be considered when evaluating the endurance life of the first wall of tokamak reactors [l] and the reaction chamber of inertial confinement systems [2]. The understanding of the interaction of fatigue crack growth and radiation damage is then necessary for an accurate estimation of first wall operating lifetimes. Moreover, to assess the maximum allowable crack length in a structure, reliable fatigue crack growth data under appropriate reactor operation conditions are needed. The anticipated operating conditions for a fusion reactor first wall vary according to reactor design. In general neutron fluxes of - 10’7-10’s n mm2 s-’ of 14 MeV are to be expected. The corresponding displacement damage rates for austenitic stainless steels are estimated to be 10~7-10-6 dpa s-l. Adequate simulation of neutron irradiation is obtained by light ions produced by a particle accelerator. Previous studies have shown that protons and deuterons can generate comparable recoil energy spectra together with realistic ratios of helium production to displacement damage [3-51. The effect of helium produced by transmutation reaction is studied by a-particle irradiation experiments. The relatively short range of these particles in metals and alloys and the difficulties of extracting the thermal energy deposited in the specimen, require the use of thin specimens (= 200 pm). The uncertainties associated with post-irradiation experiments, in particular the fact that tests may not always adequately represent the synergistic effects associated with irradiation under cyclic stressing, have lead to attempts to perform experiments directly under irradiation [6-111. No data have as yet been published on fatigue crack growth under light ion irradiation. 0022-3115/88/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
In the present work we will discuss the influence of light ions irradiation on fatigue crack propagation in Type 316 stainless steel. Thin miniaturized specimens were irradiated in a suitable irradiation chamber [ll] using a variable energy cyclotron. Fatigue crack growth under cyclic tensile stress was measured under simultaneous 18 MeV proton irradiation producing a displacement damage rate of the order of lo-’ dpa s-l. The same type of specimens were implanted with 38 MeV a-particles to obtain up to 800 appm helium. Tests after implantation were conducted under an inert atmosphere using a conventional mechanical testing machine [12]. Thin miniaturized specimens developed for these experiments, have been extensively studied, demonstrating the validtiy and the reproducibility of the data obtained with this specimen geometry on unirradiated material at room temperature [13,14] and at elevated temperature [12]. 2. Experimental The specimens used for irradiation studies (fig. 1) were of the single edge notch (SEN) type. Specimens were milled with longitudinal axes parallel to the rolling direction from solution annealed sheets of Type 316 stainless steel with the following nominal composition (wt%): 0.08 C, 0.006 S, 0.6 Si, 12.0 Ni, 17.0 Cr, 0.2 Cu, 1.5 Mn, 2.5 MO, balance Fe. The fatigue crack notches
Fig. 1. Specimen
geometry
(dimensions
in mm).
964
P. Fenicr / Influence of lighr ion irradiatron on fatigue truck propagarion
were formed by spark erosion, and were 0.1 mm wide, with end radii of 0.05 mm, and either 1 or 2 mm in length. The sheets were 0.15 mm thick, with an average grain diameter of - 0.01 mm. In-beam testing was carried out using a specially designed irradiation facility [ll]. Specimen temperature was monitored by an infrared pyrometer, with a spot diameter of 0.8 mm. Crack growth was measured by means of a remotely controlled long focal length optical microscope, with a magnification of 40 X , equipped with a video camera and recorder. Measurements of crack length were made on a high resolution television screen during experiments, using a videomicrometer at intervals of 0.20-0.25 mm with a precision of kO.02 mm. The specimen was at an angle of 15” to the beam axis in order to allow direct observation with the pyrometer and microscope through quarz windows. The loading waveform was approximately triangular, with a frequency generally of 60 cpm. During irradiation both specimen surfaces were continuously cooled by two opposing high velocity helium jets. The helium, generally at atmospheric pressure, was supplied by a closed purification system giving < 0.1 ppm oxygen and < 1 ppm each of the residual impurities up to a maximum of < 3 ppm total. The irradiation chamber used for a-particle implantation was much simpler. A small aluminium vacuum chamber in which specimens are mounted on a water cooled copper sample holder was utilized. In order to produce a homogeneous implantation through the specimen thickness, a rotating beam energy degrader was used. Implantations were carried out at temperatures Q 50 o C. Subsequent fatigue testing was performed at elevated temperature using a Schenck-Trebel mechanical testing machine operated under closed loop conditions in a load controlled tension-tension mode. A triangular loading waveform was used with a frequency of 60 cpm. Crack growth was measured using the equipment for in-beam tests already described. A small environmental cell was constructed, enclosing the specimen and grips, to allow testing in an inert helium atmosphere to avoid undesirable effects due to crack tip reactions. Irradiations were performed at the Cyclotron Laboratory of Ispra using a Scanditronix MC-40 Variable Energy Cyclotron.
where u is the applied stress, a is the average crack length between two successive measurements and M’ is the specimen width. The average crack growth per cycle da/dN has been plotted against the stress intensity factor range AK, following a power law of the type [17]: da/dN
=
I”,
(2)
where c and n are constants. factor range, given by: AK=K,,,(~
AK is the stress intensity
-R).
where R is the load ratio. Crack growth rates at 500°C in 316 stainless steel specimens, pre-implanted with 400 appm and 800 appm of helium by 38 MeV a-particles, are presented in fig. 2. All tests have been carried out in inert helium atmosphere. Results of these tests, published elsewhere [12], have demonstrated that crack growth below about 10-s mm/cycle was generally slightly lower than that for unimplanted specimens tested under similar conditions, indicated by the solid line. With further increasing growth rates a gradual deviation from this line of the data for pre-implanted specimens has been measured. In fact, the pre-implanted specimens were more resistant to fatigue crack growth in the final stage, as shown in fig. 2. Up to now no other fatigue crack growth data have been reported on specimens implanted with a-particles.
10
-,
3. Results Crack growth rates have been calculated using the secant method recommended in ASTM Test Method E 647-83 [15]. The appropriate stress intensity factor K was obtained from [16]:
n
Arr= 180 MPa T = 500 “C
10 -5
102
10
AK (MN
m 3’2]
Fig. 2. Fatigue crack growth in 316 steel implanted with 400 appm helium (O), 800 appm (m) helium and in unimplanted material (solid line).
P. Fenici / Injiuence
of light ion irradiation on fatigue
However, some data do exist for specimens containing helium produced in reactor irradiation, with a maximum helium concentration of - 300 appm [18]. Tests were performed at 430 and 590” C and no helium bubble formation occurred in both cases. At 430 o C a small reduction in fatigue crack growth rates was measured, whilst at 590 o C an increase was observed. It was concluded that in this temperature range, the fatigue behaviour is controlled by the influence on dislocation motion of helium atoms in solution. The data reported in our work suggest that also at 5OO’C the effect of helium is to slightly reduce the fatigue crack growth rate. This is particularly evident at higher values of AK, as can be seen in fig. 2. The final failure of implanted specimens is delayed with respect to unimplanted specimens, presumably as a result of an increased ultimate tensile stress due to the presence of helium in solution. Moreover, low cycle fatigue experiments performed on 20% cold-worked 316 steel after irradiation in the HFIR reactor up to 15 dpa, producing up to 800 appm helium, have shown little effect on fatigue life at 550 a C [19]. Reverse bending fatigue tests in solution annealed Type 316 steel with up to 3000 appm of cyclotron implanted helium have demonstrated that, below 600 o C, only a small reduction in fatigue life occurs [8]. At higher temperatures a severe reduction in fatigue life has been observed as a result of helium bubble growth at grain boundaries. In the test under 18 MeV proton irradiation, the average irradiation temperature was kept at c 500’ C whilst the damage rate was of the order of - 6 x lo-’ The beam current homodpa s-t for all experiments. geneity in the horizontal axis was 55% on the entire width of the specimen. The load ratio R and the applied stress range Au were changed for the different experiments. The frequency was generally 60 cpm. Crack growth rates are generally lower than those for unirradiated specimens tested under similar conditions, indicated always by the solid line in the figure. In fig. 3 a typical result is presented: crack growth propagations below about 1O-3 mm/cycle are generally slightly lower than those obtained for unirradiated material. With further increasing growth rates, a gradual deviation of the data has been observed for the irradiated specimen. Increasing the applied stress range Au to more than 200 MPa has only a slight effect: crack growth rates were still lower than those measured for unirradiated material (fig. 3). Decreasing the applied stress range Au to 150 MPa resulted in a further decrease in crack growth rates. The effect of varying the load ratio R on crack growth rates has been studied in unirradiated material [13] and it was difficult to discern any significant influence. This was true also in the present case under proton irradiation. An effect is noticeable when varying the applied stress range Au, at comparable values of irradiation temperature and damage rate. It is worth noticing that in specimens irradiated with 18 MeV
crack propagation
965
AK [MN m “2] Fig. 3. Influence of irradiation on crack growth rate in type 316 steel at 490 o C, with Au = 184 MPa, R = 0.06 (0) and with Au = 202 MPa, R = 0.09 (A). protons at 500 o C and at a damage rate of 6.4 x lo-’ dpa s-’ with an applied stress range of 140 MPa, after 6 days of continuous irradiation no crack initiation has occurred. The data reported in the present work suggest that at 500° C the irradiation hardening has a slight influence on fatigue crack propagation already at low fluences ( < 1 dpa). The test have demonstrated the ability of the fatigue cyclic loading apparatus and of the optical crack length measurements system to function under irradiation. No data have as yet been published of fatigue crack growth under light ion irradiation. Recently a fatigue crack growth experiment carried out under irradiation in the Oak Ridge Research Reactor has been described [6]. Preliminary results for low fluences have indicated no effect of dynamic irradiation on crack growth rate in 20% cold-worked 316 stainless steel at 460°C. Results of post-irradiation tests are equivocal as to the effect on fatigue behaviour. Work on austenitic stainless steels [20-231 has shown that the effect of irradiation on fatigue crack propagation is dependent on test temperature, fluence, energy spectrum and stress intensity factor range. In general, higher temperature and fluence tended to promote increased crack growth rates whilst lower temperature and fluence tended to results in a decrease. The data reported here on in-beam tests seem to suggest the same behaviour. Insufficient crack initiation data exist to draw any firm conclusions, but the tendency was for rather slower initiation with irradiated and implanted specimens [12]. Evidence of this effect may be seen in fig. 4 in the crack
P. Fenicr / Influence of light ion irradration on fatigue crack propagation
966
have shown a very strong, detrimental influence of helium on fatigue life. The difference in behaviour is due to the lack of helium bubble formation at grain boundaries below 600 o C. Relatively high damage rates of the order of 10 ’ dpa s-r, with ratios of displacement damage to helium production rate realistic for fusion reactor materials, have been achieved by irradiation with 18 MeV protons, In-beam proton irradiation has a small effect on resistance to crack growth of Type 316 stainless steel at 500 o C. The present results tend to suggest that irradiation hardening slightly influences the fatigue crack growth already at low fluences (< 1 dpa). The proton irradiation data are in agreement also with the preliminary results obtained with low fluence neutron irradiations.
Impian tee
; I I I Irradiated : : : I1 I
Unirfadiated
The assistance of Mr. J. de Greeff and of the cyclotron laboratory staff is gratefully acknowledged. 1
0
20
1
I
I
60
80
References
Fig. 4. Fatigue crack growth curves of 316 stainless steel at - 500 ’ C showing slower growth and longer life due to irradiation.
Ul B.A. Cramer and J.W. Davis, Nucl. Engrg. Des. 58 (1980)
growth curves which show that fatigue life was shorter for the unirradiated 316 steel. It appears that about 20 or 30% of the crack extension occurred in the first 80% of the total cycles. On examining the corresponding crack growth rate data, shown in figs. 2 and 3, the effect on fatigue life is not readily evident.
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4. Conclusions Fatigue crack growth appears to be a limiting factor for the life-time of the first wall of a tokamak type fusion reactor. Since post-irradiation experiments do not adequately simulate possible interaction effects between fatigue and radiation damage, a considerable advantage is obtained if fatigue tests are conducted directly under irradiation. A suitable instrumentation has been developed to perform fatigue crack growth measurements in beam and after a-particle implantation using a variable energy cyclotron. This equipment, capable of fully remote operation for experiments made under irradiation, has demonstrated the ability of the whole system to function under irradiation. To study the effect of helium on mechanical properties in the presence of relatively low displacement damage, u-particle implantation experiments have been carried out in thin miniaturized specimens. Using this technique it was found that helium concentrations of up to 800 appm had only a slight influence on fatigue crack growth at 500°C in Type 316 stainless steel. This is in contrast with experiments at higher temperatures which
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P. Fenici / Influence of light ion irradiation on fatigue crack propagation
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