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Development of miniature bending fatigue specimens
43X
Journal of Nuclear Materials 179-181 (1991) 43X-440 North-Holland
Development
of miniature
G.R. Rao ‘, A. Rowcliffe
bending
fatigue specimens
* and B.A. Chin 1
1 Materials Engmeering, Auburn University, Auburn, AL 36830, USA .’ Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA
Two new miniaturized bending fatigue specimens have been designed and developed to aid in the scoping of materials for fusion first-wall and blanket structural applications. One of these is rectangular in shape with a gauge section 6.35 mm in length, while the other is cut from a 3 mm transmission electron microscopy (TEM) disk and has a gauge length of 1.5 mm. Test results for unirradiated annealed type 316 stainless steel tested at room temperature, 550°C and 650°C are presented. A good correlation between miniature and standard subsize fatigue specimen results was obtained. The miniature specimen results show the same dependence of strain range on cycles to failure as the standard subsize specimens with the miniature-disk specimen results falling below all the other results. The results indicate that these specimens provide reliable data that can be used to scope fatigue properties for fusion applications.
1. Introduction Small-scale specimens have been used to test irradiated materials to overcome size constraints within the reactor, fluence gradients and gamma heating, and to reduce potential exposure hazards to personnel. The fusion program has provided a further impetus for the development of such specimens. Currently, the study of the effects of high energy neutron bombardment requires accelerator based irradiation. Small-scale, lowvolume specimens are an attractive means of obtaining useful properties data from candidate first-wall materials, since irradiation volume is quite limited in such sources. Fatigue is an important factor in fusion reactor design since most Tokamak fusion reactors will operate in a cyclic mode. Moreover, high energy neutron bombardment will be involved. It is necessary to have a detailed knowledge of the simultaneous effects of both factors. The purpose of this study was to develop two miniature bending fatigue specimens to test the resistance to cyclic thermal stresses which would be experienced by the first wall and blanket materials in Tokamak fusion reactors. 2. Experimental procedure The first specimen is termed the “rectangular specimen”, and is shown in fig. 1. It has overall dimensions of 30.1625 x 4.7625 x 0.762 mm with a gauge length of 6.35 mm. The second specimen, termed the “miniaturedisk” specimen, shown in fig. 2, has an overall diameter of 3 mm and is cut from a standard TEM disk. The gauge section is formed by two circular radii of 1.5 mm with a minimum width of 1 mm and a thickness of 0.254 mm. The rectangular specimens were fabricated by rnilling. The disk specimens were obtained by electrical discharge machining (EDM). Both were vacuum an-
I,
0.762
I
I
t
f Fig. 1. The rectangular specimen. Dimensions are in mm.
nealed at 1050°C for one hour prior to electropolishing using a 95% ethanol, 5% perchloric acid electrolyte at 0°C. All specimens were made from type 316 stainless steel (reference heat 8092297) obtained from Oak Ridge National Laboratory. Fatigue tests using the two specimens were performed on a cantilever flexural fatigue machine. A Wheatstone Bridge network of four strain gauges attached at the end of the cantilevered specimen holder forms the load cell and is calibrated to monitor the bending load. The test is terminated when the load decays to a specified threshold level due to crack initiation and growth. Further details on the machine can be found in ref. [3]. 6.3
0.254 Fig. 2. The miniature-disk specimen. Dimensions are in mm.
0022-3115/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
ofminiaturebending fatigue
G.R Rao et al. / Development
439
specimens
3. Results and discussion The fatigue data were plotted as total strain range versus cycles to failure on logarithmic axes. Strain values were determined by measuring the load acting at the cantilever beam free end and subsequently calculating the corresponding strain ranges at the position of maximum stress in the gauge area of the specimens [3]. Fig. 3 shows the data obtained for the rectangular and miniature-disk specimens at room temperature, 550°C and 650°C. Data points for the miniature-disk specimens fell below those for the rectangular specimens. The room temperature results were analyzed using a Coffin-Manson power law of the form: de, = AN;*,
-t BN;$.
(2)
The difference between the 55O’C and 650°C results appears to be vague and there is a fair amount of scatter about the best-fit curve. At the present time, strain ranges for the miniature-disk specimens have been calculated assuming the specimen to be a regular cantilever beam. End effect corrections have not been taken into consideration, which could slightly modify the current strain-range values. The rectangular specimen results show a similar trend. The room temperature data fit a power law equation similar to eq. (1). The 550°C and 650°C data follow eq. (2) indicated above. However, unlike the elevated temperature results for the disk specimen, there is an obvious degradation of fatigue life at elevated
‘~~,~,J’
tc?
’
~ll,cl’
ll,ll.
10’
105
Fig. 4. Comparison
d
10'
Fig. 3. Rectangular
10'
id rti CYCLES TO FAILURE
and miniature-disk
specimen
results.
10’
lllllt’
’
10”
of room temperature
clllll’
10’
j
“‘a
10’
results.
temperatures for the rectangular specimen. A distinction can be made between the 550°C and 650°C results with the 650°C data falling below the 550°C data and both lying below the room temperature results. This is hypothesized to be caused by a greater surface area in the vicinity of the crack for environmental attack for the rectangular specimen, in spite of the argon atmosphere. The room temperature data for both specimens are compared with other room temperature results in fig. 4 obtained for type 316 stainless steel. The SS-1 specimens were tested on the fatigue test machine used in this study. Nagata et al. used axially fatigued hourglass specimens loaded with a cyclic triangular waveform [4]. The data attributed to Boiler were obtained from ref. 151. The values of the power law exponents and constants are given in table 1, indicating a similar trend for the specimens. The elevated temperature results for the rectangular and miniature disk specimens are compared with results obtained by Liu and Grossbeck at ORNL using subsize hourglass fatigue specimens made of 20% cold-worked
Table 1 Values of the power law exponents
,oj4
’
CYCLES TO FAILURE
(1)
where Ar, is the total strain range, N, is the number of failure cycles, and A and (Yare constants. The elevated temperature data for the disk specimens closely follow the room temperature data. However, the data points were found to fit a power law equation of the type: AC, = AN;”
lo4 10’
and constants
Data
Temp.
A
a
B
B
Rectangular Disk S-1 Nagata et al. Boiler Grossbeck Grossbeck Rectangular Rectangular Disk
room room room room room 550°c 650°C 550°C 65O’C 550°C & 650°C
0.0542 0.80 0.076 0.30 0.2143 0.016 0.014 0.029 0.016 0.295
0.15 0.18 0.18 0.40 0.12 0.12 0.12 0.12 0.12 0.12
-0.0418 0.66 0.54 - 0.098 0.78 6.19
_ _ 0.5 0.5 0.5 0.5 0.5 0.5
G.R. Rao et al. / Development
of mmuzture
bending
fatque specrmens
4. Conclusions
I
,
/ ,,,,,,,
30'
I
,
,,,,,,l
106
,111111
100
,
a
,‘(
10’
CYCLES TO FAILURE
Fig. 5. Comparison
of elevated
temperature
The following conclusions resulted from this study: (1) Two new small-sized bending fatigue specimens were successfully developed and tested using type 316 stainless steel. A fatigue test machine was also developed to test the specimens. (2) The rectangular specimen generated good results which correlate favorably with data obtained using other specimens made of type 316 stainless steel. The miniature-disk specimen data follow the same trend but the data points consistently fell below the other results. (3) The difference in results obtained from the two designs, in spite of using the same material in the same condition, can be explained in terms of geometry and size.
results.
Acknowledgements type 316 stainless steel [6,7] in fig. 5. The rectangular and disk specimen data fell below those obtained by Liu and Grossbeck but follow the same trend. The values for the material constants are shown in table 1. The correlation of results is evident for all results. The difference in the behavior at room and elevated temperatures can be explained in terms of the elastic and plastic components of the total strain range. The elastic component becomes significant at elevated temperatures leading to the observed nature of the curve. Both specimens have yielded reliable results and are suitable for scoping fatigue properties of materials. The miniature-disk specimen results are very promising and are self-contained within their data set. The advantages of the miniature-disk specimen are evident. Microstructural examination subsequent to testing is made simple since the specimen itself can be thinned. The small volume of each specimen is advantageous since a greater number of specimens can be irradiated and tested, yielding a better understanding of material behavior.
This work was supported by the Department of Energy, Office of Fusion Energy, under grant number DEFGO585ER52139C. References 111 W.R.
Corwin and G.E. Lucas, The Use of Small-Scale Specimens for Testing Irradiated Material, ASTM-STP 888 (1986) pp. 1-2. VI G.E. Lucas, J. Nucl. Mater. 117 (1983) pp. 327-339. [31G.R. Rao, M.S. Thesis, Auburn University, 1988, pp. 23-38. J. Nucl. Mater. [41N. Nagata, M. Furuya and R. Watanabe, 85 & 86 (1979) p. 839. [51C. Boller and T. Seeger, Materials Data for Cyclic Loading, Part C: High Alloy Steels, Material Science Monographs (Elsevier, Amsterdam 1987) p. 155. Use of Subsize Fatigue [61 K.C. Liu and M.L. Grossbeck, Specimens for Reactor Irradiation Testing, ASTM-STP 888 (1986) pp. 276-289. Progress [71 M.L. Grossbeck and K.C. Liu, ADIP Semiannual Report, DOE/ER-0045/8 (March 1982) pp. 1366140.
Journal of Nuclear Materials 179-181 (1991) 43X-440 North-Holland
Development
of miniature
G.R. Rao ‘, A. Rowcliffe
bending
fatigue specimens
* and B.A. Chin 1
1 Materials Engmeering, Auburn University, Auburn, AL 36830, USA .’ Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA
Two new miniaturized bending fatigue specimens have been designed and developed to aid in the scoping of materials for fusion first-wall and blanket structural applications. One of these is rectangular in shape with a gauge section 6.35 mm in length, while the other is cut from a 3 mm transmission electron microscopy (TEM) disk and has a gauge length of 1.5 mm. Test results for unirradiated annealed type 316 stainless steel tested at room temperature, 550°C and 650°C are presented. A good correlation between miniature and standard subsize fatigue specimen results was obtained. The miniature specimen results show the same dependence of strain range on cycles to failure as the standard subsize specimens with the miniature-disk specimen results falling below all the other results. The results indicate that these specimens provide reliable data that can be used to scope fatigue properties for fusion applications.
1. Introduction Small-scale specimens have been used to test irradiated materials to overcome size constraints within the reactor, fluence gradients and gamma heating, and to reduce potential exposure hazards to personnel. The fusion program has provided a further impetus for the development of such specimens. Currently, the study of the effects of high energy neutron bombardment requires accelerator based irradiation. Small-scale, lowvolume specimens are an attractive means of obtaining useful properties data from candidate first-wall materials, since irradiation volume is quite limited in such sources. Fatigue is an important factor in fusion reactor design since most Tokamak fusion reactors will operate in a cyclic mode. Moreover, high energy neutron bombardment will be involved. It is necessary to have a detailed knowledge of the simultaneous effects of both factors. The purpose of this study was to develop two miniature bending fatigue specimens to test the resistance to cyclic thermal stresses which would be experienced by the first wall and blanket materials in Tokamak fusion reactors. 2. Experimental procedure The first specimen is termed the “rectangular specimen”, and is shown in fig. 1. It has overall dimensions of 30.1625 x 4.7625 x 0.762 mm with a gauge length of 6.35 mm. The second specimen, termed the “miniaturedisk” specimen, shown in fig. 2, has an overall diameter of 3 mm and is cut from a standard TEM disk. The gauge section is formed by two circular radii of 1.5 mm with a minimum width of 1 mm and a thickness of 0.254 mm. The rectangular specimens were fabricated by rnilling. The disk specimens were obtained by electrical discharge machining (EDM). Both were vacuum an-
I,
0.762
I
I
t
f Fig. 1. The rectangular specimen. Dimensions are in mm.
nealed at 1050°C for one hour prior to electropolishing using a 95% ethanol, 5% perchloric acid electrolyte at 0°C. All specimens were made from type 316 stainless steel (reference heat 8092297) obtained from Oak Ridge National Laboratory. Fatigue tests using the two specimens were performed on a cantilever flexural fatigue machine. A Wheatstone Bridge network of four strain gauges attached at the end of the cantilevered specimen holder forms the load cell and is calibrated to monitor the bending load. The test is terminated when the load decays to a specified threshold level due to crack initiation and growth. Further details on the machine can be found in ref. [3]. 6.3
0.254 Fig. 2. The miniature-disk specimen. Dimensions are in mm.
0022-3115/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
ofminiaturebending fatigue
G.R Rao et al. / Development
439
specimens
3. Results and discussion The fatigue data were plotted as total strain range versus cycles to failure on logarithmic axes. Strain values were determined by measuring the load acting at the cantilever beam free end and subsequently calculating the corresponding strain ranges at the position of maximum stress in the gauge area of the specimens [3]. Fig. 3 shows the data obtained for the rectangular and miniature-disk specimens at room temperature, 550°C and 650°C. Data points for the miniature-disk specimens fell below those for the rectangular specimens. The room temperature results were analyzed using a Coffin-Manson power law of the form: de, = AN;*,
-t BN;$.
(2)
The difference between the 55O’C and 650°C results appears to be vague and there is a fair amount of scatter about the best-fit curve. At the present time, strain ranges for the miniature-disk specimens have been calculated assuming the specimen to be a regular cantilever beam. End effect corrections have not been taken into consideration, which could slightly modify the current strain-range values. The rectangular specimen results show a similar trend. The room temperature data fit a power law equation similar to eq. (1). The 550°C and 650°C data follow eq. (2) indicated above. However, unlike the elevated temperature results for the disk specimen, there is an obvious degradation of fatigue life at elevated
‘~~,~,J’
tc?
’
~ll,cl’
ll,ll.
10’
105
Fig. 4. Comparison
d
10'
Fig. 3. Rectangular
10'
id rti CYCLES TO FAILURE
and miniature-disk
specimen
results.
10’
lllllt’
’
10”
of room temperature
clllll’
10’
j
“‘a
10’
results.
temperatures for the rectangular specimen. A distinction can be made between the 550°C and 650°C results with the 650°C data falling below the 550°C data and both lying below the room temperature results. This is hypothesized to be caused by a greater surface area in the vicinity of the crack for environmental attack for the rectangular specimen, in spite of the argon atmosphere. The room temperature data for both specimens are compared with other room temperature results in fig. 4 obtained for type 316 stainless steel. The SS-1 specimens were tested on the fatigue test machine used in this study. Nagata et al. used axially fatigued hourglass specimens loaded with a cyclic triangular waveform [4]. The data attributed to Boiler were obtained from ref. 151. The values of the power law exponents and constants are given in table 1, indicating a similar trend for the specimens. The elevated temperature results for the rectangular and miniature disk specimens are compared with results obtained by Liu and Grossbeck at ORNL using subsize hourglass fatigue specimens made of 20% cold-worked
Table 1 Values of the power law exponents
,oj4
’
CYCLES TO FAILURE
(1)
where Ar, is the total strain range, N, is the number of failure cycles, and A and (Yare constants. The elevated temperature data for the disk specimens closely follow the room temperature data. However, the data points were found to fit a power law equation of the type: AC, = AN;”
lo4 10’
and constants
Data
Temp.
A
a
B
B
Rectangular Disk S-1 Nagata et al. Boiler Grossbeck Grossbeck Rectangular Rectangular Disk
room room room room room 550°c 650°C 550°C 65O’C 550°C & 650°C
0.0542 0.80 0.076 0.30 0.2143 0.016 0.014 0.029 0.016 0.295
0.15 0.18 0.18 0.40 0.12 0.12 0.12 0.12 0.12 0.12
-0.0418 0.66 0.54 - 0.098 0.78 6.19
_ _ 0.5 0.5 0.5 0.5 0.5 0.5
G.R. Rao et al. / Development
of mmuzture
bending
fatque specrmens
4. Conclusions
I
,
/ ,,,,,,,
30'
I
,
,,,,,,l
106
,111111
100
,
a
,‘(
10’
CYCLES TO FAILURE
Fig. 5. Comparison
of elevated
temperature
The following conclusions resulted from this study: (1) Two new small-sized bending fatigue specimens were successfully developed and tested using type 316 stainless steel. A fatigue test machine was also developed to test the specimens. (2) The rectangular specimen generated good results which correlate favorably with data obtained using other specimens made of type 316 stainless steel. The miniature-disk specimen data follow the same trend but the data points consistently fell below the other results. (3) The difference in results obtained from the two designs, in spite of using the same material in the same condition, can be explained in terms of geometry and size.
results.
Acknowledgements type 316 stainless steel [6,7] in fig. 5. The rectangular and disk specimen data fell below those obtained by Liu and Grossbeck but follow the same trend. The values for the material constants are shown in table 1. The correlation of results is evident for all results. The difference in the behavior at room and elevated temperatures can be explained in terms of the elastic and plastic components of the total strain range. The elastic component becomes significant at elevated temperatures leading to the observed nature of the curve. Both specimens have yielded reliable results and are suitable for scoping fatigue properties of materials. The miniature-disk specimen results are very promising and are self-contained within their data set. The advantages of the miniature-disk specimen are evident. Microstructural examination subsequent to testing is made simple since the specimen itself can be thinned. The small volume of each specimen is advantageous since a greater number of specimens can be irradiated and tested, yielding a better understanding of material behavior.
This work was supported by the Department of Energy, Office of Fusion Energy, under grant number DEFGO585ER52139C. References 111 W.R.
Corwin and G.E. Lucas, The Use of Small-Scale Specimens for Testing Irradiated Material, ASTM-STP 888 (1986) pp. 1-2. VI G.E. Lucas, J. Nucl. Mater. 117 (1983) pp. 327-339. [31G.R. Rao, M.S. Thesis, Auburn University, 1988, pp. 23-38. J. Nucl. Mater. [41N. Nagata, M. Furuya and R. Watanabe, 85 & 86 (1979) p. 839. [51C. Boller and T. Seeger, Materials Data for Cyclic Loading, Part C: High Alloy Steels, Material Science Monographs (Elsevier, Amsterdam 1987) p. 155. Use of Subsize Fatigue [61 K.C. Liu and M.L. Grossbeck, Specimens for Reactor Irradiation Testing, ASTM-STP 888 (1986) pp. 276-289. Progress [71 M.L. Grossbeck and K.C. Liu, ADIP Semiannual Report, DOE/ER-0045/8 (March 1982) pp. 1366140.