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An electric potential drop technique for characterizing part-through surface cracks
jou.roalof
Journal of Nuclear Materials 191-194 (1992) 1038-1041 North-Holland
gUClOar
An electric potential drop technique for characterizing part-through surface cracks M. Enmark, G. Lucas and G.R. Odette Department of Chemzcal and Nuclear Engmeenng, Unwersl~. of Cahfonua, Santa Barbara, Santa Barbara, CA 93106, USA
The development and venficatmn of fracture criteria - including catastrophic fadure and leak-before-break phenomena are ~mportant to the assessment of fusion reactor structural integrity. Rehable assessments wdl reqmre appropriate measures to characterize the m~tiatton, stable growth and mstabthty of part-througb aurface cracks m thin-walled structures This paper describes procedures we are developing for momtormg the evoluttoa of part-through cracks using electropotentml drop techmques A cahbratton between potential drops across mulUplc probes placed across the crack plane and crack size and shape was developed for alummuw, and 410 steel speomens. The cahbrauon techmque was then succes3fully apphed to momtor surface crack growth in an HT-9 specimen.
1. Introduction One of the major hfetime hmits for fusion reactor structural alloys is radmtlon-mduced degradation of the fracture toughness [1,2] However, despite substantml reductions in fracture toughness, neither linear elastic ( L E F M ) nor elestlc-plastlc fracture mechanics (EPFM) may be directly applicable to predictions of catastrophic failure because the dimensions of the structure {e.g. first wall) may be small relative to those required to satisfy constraint conditions [3]. Loss of constraint on the crack tip plastic zone resulting from decreasing s p e c i m e n / c o m p o n e n t dimensions can lead to nonumque crack tip fields that are no longer well described by the asymptotic L E F M / E P F M solutions, hence, greater remote stresses and d~splacements can be apphed before fracture or failure condmons are achieved [4]. In situations where components contain part-through surface cracks vs the through-crack geometries typical of test p~eces used in fracture mechanics (e g. compact tension or single edge-notched bend bar), the stress fields can be relaxed along the crack front, leading to fields that further deviate from the elastic and small scale yielding f, elds calculated for L E F M and E P F M conditions [5] ~,~e have previously examined the deviation of fracture conditions from L E F M with decreasing specimen and crack t~p ligament size m through-crack, threepoint bend specimens of HT-9 [6] The objectwe of this work is to extend this approach to develop failure criteria for part-through surface cracks This requires developing a rehable technique for monitoring crack size and shape during monotomc loading. A number of
approaches to this problem have been examined meluding laser mterferometry for transparent materials [7], ac potential drop [8], and pulsed potential drop [9]. Each has advantages and disadvantages Our approach is based on a multiple probe dc electropotential drop ( E P D ) techmque, which offers adequate sensitivity and relative slmphctty for remote testing of radioactive matermls [10]. This paper describes the results of this effort
2. Experimental procedure To calibrate the method, E P D and optical crack growth measurements were first carried out on a 6061 aluminum and a 410 t e m p e r e d martensmc stainless steel, which has a composition slmdar to HT-9. The tests were c a m e d out on three-pomt bend specimens illustrated m fig. 1 with the dimensions gwen m table 1. Cracks were grown from trlangu!ar starter notches of depth a~) and base 2cu electro-&scharge machined ( E D M ' d ) into the s p e o m e n . Different starter notch sizes were used to obtain two different surface crack geometries, a() = 0.5 mm, and 2c 0 = 3 8 ram, and a 0 = 0.76 mm, and 2c o = 2 5 mm The triangular notches grew into semlelhptlcal cracks under fatigue loading as shown m fig. 1. Four dc potential drop voltage leads were fixed at 2 54 mm intervals on opposite sides of the crack plane as shown in fig. 1. In the alummum specimens, voltage probes were attached by peening alummum lead wires into predrdled holes. However, this proved an unreliable means of attachment. Hence, in the 410 steel
0022-3115/92/$05 00 © 1992 - Elsevier Science Pubhshers B V All rights reserved
M Enmark et al / EPDforpart-through cracks 08
Voltage PrObe Posd=ons
1039
i
i
i
i
07
"1"
0.6
~ i [] 5 1ram tlack aluminumI
I
0.5 0
s¢
0.2
~
W
0.1
:',
Fig 1. Schematic illustration of part-through crack specimen
O0 00
Fig 2 Summary of optical measurements of crack growth m aluminum 410 steel specimens.
fracture surface. This data was used to construct the E P D - a ( x ) calibration curves as described below. The calibration was tested on a specimen of the E S R heat of HT-9 with dimensions given m table 1 T h e initial semielhpttcal faUgue precracked surface flaw had dimensions a o = 1 52 m m and 2c o = 30 mm (note that the initial a/2c ratio of 0.05 in this case differed appreciably from the range of 0.13 to 0.30 used m the calibraUon) F o u r sets of 410 steel wire voltage probes were spot welded as previously described, and potential differences were recorded during subsequent fatigue crack growth. The calibration curves developed on the aluminum and 410 steel were used to predict the crack shape and size m the HT-9.
Table l Part-through crack specimen dimensions
6061 aluminum 410 steel HT-9
n , . . 0.6 0.8 1.0
a/t
specimens, leads were attached by spot welding into precmely located dimples on the specimen surface, lead;ng to better adhes,on and reduced voltage uncertamty. Cracks were subsequently extended in three-point bending under fatigue conditions with a maximum stress intensity range ( A K ) of 25 MPav/-m. All tests were performed at ambient temperature. The voltage, II, across each set of probes was m o m t o r e d at a constant current of 15 A. Periodic elevation of the maxim u m A K to 35 M P a v ~ for 600 cycles was used to mark the actual crack growth with distractive large striations on the fracture surface over the entire crack front. C~acks were extended beyond the last set of probes. The specimens were then broken and crack depth profiles a(x) were measured optically on the
Material
. . . . 02 0.4
Dimensions [mm] 3. Results
L
S
W
t
190 190 190 190
114 3 114 3 114 3 114 3
50 8 50 8 50.8 50.8
5 08 4 57 6 35 5 08
|
A summary of the optical measurements of the crack growth for the 410 steel s p e o m e n s 1s gwen m fig. 2. After an mtual transient the crack shape (a/2c) evolves with depth ( a / t ) m an envelope that is rela-
!
I
EstimatedUncertainty ~
o
[]
5 l m m thick Aluminum
•
4 6mm thick Steel
o
6 4mm thick Steel Polynomial Fit
00
i
i
i
02
04
06
08
a(x)/t Fig. 3. A comparison of normahzed potential plotted against a(.c)/t for the aluminum and 410 ,teel samples (two thncknesses)
M Enmark et al. / EPD.forpart-through cracks
1040
timely independent of initial crack shape and size. Consistent with previous observations the asymptotic curve extrapolates to a point circle of a/c = 1 at a / t = 0, as is the case here [ll] As the crack depth approaches the neutral axis in bending, the advance slows in the through thickness direction but not in the c direction leading to a decreasing a/2c ratio. A similar crack evolution pattern was observed in the aluminum specimens. Normalizing the potential by a reference potential and the crack Ic ~gth by a specimen dimension minimizes the dependence of the calibration curve on material and specimen s~ze and variations in test current and temperature [10,11]. In through-crack specimens, crack length is typically normalized by specimen width; and hence by analogy, we normalized crack length m the through-thickness direction, a(x), against specimen thickness t. Consistent with previous results we found for through-crack specimens, the voltage at an a(x)/t of one half the specimen thickness is a good normalizing parameter. The normalized potentml for the central probe plotted against a / t for the aluminum and 410 steel sampies (two thicknesses) is shown in fig. 3. The error bars shown m fig. 3 are based on potential and crack depth measurement uncertainties and fluctuations observed during the test. The hne is a third order, least squares polynomial fit. Within the data scatter, this normahzat~on appears to reasonably collapse the potential vs crack length data for dissimilar materials and for specimens of two thicknesses. The normalized potential vs a / t for the 410 steel specimen was found to vary at different probe positions as illustrated in fig. 4 T h e slope oi" V/V(0 5) decreases with increasing x / W . This suggests some three-dimensional crack shape effects. We are attemptmg to understand thcsc effects based on fimte element analyses of the three-dimensional potential fields. Fig. 5 compares crack measurements in HT-9 based on the EPD calibration to post-test optical measuremcnts at three depths The error bars on the EPD
2
!
|
j s
~ - x/W= 010 1
"
......
x/W=
0 15
0 00
02
04
x
0.6
"~
04 O2 00 -0.5
,1 . . . . . . -03
-01
0,1
0,3
0 5
x/W Fig• 5. Positions of the crack as predicted by the potential drop data (data points) compared to post-test optical measurements of the crack front (lines) m a fatigue-cracked HT-9
specimen
points reprcsent the magnitude of the data scatter about the calibration curves. As was obvious from the beginning of the test, the crack was long relative to the probe positions and additional calibrated probe positions for these long cracks will be needed in future work; moreover, the crack grew asymmetrically; thus additional probe positions on either side of center a n d / o r a greater probe spacing are needed to capture such asymmetry. However overall the predicted crack front position is in very good agreement with the actual, within the data scatter. Thus it is clear that improved crack position predicuons will require reduced data scatter. One of the mare sources of scatter is beheved to be temperature fluctuations, which are not completely compensated in the normalization scheme we used. A better way of compensating for temperature fluctuations might bc to normalize by a potential gradient, V0, at a posit~on far removed from the ends and the surface crack where the gradient does not vary with crack growth. While this was not feasible m our previous work on through crack specimens (specimen geometries were such that a sigmficant region of uniform potential gradient was not accessible [10]), it should be feasible in the part-through crack geometry and this approach will be examined m future work.
000
. . . . x/W= 005
~"
~
08
|
-x
•
10
0'6
08
a(x)/t Fig. 4 A composite of the fits of normalized potential versus a(x)/t at all probe positions
4. Conclusions
Wc have evaluated a multiple probe dc potential drop techmque to monitor the depth and shape of growing surface cracks under bending fatigue• Sensitivity to material, test temperature, and specimen dimensions can be minimized by normalizing the EPD to values at a(x)/t = 0.5. However, there is a dependence of the calibration on probe position along the crack mouth The calibrations obtained with aluminum and 410 steel specimens reasonably predicted crack depth and shape in a propagating surface crack in HT-9.
M Enmark et al. / EPD for part-through cracks Future work wdl address reducing the magnitude of thc residual uncertainty.
Acknowledgements T h e authors gratefully acknowledge support from the U S D e p a r t m e n t of Energy under which this work was performed: grant no. DE-FGO3-87ER52143, W.F. Wiffen, contract monitor.
References [1] R.H. Jones, W. Wolfer, N Ghonetm and G.E. Lucas, DOE/ER-0046/13, vol 2 (1983) [2] G R. Odette and G.E, Lucas, J Nucl Mater. 179-!81 (1991) .~72.
1041
[3] G.R. Odette and G.E. Lucas, J. Nucl. Mater. 117 (1983) 276. [4] G.R. Odette, B.L. Chao and G.E. Lucas, m these Proceedings (ICFRM-5), J. Nuci. Mater. 191-194 (1992) 827. [5] D. Parks, ASTM-STP 1060. Am. Soc. for Testing and Materials (1990) 9. [6] G.R. Odette, G.E. Lucas, R. Maiti and J.W. Sheckherd, J. Nucl. Mater. 133&134 (1986) 849. [7] W.A. Troha, T. Nicholas and A. Grandt, ASTM-STP 1060, Am. Soc. for Testmg and Materials (1990) 260. [8] N. Marchand, W. Dorner and B. llschner, ~id., p. 237. [9] G. Baudin and H. Pohcella, A Pulsed dc PD Techmque ApplicaUons to Three-Dimensional Crack Fronts, Adv. in Crack Length Measurement (1982) 159. [1O] C. Elliott, M Enmark, G.E. Lucas, G.R. Odette and A. Rowcliffe, J. Nucl. Mater. 179-181 (1991) 434. [11] W.S. Pierce and J L. Shannon, J. Test Eval. 6 (1978) 183. [12] M.D. Halhday and C J. Beevers, The dc Electrical Potential Method for Crack Length Measurement, The Measurement of Crack Length and Shape During Fracture and Fatigue (1980) 85.
Journal of Nuclear Materials 191-194 (1992) 1038-1041 North-Holland
gUClOar
An electric potential drop technique for characterizing part-through surface cracks M. Enmark, G. Lucas and G.R. Odette Department of Chemzcal and Nuclear Engmeenng, Unwersl~. of Cahfonua, Santa Barbara, Santa Barbara, CA 93106, USA
The development and venficatmn of fracture criteria - including catastrophic fadure and leak-before-break phenomena are ~mportant to the assessment of fusion reactor structural integrity. Rehable assessments wdl reqmre appropriate measures to characterize the m~tiatton, stable growth and mstabthty of part-througb aurface cracks m thin-walled structures This paper describes procedures we are developing for momtormg the evoluttoa of part-through cracks using electropotentml drop techmques A cahbratton between potential drops across mulUplc probes placed across the crack plane and crack size and shape was developed for alummuw, and 410 steel speomens. The cahbrauon techmque was then succes3fully apphed to momtor surface crack growth in an HT-9 specimen.
1. Introduction One of the major hfetime hmits for fusion reactor structural alloys is radmtlon-mduced degradation of the fracture toughness [1,2] However, despite substantml reductions in fracture toughness, neither linear elastic ( L E F M ) nor elestlc-plastlc fracture mechanics (EPFM) may be directly applicable to predictions of catastrophic failure because the dimensions of the structure {e.g. first wall) may be small relative to those required to satisfy constraint conditions [3]. Loss of constraint on the crack tip plastic zone resulting from decreasing s p e c i m e n / c o m p o n e n t dimensions can lead to nonumque crack tip fields that are no longer well described by the asymptotic L E F M / E P F M solutions, hence, greater remote stresses and d~splacements can be apphed before fracture or failure condmons are achieved [4]. In situations where components contain part-through surface cracks vs the through-crack geometries typical of test p~eces used in fracture mechanics (e g. compact tension or single edge-notched bend bar), the stress fields can be relaxed along the crack front, leading to fields that further deviate from the elastic and small scale yielding f, elds calculated for L E F M and E P F M conditions [5] ~,~e have previously examined the deviation of fracture conditions from L E F M with decreasing specimen and crack t~p ligament size m through-crack, threepoint bend specimens of HT-9 [6] The objectwe of this work is to extend this approach to develop failure criteria for part-through surface cracks This requires developing a rehable technique for monitoring crack size and shape during monotomc loading. A number of
approaches to this problem have been examined meluding laser mterferometry for transparent materials [7], ac potential drop [8], and pulsed potential drop [9]. Each has advantages and disadvantages Our approach is based on a multiple probe dc electropotential drop ( E P D ) techmque, which offers adequate sensitivity and relative slmphctty for remote testing of radioactive matermls [10]. This paper describes the results of this effort
2. Experimental procedure To calibrate the method, E P D and optical crack growth measurements were first carried out on a 6061 aluminum and a 410 t e m p e r e d martensmc stainless steel, which has a composition slmdar to HT-9. The tests were c a m e d out on three-pomt bend specimens illustrated m fig. 1 with the dimensions gwen m table 1. Cracks were grown from trlangu!ar starter notches of depth a~) and base 2cu electro-&scharge machined ( E D M ' d ) into the s p e o m e n . Different starter notch sizes were used to obtain two different surface crack geometries, a() = 0.5 mm, and 2c 0 = 3 8 ram, and a 0 = 0.76 mm, and 2c o = 2 5 mm The triangular notches grew into semlelhptlcal cracks under fatigue loading as shown m fig. 1. Four dc potential drop voltage leads were fixed at 2 54 mm intervals on opposite sides of the crack plane as shown in fig. 1. In the alummum specimens, voltage probes were attached by peening alummum lead wires into predrdled holes. However, this proved an unreliable means of attachment. Hence, in the 410 steel
0022-3115/92/$05 00 © 1992 - Elsevier Science Pubhshers B V All rights reserved
M Enmark et al / EPDforpart-through cracks 08
Voltage PrObe Posd=ons
1039
i
i
i
i
07
"1"
0.6
~ i [] 5 1ram tlack aluminumI
I
0.5 0
s¢
0.2
~
W
0.1
:',
Fig 1. Schematic illustration of part-through crack specimen
O0 00
Fig 2 Summary of optical measurements of crack growth m aluminum 410 steel specimens.
fracture surface. This data was used to construct the E P D - a ( x ) calibration curves as described below. The calibration was tested on a specimen of the E S R heat of HT-9 with dimensions given m table 1 T h e initial semielhpttcal faUgue precracked surface flaw had dimensions a o = 1 52 m m and 2c o = 30 mm (note that the initial a/2c ratio of 0.05 in this case differed appreciably from the range of 0.13 to 0.30 used m the calibraUon) F o u r sets of 410 steel wire voltage probes were spot welded as previously described, and potential differences were recorded during subsequent fatigue crack growth. The calibration curves developed on the aluminum and 410 steel were used to predict the crack shape and size m the HT-9.
Table l Part-through crack specimen dimensions
6061 aluminum 410 steel HT-9
n , . . 0.6 0.8 1.0
a/t
specimens, leads were attached by spot welding into precmely located dimples on the specimen surface, lead;ng to better adhes,on and reduced voltage uncertamty. Cracks were subsequently extended in three-point bending under fatigue conditions with a maximum stress intensity range ( A K ) of 25 MPav/-m. All tests were performed at ambient temperature. The voltage, II, across each set of probes was m o m t o r e d at a constant current of 15 A. Periodic elevation of the maxim u m A K to 35 M P a v ~ for 600 cycles was used to mark the actual crack growth with distractive large striations on the fracture surface over the entire crack front. C~acks were extended beyond the last set of probes. The specimens were then broken and crack depth profiles a(x) were measured optically on the
Material
. . . . 02 0.4
Dimensions [mm] 3. Results
L
S
W
t
190 190 190 190
114 3 114 3 114 3 114 3
50 8 50 8 50.8 50.8
5 08 4 57 6 35 5 08
|
A summary of the optical measurements of the crack growth for the 410 steel s p e o m e n s 1s gwen m fig. 2. After an mtual transient the crack shape (a/2c) evolves with depth ( a / t ) m an envelope that is rela-
!
I
EstimatedUncertainty ~
o
[]
5 l m m thick Aluminum
•
4 6mm thick Steel
o
6 4mm thick Steel Polynomial Fit
00
i
i
i
02
04
06
08
a(x)/t Fig. 3. A comparison of normahzed potential plotted against a(.c)/t for the aluminum and 410 ,teel samples (two thncknesses)
M Enmark et al. / EPD.forpart-through cracks
1040
timely independent of initial crack shape and size. Consistent with previous observations the asymptotic curve extrapolates to a point circle of a/c = 1 at a / t = 0, as is the case here [ll] As the crack depth approaches the neutral axis in bending, the advance slows in the through thickness direction but not in the c direction leading to a decreasing a/2c ratio. A similar crack evolution pattern was observed in the aluminum specimens. Normalizing the potential by a reference potential and the crack Ic ~gth by a specimen dimension minimizes the dependence of the calibration curve on material and specimen s~ze and variations in test current and temperature [10,11]. In through-crack specimens, crack length is typically normalized by specimen width; and hence by analogy, we normalized crack length m the through-thickness direction, a(x), against specimen thickness t. Consistent with previous results we found for through-crack specimens, the voltage at an a(x)/t of one half the specimen thickness is a good normalizing parameter. The normalized potentml for the central probe plotted against a / t for the aluminum and 410 steel sampies (two thicknesses) is shown in fig. 3. The error bars shown m fig. 3 are based on potential and crack depth measurement uncertainties and fluctuations observed during the test. The hne is a third order, least squares polynomial fit. Within the data scatter, this normahzat~on appears to reasonably collapse the potential vs crack length data for dissimilar materials and for specimens of two thicknesses. The normalized potential vs a / t for the 410 steel specimen was found to vary at different probe positions as illustrated in fig. 4 T h e slope oi" V/V(0 5) decreases with increasing x / W . This suggests some three-dimensional crack shape effects. We are attemptmg to understand thcsc effects based on fimte element analyses of the three-dimensional potential fields. Fig. 5 compares crack measurements in HT-9 based on the EPD calibration to post-test optical measuremcnts at three depths The error bars on the EPD
2
!
|
j s
~ - x/W= 010 1
"
......
x/W=
0 15
0 00
02
04
x
0.6
"~
04 O2 00 -0.5
,1 . . . . . . -03
-01
0,1
0,3
0 5
x/W Fig• 5. Positions of the crack as predicted by the potential drop data (data points) compared to post-test optical measurements of the crack front (lines) m a fatigue-cracked HT-9
specimen
points reprcsent the magnitude of the data scatter about the calibration curves. As was obvious from the beginning of the test, the crack was long relative to the probe positions and additional calibrated probe positions for these long cracks will be needed in future work; moreover, the crack grew asymmetrically; thus additional probe positions on either side of center a n d / o r a greater probe spacing are needed to capture such asymmetry. However overall the predicted crack front position is in very good agreement with the actual, within the data scatter. Thus it is clear that improved crack position predicuons will require reduced data scatter. One of the mare sources of scatter is beheved to be temperature fluctuations, which are not completely compensated in the normalization scheme we used. A better way of compensating for temperature fluctuations might bc to normalize by a potential gradient, V0, at a posit~on far removed from the ends and the surface crack where the gradient does not vary with crack growth. While this was not feasible m our previous work on through crack specimens (specimen geometries were such that a sigmficant region of uniform potential gradient was not accessible [10]), it should be feasible in the part-through crack geometry and this approach will be examined m future work.
000
. . . . x/W= 005
~"
~
08
|
-x
•
10
0'6
08
a(x)/t Fig. 4 A composite of the fits of normalized potential versus a(x)/t at all probe positions
4. Conclusions
Wc have evaluated a multiple probe dc potential drop techmque to monitor the depth and shape of growing surface cracks under bending fatigue• Sensitivity to material, test temperature, and specimen dimensions can be minimized by normalizing the EPD to values at a(x)/t = 0.5. However, there is a dependence of the calibration on probe position along the crack mouth The calibrations obtained with aluminum and 410 steel specimens reasonably predicted crack depth and shape in a propagating surface crack in HT-9.
M Enmark et al. / EPD for part-through cracks Future work wdl address reducing the magnitude of thc residual uncertainty.
Acknowledgements T h e authors gratefully acknowledge support from the U S D e p a r t m e n t of Energy under which this work was performed: grant no. DE-FGO3-87ER52143, W.F. Wiffen, contract monitor.
References [1] R.H. Jones, W. Wolfer, N Ghonetm and G.E. Lucas, DOE/ER-0046/13, vol 2 (1983) [2] G R. Odette and G.E, Lucas, J Nucl Mater. 179-!81 (1991) .~72.
1041
[3] G.R. Odette and G.E. Lucas, J. Nucl. Mater. 117 (1983) 276. [4] G.R. Odette, B.L. Chao and G.E. Lucas, m these Proceedings (ICFRM-5), J. Nuci. Mater. 191-194 (1992) 827. [5] D. Parks, ASTM-STP 1060. Am. Soc. for Testing and Materials (1990) 9. [6] G.R. Odette, G.E. Lucas, R. Maiti and J.W. Sheckherd, J. Nucl. Mater. 133&134 (1986) 849. [7] W.A. Troha, T. Nicholas and A. Grandt, ASTM-STP 1060, Am. Soc. for Testmg and Materials (1990) 260. [8] N. Marchand, W. Dorner and B. llschner, ~id., p. 237. [9] G. Baudin and H. Pohcella, A Pulsed dc PD Techmque ApplicaUons to Three-Dimensional Crack Fronts, Adv. in Crack Length Measurement (1982) 159. [1O] C. Elliott, M Enmark, G.E. Lucas, G.R. Odette and A. Rowcliffe, J. Nucl. Mater. 179-181 (1991) 434. [11] W.S. Pierce and J L. Shannon, J. Test Eval. 6 (1978) 183. [12] M.D. Halhday and C J. Beevers, The dc Electrical Potential Method for Crack Length Measurement, The Measurement of Crack Length and Shape During Fracture and Fatigue (1980) 85.