- Email: [email protected]
The reasonable design of Charpy-size specimen for fracture toughness test in nuclear surveillance
hr. .I. Pres.
0308-0161(94)00015-8
ELSEVIER
Ves. & Plpg
62 (1995)
219-225
0 1995 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 030%0161/95/$09.50
The reasonable design of Charpy-size specimen for fracture toughness test in nuclear surveillance X. IP. Zhang & Y. W. Shi Department
of Mechanical
Engineering, Wan Jiaotong University, People’s Republic o,f China
Xi’an,
Shaanxi
Province,
710049,
(Received 1 January 1994;accepted21 March 1994)
Use of a precracked and side-groovedCharpy-size specimenis an economical and convenient single-specimenmethod of evaluating the elastic-plastic fracture toughnessof nuclear pressurevesselsteels.This paper hasstudied the influence of side-groovedepth on fracture toughnessand stable increment of crack on several pressurevesselsteelsin detail. Test resultsare comparedwith those of large-sizespecimenswhich the National Standard of China (i.e. GB 2038) requires. The researchresults indicate that using a precracked Charpysize specimenwith side-groovedepth 30% and adopting the energy before the maximum load of a three-point bending test curve, the elastic-plastic fracture toughnessof materialscan be evaluated conservatively when the crack begins to propagate, and the reasonabledesignof a Charpy-size specimenfor fracture toughnesstestsin nuclear surveillancehasbeen established.At the sametime, the constraint effect and thickening action of the side-grooveon a Charpy-size specimenhave also been discussed,and theoretically explained.
1 INTRODUCTION
vessel and retrieved periodically to monitor the degradation of toughness and other mechanical properties of pressure vessel materials due to neutron irradiation. In the past, the surveillance specimens that were put into pressure vessels were almost always small specimens for conventional tensile tests and impact tests, such as round tensile specimens, and Charpy specimens, respectively.’ Along with fracture mechanics development, the surveillance of fracture toughness of pressure vessel materials and its welded joints becomes more important for evaluating the safety and integrity of nuclear pressure vessels in modern times. Fracture toughness is as important to monitor as the activity of defects and cracks in a pressure vessel. In surveillance programmes, fracture toughness tests and dynamic tearing tests using compact tensile specimens and precracked Charpy-size specimens have been found to be a practical way of monitoring pressure vessel materials in some nuclear power stations.2” In the surveillance test of fracture toughness,
During the operating process, pressure vessels and other components of nuclear power stations become embrittled due to neutron irradiation and heat-ageing at high temperature as well as strain-ageing. In order to evaluate the structural integrity of nuclear power stations, it becomes more important to research and estimate the embrittlement of pressure vessel material properties, which change with operating time or neutron irradiating dose. In recent ye:ars, an important content in surveillance programmes is to monitor periodically the irradiated embrittlement of pressure vessel materials and its welded joints, to put forward tolerance limits on defects in a pressure vessel under operating state, and to predict the safety and the future lifetime of the reactor pressure vessel. Due to the hi,gh costs of space at the reactor core, the surveillance specimens’ size should be small. In a nuclear power station, a great number of small surveillance specimens are put into the pressure 219
220
X. P. Zhang, Y. W. Shi
the method of using a small single specimen is the most reasonable one. In order to avoid adopting the complicated physical method for monitoring initial growth of a crack, more research work has been done. It has been pointed out that a precracked Charpy-size specimen with a deep side-groove is a cheap single-specimen method for estimating elastic-plastic fracture toughness of nuclear pressure vessel steels.6,7 In this method, the fracture toughness of the maximum load on a load versus load-point displacement curve is regarded as the initial fracture toughness. Moreover, the effects of side-groove depth and geometric shape on elastic-plastic fracture toughness parameters in precracked Charpy-size specimens have also been studied.8,9 However, some results for a mild steel indicated that slow crack propagation exceeded 1 mm if the fracture toughness at maximum load was regarded as the critical value of initiation and with the side-groove depth reaching 40 to 60% of the specimen thickness.” This paper has studied the effects of sidegroove depth on fracture toughness of maximum load (JJ on the load versus load-point displacement curve and the relative growth of crack depth (Aa,,,) corresponding to the maximum load for several common pressure vessel steels by using precracked Charpy-size specimens. Comparison with that of large specimens which GB 2038” requires has also been made. In the end, the reasonable design of a Charpy-size specimen for fracture toughness tests in nuclear surveillance has been established. Furthermore, Table Materials A508CL3-A A508CL3-B BHW3.5
1. Chemical
A508CL3-A A508CL3-B BHW3.5 A508CL3-A’
2 TEST MATERIALS
AND PROCEDURE
2.1 Materials and specimen preparation The materials for tests are typical heavy-section nuclear pressure vessel steels A508CL3-A, A508CL3-A’ and A508CL3-B produced by two plants, as well as BHW3.5 steel. The chemical composition and mechanical properties of four test materials are given in Tables 1 and 2. A508CL3-A and A508CL3-A’ steels have the same chemical composition, the only difference is that A508CL3-A’ has undergone a special heat treatment which was equivalent to the embrittlement of pressure vessel steel irradiated at a 3 X 101’ n/cm2 neutron dose.‘2,J3 The standard precracked Charpy-size specimen is shown in Fig. l(a). The length of fatigue crack (a/w) is 0.45 to O-55. After precracking, the side-grooves with different relative depth were machined on both sides of the specimens in the shape of a V-notch (60” angle) and with a root radius of O-1mm, as shown in Fig. l(b). The orientation of all specimens is in L-S.
composition
of test materials
(wt%)
C
Mn
Si
S
P
Ni
Cr
MO
V
Cu
Al
0.14 0.20 0.14
1.29 1.29 1.32
0.27 0.24 0.34
0.006 0.007 0.012
0.004 0.008 O-019
0.98 0.75 0.86
0.05 0.06 0.39
0.51 0.50 0.27
0.03 0.004
0.08
0.012
Table Materials
this paper also discusses the constraint effect and thickening action of the side-groove on Charpysize specimens. Methods to calculate the constraint coefficient, equivalent thickness and additional thickness of side-grooved specimens are put forward.
2. Mechanical
fly WV 540 515 586 893
properties
auTs(MPa) 642 650 664 953
“S,( %) is percentage elongation. “@(%) is percentage reduction in area. ‘Strain hardening exponent. dVickers hardness.
of test materials S5 (%y
Cp(%)”
nc
HVd
22 24 25 20
73 66 68 69
6.5 6.9 8.0 17.7
220-230 245-255 260-275 365-380
221
Charpy-size specimen for fracture toughness test in nuclear surveillance
requires, B = 20 mm, W = 24 mm, the span is 96 mm (4W), a/W = 0.5, and the J, resistance curves were measured with blunting line as follows:
.Fat igue crack flotch
J = 3a, * Aa
(4
where (T, is flow stress, gf = 0*5(a, + (T&, (TVis yield stress, (TUT, is ultimate tensile strength, and Aa is the stable propagation length of the crack. The valid Jlc can be obtained from JR resistance curves.
Bn
(b) Fig. 1. Precracked Charpy-size specimens (unit: mm): standard specimen; (b) with side-groove.
(a)
3 RESULTS
AND
DISCUSSION
3.1 Effect of side-groove toughness parameters 2.2 Experimental
(4)
depth
on fracture
procedure
Tests were carried out at room temperature in an Instron 1195 electronic tensile testing machine with a loading speed of 0.5 mm/min. The span between supports was 40 mm. During the testing process, the curve of load versus lo,ad-point displacement was recorded. The specimen was unloaded when the load reached the mlaximum point, and then the specimens were broken at liquid nitrogen temperature. The initial crack depth (a,) and the relative growth of crack depth (Au,,,) corresponding to the maximum load were measured by means of a five points average method. The relative depth of the side-groove is defined as follows: d, = [(B -&JIB]
x 100%
(1)
where B is the specimen thickness, and 13, is the net thickness of cross section of specimen, as shown in Fig. 1. The maximum load a.bsorbed energy, &, can be calculated from the load versus load-point displacement curve, and the J-integral value of maximum load, J,, can be expressed as: Jm= alIIRl(~
- a,)]
(2)
where W is specimen width. The elastic-plastic fracture toughness K,, can be calculated by K,, = d[E * J,/(l - Y*)]
(3)
where E is Young’s modulus of elasticity, and Y is Poisson’s ratio. For large standard specimens, which GB2038
Figure 2 shows the results of fracture toughness of maximum load (Jm) changing with side-groove depth (dS). For the four kinds of materials, it can be seen that the experimental curves have the same trend, where J, values decrease rapidly when the side-groove depth increases. As soon as the d, reaches the critical value (d,),, the J, value also achieves the critical value (J&, and stays constant. For a certain material, when the d, value is larger than the (d,), value, the J,,, value will keep constant, which is the so-called platform value (J,&. The characteristic values of experimental curves for the four kinds of materials, (d&, (J& and (J&, are shown in Table 3. The fracture toughness values corresponding to a side-groove depth of 30%, (Jm)30, are also shown in Table 3. Figure 3 shows the results of Au, changing with side-groove depth. The Aa, values drop gradually when the side-groove depth increases, and the curves have no abrupt changes. At the critical side-groove depth, the critical lengths of crack increment for the four materials are also shown in Table 3. For precracked Charpy-size specimens with a side-groove, the slow crack growth is very short when the side-groove depth reaches the critical value (d,),. For the specimen with critical side-groove depth, the slow crack growth is 4.5, 4.7, 2.7 and 4.5% of the initial crack length a, for A508CL3-A, A508CL3-B, BHW35 and A508CL3-A’ steels, respectively, when it is assumed that a, = 5 mm. It is clear that the crack growth is very small, and the maximum load point almost approaches the initiation.
X. P. Zhang, Y. W. Shi
222
1
d””
o A508CL3-A
600-o l
A508CL3-B
o A508CL3-A .
A508CL3-B
il; , , , , , , 1
100 t
t O0
I 10
I 20
I 30
I 40
I 50
, 60
-I
ds (%) 10
20
30
40
50
60
d, (o/o)
(a)
.I”” I
I
Ob 0
J ’ 0
I lo
I
I
20
30
I
40
I
*
50
60
d,
(%)
o A508CL3-A'
1
1
I
I
I
1
10
20
30
40
50
60
ds (%) (b) Fig. 3. Results of Aa, vs. side-groove depth: (a) A508CL3-A and A508CL3-B steels; (b) BHW35 and A508CL3-A’ steels.
(b) Fig. 2. Resultsof J,,, vs. side-groovedepth: (a) A508CL3-A
and A508CL3-B steels;(b) BHW35 and A508CL3-A’ steels.
3.2 Thickening action and constraint of sidegroove in precracked Charpy-size specimen
The side-groove in a specimen is equivalent Table
Materials A508CL3-A A508CL3-B BHW35 A508CL3-A’
(d&W) 25 30 20 20
to
3. The characteristic
(.hJc @J/m*) 305.1 269.0 148.0 182.8
thickening the specimen, and it can strengthen the level of stress triaxiality at the crack tip along the thickness direction of the specimen. It also changes the stress state at the crack tip, and makes the mixed stress state in a small Charpy-size specimen become more planevalues of test results
(A& @J/m’) 299.8zt 14.8 265.0f 10.6 149.8f 15.8 174.0f 12.5
GAI @J/m’) 303.5 269.0 157.2 179.3
WA
6-d 226 234 134 202
Charpy-size
specimen folr fracture
strain state. The research results on a 1CrMoV steeP9 indicated that the region of plane strain got to 88% of the whole thickness of the specimen when the side-groove depth in a precracked Charpy-size specimen was 20%. By contrast, the region of plane strain of a standard specimen was only 25% of the whole thiclkness of the specimen, i.e. in the centre of the specimen there existed a narrow region of plane strain. The effect of a side-groove on thickening the precracked Charpy-size specimen may be expressed in terms of equivalent thickness B, as follows:8’9 B, = B,[l + 0*67(B,/B)(l
toughness
test in nuclear surveillance 26
; E *
24
^, es
20 '
16
& 12 8
- B,/B)]I
(5) The curves for B, and B, changing with d, are shown in Fig. 4. Considering that the different materials have different sensitivities to sidegrooves, eqn (5) should be revised to include the strain hardening exponent y1 expressed by the Ramberg-Osgood equation and the ratio of ((T~/(T& as follows: (Be), = Bn[l + @Wqk,,,) x (WW - BnIWI (6) where (B,), is the revised equivalent thickness. Using eqn (6), curves of (B,), and B, with respect to d, are shown in Fig. 5. If B, is subtracted from both sides of eqn (6), we obtain AB = (B,), - B, = 0~67B,(na,/a,,,) x K&W - (WB2)1 (7) where AB is the additional thickness of the specimens caused by the constraint effect of side-grooves. We then calculate the first deriva-
I
1
223
ds l%:, Fig. 5. Curves of (B,), and B, vs. side-groove
tive of AB with respect to B, using eqn (7), and the maximum value of B, can be obtained from d(AB)/dB,
= 1.34(B,/B) - 2.01(B,/‘B)’ = 0
Therefore, when B, equals 6.67 mm (i.e. d, equals 33% of the specimen thickness), the maximum additional thickness of the precracked Charpy-size specimen is obtained, as shown in Fig. 5. For A508CL3-A, A508CL3-B, BHW35 and A508CL3-A’ steels, the maximum values of additional thicknesses are 5.4, 5.3, 9.4 and 22.0 mm, respectively, and the equivalent thicknesses are 12.1, 12.0, 16.1 and 28.7 mm, respectively. It is found that the constraint effect of the side-groove is explicit in specimens. 3.3 Constraint
coefficient
It is evident that, at the crack tip, there exists the constraint effect from the side-groove. In order to evaluate the level of plastic constraint of side-grooves at a crack tip and make the constraint effect of side-grooves quantitative and convenient, the constraint effect of a side-groove can be expressed in terms of the constraint coefficient (C). For precracked Charpy-size specimens with side-grooves, the C value can be written as follows: C = P, +S/[B, . W’ . cruTS* (1 -a/W)‘]
ds
Fig. 4. (‘urves of B, and B, vs. side-groove
Co,b)
depth.
depth.
(8)
where Pmis the maximum load on the load versus load-point displacement curve. The relation of C value changing with d, is shown in Fig. 6. It is found that C values increase gradually when the side-groove depth increases, i.e. the plastic
224
X. P. Zhang, Y. W. Shi
J-integral values keeping steady. The experimental results have proved this.
C.L” O A508CL3-A
c 2.00
A508CL3-B
l
3.4 Effect of material depth of side-groove
I-
1.20'
0
10
20
30
40
50
60
ds t%) (a)
o BWH35
G 2.00
le2*L 0
l
I 10
1 20
I 30
A508CL3-A'
I 40
I 50
I 60
embrittlement
on critical
With neutron irradiation and heat-ageing as well as strain-ageing, the plasticity and toughness of nuclear pressure vessel steels would decrease gradually. Comparing A508CL3-A steel with the A508CL3-A’, which had undergone a heat treatment simulating neutron irradiation, the strength of the latter steel increased and the plasticity as well as fracture toughness decreased, as shown in Fig. 2 and Table 3. For A508CL3-A steel, (d,), is 25% and (J,& equals 305.1 kJ/m2; while (d,), is 20% and (J& equals 1828 kJ/m2 for the A508CL3-A’ steel. If the side-groove depth in the precracked Charpy-size specimen is selected as 25% for A508CL-A’ steel, then the & value corresponding to the d, (i.e. (Jm)&, equals 179.3 kJ/m’, and the difference between (J& and (J& is less than 2%. If a side-groove depth of 30% in a precracked Charpy-size specimen is adopted for all test materials which have different strengths, and the J, value corresponding to d, of 30% is expressed as (Jm)3o, the difference between (J&, and (J& is very small, as shown in Table 3. Therefore, a constant side-groove depth of 30% is suitable for the whole surveillance process of neutron irradiation embrittlement of a nuclear pressure vessel.
ds 1%) (b) Fig. 6. Relation of constraint coefficient vs. side-groove depth: (a) A508CL3-A and ASOSCL3-Bsteels;(b) BHW3.5 and A508CL3-A’ steels.
constraint increases. When the side-groove depth reaches and exceeds the critical value (d,),, the C value becomes constant. The critical J-integral values depend upon the stress state at the crack tip. For the precracked Charpy-size specimen with side-grooves, C values can be used to evaluate the stress state under the action of side-groove constraint quantitatively. That the C value becomes constant, means the stress state is steady at the crack tip along the thickness direction of the specimen. Correspondingly, the maximum load f-integral values (Jm) become constant. This is due to the constancy of C values, which results in the maximum load
3.5 Valid
elastic-plastic
fracture
toughness
The JR resistance curves for A508CL3-A, A508CL-B, BHW35 and A508CL3-A’ steels were measured, and the results of valid Jlc are shown in Table 4. According to the slope of the JR resistance curves, the tearing modulus, TM (no dimension), can be obtained as follows: TM= (Elaf)(dJlda) (9) The TM values and the K,, values converted from valid JIc by eqn (3) are also shown in Table 4. In Table 4, J,,, values corresponding to the critical side-groove depth in a precracked Charpy-size specimen, and the K,, values converted from the J, are shown for comparison with valid fracture toughness values. It can be observed that J, values are a bit smaller than valid JIc values. For A508CL3-A, A508CL3-B, BHW35 and A508CL3-A’ steels, the K,, values converted
Charpy-size Table
Materials A508CL3-A A508CL3-B BHW35 A508CL3-A’
specimen for fracture
4. The comparison
Valid .I,= (kJ/m’) 322.4 304.7 203.0 210.4
between
valid &(&)
’ gt%) 270.1 26;!.6 214.4 218.2
225
toughness test in nuclear surveillance
TM
(Jr&
4 CONCLUSIONS (1) Side-grooves in precracked Charpy-size specimens have the action of thickening the specimen, and for a certain material there exists an optimal side-groove depth at which the specimen gains the maximum additional thickness. (2) The critical value of side-groove depth, (d,),, determined by curves of J, versus d,, is the same as that determined through curves of C values versus d,. In fact, the tendency of J, versus d, depends upon that of C values versus 4. (3) Constraint of the side-grooves may be quantitatively evaluated by means of constraint coefficient (C) values while the side-groove depth changes. (4) The systematic studies of the effect of side-groove depth on fracture toughness parameters indicate that, according to the maximum load absorbed energy of load versus load-point displacement curve, the conservative vallues of elastic-plastic fracture toughness for initiation of ductile fracture can be obtained. In the experimental process, it is not necessary to monitor crack propagation by means of a special and complicated method. It is a single-specimen method with a small specimen size that is especially suitable for surveillance of the fracture toughness of a nuclear pressure vessel subject to embrittlement by neutron irradiation.
K,,
(kJ/m’)
331.0 305.3 298.2 295.4
from J, values are smaller than valid K,, values by 2.7, 5.7, 13.8 and 6.5%, respectively. Nevertheless, according to the maximum load absorbed energy E,, the conservative values for elastic-plastic fracture toughness of pressure vessel materials can be obtained by mea.ns of a precracked Charpy-size specimen with a deep side-groove. This is a convenient single-specimen method that is especially suitable for surveillance of embrittlement of a nuclear pressure vessel due to neutron irradiation.
and J,(converted
K,,)
(converted from (Jm)c) (MPaV&)
305.1 269.0 148.0 182.8
262.8 247.6 184.6 204.4
ACKNOWLEDGEMENT The authors greatly acknowledge the financial support from the National Nuclear Safety Bureau of China.
REFERENCES 1. ASTM El8582, Standard Practice for Conducting Surveillance Tests for Light-water Cooled Nuclear Power Reactor Vessels.E706(1F), ASTM, Philadelphia, 1982,pp. 350-9. -.3 ASTM E636-83, Standard Practice for Conducting Supplemental Surveillance Tests for Nuclear Power Reactor Vessels,E706(1H), ASTM, Philadelphia, 1983, pp. 318-32. 3. Varga, T. & Njo, D. H., Selectionof specimentypes for irradiation surveillance programs. Radiation Embrittlement and Surveillance of Nuclear Reactor Vessels, ASTM STP819,1983,pp. 166-73.
Pressure
4. Davies, L. M. & Ingham, T., Overview of studiesin the United Kingdom on neutron irradiation embrittlement of pressurevessel steels. Radiation Embrittlement and Surveillance
of Nuclear
Reactor
Pressure
Vessel Steels.
ASTM STP909,1986,pp. 13-33. 5. Ahlstrand, R. Torronen, K., Valo, M. & Bars, B., Surveillance programs and irradiation embrittlement researchof the Loviisa nuclear power plant. Radiation Embrittlement and Surveillance of Nuclear Reactor Pressure Vessel Steels, ASTM STP909, 1986,pp. 55-69. 6. Ritchie, R. O., Server, W. L. & Wullaert, R. A., Znt. J. Fracture, 14 (1978) 139-42. 7. Server, W. L., Wullaert, R. A. & Ritchie, R. O., J. Eng. Mater. and Technol., Trans. ASME, 102 (1980) 192-9. 8. Neale, B. K., Znt. J. Pres. Ves. and Piping, 10 (1982)
375-98. 9. Neale, B. K., Znt. J. Pres. Ves. and Piping, 12 (1983) 207-27. 10. Willoughby, A. A., Int. .T. Fracture, 15 (1979) 25-126. 11. GB 2038-80, Test Method of Fracture Toughnessfor Ductile Metallic Materials by Means of J-resistant Curve, National Standard of China, 1982. 12. Kussmaul, K., West German research programs on irradiation effects on reactor pressure vessels. In Radiation Reactor
Embrittlement and Surveillance of Nuclear Pressure Vessels, ASTM STP819, 1983,
pp. 16-28. 13. Susokida, H., Satoh, M., Ando, K., Sakaguchi, Y., Fukuhara, A., Tanabe, J. & Ando, Y. Evaluation of the criterion for lateral expansion of steel for nuclear components. In 4th Znt. Conj on Pressure Vessel Technology, London, May 1980,C54/80, pp. 361-8.
0308-0161(94)00015-8
ELSEVIER
Ves. & Plpg
62 (1995)
219-225
0 1995 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 030%0161/95/$09.50
The reasonable design of Charpy-size specimen for fracture toughness test in nuclear surveillance X. IP. Zhang & Y. W. Shi Department
of Mechanical
Engineering, Wan Jiaotong University, People’s Republic o,f China
Xi’an,
Shaanxi
Province,
710049,
(Received 1 January 1994;accepted21 March 1994)
Use of a precracked and side-groovedCharpy-size specimenis an economical and convenient single-specimenmethod of evaluating the elastic-plastic fracture toughnessof nuclear pressurevesselsteels.This paper hasstudied the influence of side-groovedepth on fracture toughnessand stable increment of crack on several pressurevesselsteelsin detail. Test resultsare comparedwith those of large-sizespecimenswhich the National Standard of China (i.e. GB 2038) requires. The researchresults indicate that using a precracked Charpysize specimenwith side-groovedepth 30% and adopting the energy before the maximum load of a three-point bending test curve, the elastic-plastic fracture toughnessof materialscan be evaluated conservatively when the crack begins to propagate, and the reasonabledesignof a Charpy-size specimenfor fracture toughnesstestsin nuclear surveillancehasbeen established.At the sametime, the constraint effect and thickening action of the side-grooveon a Charpy-size specimenhave also been discussed,and theoretically explained.
1 INTRODUCTION
vessel and retrieved periodically to monitor the degradation of toughness and other mechanical properties of pressure vessel materials due to neutron irradiation. In the past, the surveillance specimens that were put into pressure vessels were almost always small specimens for conventional tensile tests and impact tests, such as round tensile specimens, and Charpy specimens, respectively.’ Along with fracture mechanics development, the surveillance of fracture toughness of pressure vessel materials and its welded joints becomes more important for evaluating the safety and integrity of nuclear pressure vessels in modern times. Fracture toughness is as important to monitor as the activity of defects and cracks in a pressure vessel. In surveillance programmes, fracture toughness tests and dynamic tearing tests using compact tensile specimens and precracked Charpy-size specimens have been found to be a practical way of monitoring pressure vessel materials in some nuclear power stations.2” In the surveillance test of fracture toughness,
During the operating process, pressure vessels and other components of nuclear power stations become embrittled due to neutron irradiation and heat-ageing at high temperature as well as strain-ageing. In order to evaluate the structural integrity of nuclear power stations, it becomes more important to research and estimate the embrittlement of pressure vessel material properties, which change with operating time or neutron irradiating dose. In recent ye:ars, an important content in surveillance programmes is to monitor periodically the irradiated embrittlement of pressure vessel materials and its welded joints, to put forward tolerance limits on defects in a pressure vessel under operating state, and to predict the safety and the future lifetime of the reactor pressure vessel. Due to the hi,gh costs of space at the reactor core, the surveillance specimens’ size should be small. In a nuclear power station, a great number of small surveillance specimens are put into the pressure 219
220
X. P. Zhang, Y. W. Shi
the method of using a small single specimen is the most reasonable one. In order to avoid adopting the complicated physical method for monitoring initial growth of a crack, more research work has been done. It has been pointed out that a precracked Charpy-size specimen with a deep side-groove is a cheap single-specimen method for estimating elastic-plastic fracture toughness of nuclear pressure vessel steels.6,7 In this method, the fracture toughness of the maximum load on a load versus load-point displacement curve is regarded as the initial fracture toughness. Moreover, the effects of side-groove depth and geometric shape on elastic-plastic fracture toughness parameters in precracked Charpy-size specimens have also been studied.8,9 However, some results for a mild steel indicated that slow crack propagation exceeded 1 mm if the fracture toughness at maximum load was regarded as the critical value of initiation and with the side-groove depth reaching 40 to 60% of the specimen thickness.” This paper has studied the effects of sidegroove depth on fracture toughness of maximum load (JJ on the load versus load-point displacement curve and the relative growth of crack depth (Aa,,,) corresponding to the maximum load for several common pressure vessel steels by using precracked Charpy-size specimens. Comparison with that of large specimens which GB 2038” requires has also been made. In the end, the reasonable design of a Charpy-size specimen for fracture toughness tests in nuclear surveillance has been established. Furthermore, Table Materials A508CL3-A A508CL3-B BHW3.5
1. Chemical
A508CL3-A A508CL3-B BHW3.5 A508CL3-A’
2 TEST MATERIALS
AND PROCEDURE
2.1 Materials and specimen preparation The materials for tests are typical heavy-section nuclear pressure vessel steels A508CL3-A, A508CL3-A’ and A508CL3-B produced by two plants, as well as BHW3.5 steel. The chemical composition and mechanical properties of four test materials are given in Tables 1 and 2. A508CL3-A and A508CL3-A’ steels have the same chemical composition, the only difference is that A508CL3-A’ has undergone a special heat treatment which was equivalent to the embrittlement of pressure vessel steel irradiated at a 3 X 101’ n/cm2 neutron dose.‘2,J3 The standard precracked Charpy-size specimen is shown in Fig. l(a). The length of fatigue crack (a/w) is 0.45 to O-55. After precracking, the side-grooves with different relative depth were machined on both sides of the specimens in the shape of a V-notch (60” angle) and with a root radius of O-1mm, as shown in Fig. l(b). The orientation of all specimens is in L-S.
composition
of test materials
(wt%)
C
Mn
Si
S
P
Ni
Cr
MO
V
Cu
Al
0.14 0.20 0.14
1.29 1.29 1.32
0.27 0.24 0.34
0.006 0.007 0.012
0.004 0.008 O-019
0.98 0.75 0.86
0.05 0.06 0.39
0.51 0.50 0.27
0.03 0.004
0.08
0.012
Table Materials
this paper also discusses the constraint effect and thickening action of the side-groove on Charpysize specimens. Methods to calculate the constraint coefficient, equivalent thickness and additional thickness of side-grooved specimens are put forward.
2. Mechanical
fly WV 540 515 586 893
properties
auTs(MPa) 642 650 664 953
“S,( %) is percentage elongation. “@(%) is percentage reduction in area. ‘Strain hardening exponent. dVickers hardness.
of test materials S5 (%y
Cp(%)”
nc
HVd
22 24 25 20
73 66 68 69
6.5 6.9 8.0 17.7
220-230 245-255 260-275 365-380
221
Charpy-size specimen for fracture toughness test in nuclear surveillance
requires, B = 20 mm, W = 24 mm, the span is 96 mm (4W), a/W = 0.5, and the J, resistance curves were measured with blunting line as follows:
.Fat igue crack flotch
J = 3a, * Aa
(4
where (T, is flow stress, gf = 0*5(a, + (T&, (TVis yield stress, (TUT, is ultimate tensile strength, and Aa is the stable propagation length of the crack. The valid Jlc can be obtained from JR resistance curves.
Bn
(b) Fig. 1. Precracked Charpy-size specimens (unit: mm): standard specimen; (b) with side-groove.
(a)
3 RESULTS
AND
DISCUSSION
3.1 Effect of side-groove toughness parameters 2.2 Experimental
(4)
depth
on fracture
procedure
Tests were carried out at room temperature in an Instron 1195 electronic tensile testing machine with a loading speed of 0.5 mm/min. The span between supports was 40 mm. During the testing process, the curve of load versus lo,ad-point displacement was recorded. The specimen was unloaded when the load reached the mlaximum point, and then the specimens were broken at liquid nitrogen temperature. The initial crack depth (a,) and the relative growth of crack depth (Au,,,) corresponding to the maximum load were measured by means of a five points average method. The relative depth of the side-groove is defined as follows: d, = [(B -&JIB]
x 100%
(1)
where B is the specimen thickness, and 13, is the net thickness of cross section of specimen, as shown in Fig. 1. The maximum load a.bsorbed energy, &, can be calculated from the load versus load-point displacement curve, and the J-integral value of maximum load, J,, can be expressed as: Jm= alIIRl(~
- a,)]
(2)
where W is specimen width. The elastic-plastic fracture toughness K,, can be calculated by K,, = d[E * J,/(l - Y*)]
(3)
where E is Young’s modulus of elasticity, and Y is Poisson’s ratio. For large standard specimens, which GB2038
Figure 2 shows the results of fracture toughness of maximum load (Jm) changing with side-groove depth (dS). For the four kinds of materials, it can be seen that the experimental curves have the same trend, where J, values decrease rapidly when the side-groove depth increases. As soon as the d, reaches the critical value (d,),, the J, value also achieves the critical value (J&, and stays constant. For a certain material, when the d, value is larger than the (d,), value, the J,,, value will keep constant, which is the so-called platform value (J,&. The characteristic values of experimental curves for the four kinds of materials, (d&, (J& and (J&, are shown in Table 3. The fracture toughness values corresponding to a side-groove depth of 30%, (Jm)30, are also shown in Table 3. Figure 3 shows the results of Au, changing with side-groove depth. The Aa, values drop gradually when the side-groove depth increases, and the curves have no abrupt changes. At the critical side-groove depth, the critical lengths of crack increment for the four materials are also shown in Table 3. For precracked Charpy-size specimens with a side-groove, the slow crack growth is very short when the side-groove depth reaches the critical value (d,),. For the specimen with critical side-groove depth, the slow crack growth is 4.5, 4.7, 2.7 and 4.5% of the initial crack length a, for A508CL3-A, A508CL3-B, BHW35 and A508CL3-A’ steels, respectively, when it is assumed that a, = 5 mm. It is clear that the crack growth is very small, and the maximum load point almost approaches the initiation.
X. P. Zhang, Y. W. Shi
222
1
d””
o A508CL3-A
600-o l
A508CL3-B
o A508CL3-A .
A508CL3-B
il; , , , , , , 1
100 t
t O0
I 10
I 20
I 30
I 40
I 50
, 60
-I
ds (%) 10
20
30
40
50
60
d, (o/o)
(a)
.I”” I
I
Ob 0
J ’ 0
I lo
I
I
20
30
I
40
I
*
50
60
d,
(%)
o A508CL3-A'
1
1
I
I
I
1
10
20
30
40
50
60
ds (%) (b) Fig. 3. Results of Aa, vs. side-groove depth: (a) A508CL3-A and A508CL3-B steels; (b) BHW35 and A508CL3-A’ steels.
(b) Fig. 2. Resultsof J,,, vs. side-groovedepth: (a) A508CL3-A
and A508CL3-B steels;(b) BHW35 and A508CL3-A’ steels.
3.2 Thickening action and constraint of sidegroove in precracked Charpy-size specimen
The side-groove in a specimen is equivalent Table
Materials A508CL3-A A508CL3-B BHW35 A508CL3-A’
(d&W) 25 30 20 20
to
3. The characteristic
(.hJc @J/m*) 305.1 269.0 148.0 182.8
thickening the specimen, and it can strengthen the level of stress triaxiality at the crack tip along the thickness direction of the specimen. It also changes the stress state at the crack tip, and makes the mixed stress state in a small Charpy-size specimen become more planevalues of test results
(A& @J/m’) 299.8zt 14.8 265.0f 10.6 149.8f 15.8 174.0f 12.5
GAI @J/m’) 303.5 269.0 157.2 179.3
WA
6-d 226 234 134 202
Charpy-size
specimen folr fracture
strain state. The research results on a 1CrMoV steeP9 indicated that the region of plane strain got to 88% of the whole thickness of the specimen when the side-groove depth in a precracked Charpy-size specimen was 20%. By contrast, the region of plane strain of a standard specimen was only 25% of the whole thiclkness of the specimen, i.e. in the centre of the specimen there existed a narrow region of plane strain. The effect of a side-groove on thickening the precracked Charpy-size specimen may be expressed in terms of equivalent thickness B, as follows:8’9 B, = B,[l + 0*67(B,/B)(l
toughness
test in nuclear surveillance 26
; E *
24
^, es
20 '
16
& 12 8
- B,/B)]I
(5) The curves for B, and B, changing with d, are shown in Fig. 4. Considering that the different materials have different sensitivities to sidegrooves, eqn (5) should be revised to include the strain hardening exponent y1 expressed by the Ramberg-Osgood equation and the ratio of ((T~/(T& as follows: (Be), = Bn[l + @Wqk,,,) x (WW - BnIWI (6) where (B,), is the revised equivalent thickness. Using eqn (6), curves of (B,), and B, with respect to d, are shown in Fig. 5. If B, is subtracted from both sides of eqn (6), we obtain AB = (B,), - B, = 0~67B,(na,/a,,,) x K&W - (WB2)1 (7) where AB is the additional thickness of the specimens caused by the constraint effect of side-grooves. We then calculate the first deriva-
I
1
223
ds l%:, Fig. 5. Curves of (B,), and B, vs. side-groove
tive of AB with respect to B, using eqn (7), and the maximum value of B, can be obtained from d(AB)/dB,
= 1.34(B,/B) - 2.01(B,/‘B)’ = 0
Therefore, when B, equals 6.67 mm (i.e. d, equals 33% of the specimen thickness), the maximum additional thickness of the precracked Charpy-size specimen is obtained, as shown in Fig. 5. For A508CL3-A, A508CL3-B, BHW35 and A508CL3-A’ steels, the maximum values of additional thicknesses are 5.4, 5.3, 9.4 and 22.0 mm, respectively, and the equivalent thicknesses are 12.1, 12.0, 16.1 and 28.7 mm, respectively. It is found that the constraint effect of the side-groove is explicit in specimens. 3.3 Constraint
coefficient
It is evident that, at the crack tip, there exists the constraint effect from the side-groove. In order to evaluate the level of plastic constraint of side-grooves at a crack tip and make the constraint effect of side-grooves quantitative and convenient, the constraint effect of a side-groove can be expressed in terms of the constraint coefficient (C). For precracked Charpy-size specimens with side-grooves, the C value can be written as follows: C = P, +S/[B, . W’ . cruTS* (1 -a/W)‘]
ds
Fig. 4. (‘urves of B, and B, vs. side-groove
Co,b)
depth.
depth.
(8)
where Pmis the maximum load on the load versus load-point displacement curve. The relation of C value changing with d, is shown in Fig. 6. It is found that C values increase gradually when the side-groove depth increases, i.e. the plastic
224
X. P. Zhang, Y. W. Shi
J-integral values keeping steady. The experimental results have proved this.
C.L” O A508CL3-A
c 2.00
A508CL3-B
l
3.4 Effect of material depth of side-groove
I-
1.20'
0
10
20
30
40
50
60
ds t%) (a)
o BWH35
G 2.00
le2*L 0
l
I 10
1 20
I 30
A508CL3-A'
I 40
I 50
I 60
embrittlement
on critical
With neutron irradiation and heat-ageing as well as strain-ageing, the plasticity and toughness of nuclear pressure vessel steels would decrease gradually. Comparing A508CL3-A steel with the A508CL3-A’, which had undergone a heat treatment simulating neutron irradiation, the strength of the latter steel increased and the plasticity as well as fracture toughness decreased, as shown in Fig. 2 and Table 3. For A508CL3-A steel, (d,), is 25% and (J,& equals 305.1 kJ/m2; while (d,), is 20% and (J& equals 1828 kJ/m2 for the A508CL3-A’ steel. If the side-groove depth in the precracked Charpy-size specimen is selected as 25% for A508CL-A’ steel, then the & value corresponding to the d, (i.e. (Jm)&, equals 179.3 kJ/m’, and the difference between (J& and (J& is less than 2%. If a side-groove depth of 30% in a precracked Charpy-size specimen is adopted for all test materials which have different strengths, and the J, value corresponding to d, of 30% is expressed as (Jm)3o, the difference between (J&, and (J& is very small, as shown in Table 3. Therefore, a constant side-groove depth of 30% is suitable for the whole surveillance process of neutron irradiation embrittlement of a nuclear pressure vessel.
ds 1%) (b) Fig. 6. Relation of constraint coefficient vs. side-groove depth: (a) A508CL3-A and ASOSCL3-Bsteels;(b) BHW3.5 and A508CL3-A’ steels.
constraint increases. When the side-groove depth reaches and exceeds the critical value (d,),, the C value becomes constant. The critical J-integral values depend upon the stress state at the crack tip. For the precracked Charpy-size specimen with side-grooves, C values can be used to evaluate the stress state under the action of side-groove constraint quantitatively. That the C value becomes constant, means the stress state is steady at the crack tip along the thickness direction of the specimen. Correspondingly, the maximum load f-integral values (Jm) become constant. This is due to the constancy of C values, which results in the maximum load
3.5 Valid
elastic-plastic
fracture
toughness
The JR resistance curves for A508CL3-A, A508CL-B, BHW35 and A508CL3-A’ steels were measured, and the results of valid Jlc are shown in Table 4. According to the slope of the JR resistance curves, the tearing modulus, TM (no dimension), can be obtained as follows: TM= (Elaf)(dJlda) (9) The TM values and the K,, values converted from valid JIc by eqn (3) are also shown in Table 4. In Table 4, J,,, values corresponding to the critical side-groove depth in a precracked Charpy-size specimen, and the K,, values converted from the J, are shown for comparison with valid fracture toughness values. It can be observed that J, values are a bit smaller than valid JIc values. For A508CL3-A, A508CL3-B, BHW35 and A508CL3-A’ steels, the K,, values converted
Charpy-size Table
Materials A508CL3-A A508CL3-B BHW35 A508CL3-A’
specimen for fracture
4. The comparison
Valid .I,= (kJ/m’) 322.4 304.7 203.0 210.4
between
valid &(&)
’ gt%) 270.1 26;!.6 214.4 218.2
225
toughness test in nuclear surveillance
TM
(Jr&
4 CONCLUSIONS (1) Side-grooves in precracked Charpy-size specimens have the action of thickening the specimen, and for a certain material there exists an optimal side-groove depth at which the specimen gains the maximum additional thickness. (2) The critical value of side-groove depth, (d,),, determined by curves of J, versus d,, is the same as that determined through curves of C values versus d,. In fact, the tendency of J, versus d, depends upon that of C values versus 4. (3) Constraint of the side-grooves may be quantitatively evaluated by means of constraint coefficient (C) values while the side-groove depth changes. (4) The systematic studies of the effect of side-groove depth on fracture toughness parameters indicate that, according to the maximum load absorbed energy of load versus load-point displacement curve, the conservative vallues of elastic-plastic fracture toughness for initiation of ductile fracture can be obtained. In the experimental process, it is not necessary to monitor crack propagation by means of a special and complicated method. It is a single-specimen method with a small specimen size that is especially suitable for surveillance of the fracture toughness of a nuclear pressure vessel subject to embrittlement by neutron irradiation.
K,,
(kJ/m’)
331.0 305.3 298.2 295.4
from J, values are smaller than valid K,, values by 2.7, 5.7, 13.8 and 6.5%, respectively. Nevertheless, according to the maximum load absorbed energy E,, the conservative values for elastic-plastic fracture toughness of pressure vessel materials can be obtained by mea.ns of a precracked Charpy-size specimen with a deep side-groove. This is a convenient single-specimen method that is especially suitable for surveillance of embrittlement of a nuclear pressure vessel due to neutron irradiation.
and J,(converted
K,,)
(converted from (Jm)c) (MPaV&)
305.1 269.0 148.0 182.8
262.8 247.6 184.6 204.4
ACKNOWLEDGEMENT The authors greatly acknowledge the financial support from the National Nuclear Safety Bureau of China.
REFERENCES 1. ASTM El8582, Standard Practice for Conducting Surveillance Tests for Light-water Cooled Nuclear Power Reactor Vessels.E706(1F), ASTM, Philadelphia, 1982,pp. 350-9. -.3 ASTM E636-83, Standard Practice for Conducting Supplemental Surveillance Tests for Nuclear Power Reactor Vessels,E706(1H), ASTM, Philadelphia, 1983, pp. 318-32. 3. Varga, T. & Njo, D. H., Selectionof specimentypes for irradiation surveillance programs. Radiation Embrittlement and Surveillance of Nuclear Reactor Vessels, ASTM STP819,1983,pp. 166-73.
Pressure
4. Davies, L. M. & Ingham, T., Overview of studiesin the United Kingdom on neutron irradiation embrittlement of pressurevessel steels. Radiation Embrittlement and Surveillance
of Nuclear
Reactor
Pressure
Vessel Steels.
ASTM STP909,1986,pp. 13-33. 5. Ahlstrand, R. Torronen, K., Valo, M. & Bars, B., Surveillance programs and irradiation embrittlement researchof the Loviisa nuclear power plant. Radiation Embrittlement and Surveillance of Nuclear Reactor Pressure Vessel Steels, ASTM STP909, 1986,pp. 55-69. 6. Ritchie, R. O., Server, W. L. & Wullaert, R. A., Znt. J. Fracture, 14 (1978) 139-42. 7. Server, W. L., Wullaert, R. A. & Ritchie, R. O., J. Eng. Mater. and Technol., Trans. ASME, 102 (1980) 192-9. 8. Neale, B. K., Znt. J. Pres. Ves. and Piping, 10 (1982)
375-98. 9. Neale, B. K., Znt. J. Pres. Ves. and Piping, 12 (1983) 207-27. 10. Willoughby, A. A., Int. .T. Fracture, 15 (1979) 25-126. 11. GB 2038-80, Test Method of Fracture Toughnessfor Ductile Metallic Materials by Means of J-resistant Curve, National Standard of China, 1982. 12. Kussmaul, K., West German research programs on irradiation effects on reactor pressure vessels. In Radiation Reactor
Embrittlement and Surveillance of Nuclear Pressure Vessels, ASTM STP819, 1983,
pp. 16-28. 13. Susokida, H., Satoh, M., Ando, K., Sakaguchi, Y., Fukuhara, A., Tanabe, J. & Ando, Y. Evaluation of the criterion for lateral expansion of steel for nuclear components. In 4th Znt. Conj on Pressure Vessel Technology, London, May 1980,C54/80, pp. 361-8.