Transferability of Laboratory Fracture Data to Safety Assessment of Postulated Defects in Critical NPP Components
For strength evaluation of reactor pressure vessels, large-scale benchmark tests have been conducted on determination of fracture toughness of specimens with internal defects. The obtained results provide a good check on the transferability of data on standard materials to the assessment of real and...
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Taylor, N. Nilsson, K.F. 2013-07-09T15:50:39Z 2013-07-09T15:50:39Z 2004 Transferability of Laboratory Fracture Data to Safety Assessment of Postulated Defects in Critical NPP Components / N. Taylor, K.F. Nilsson // Проблемы прочности. — 2004. — № 1. — С. 61-67. — Бібліогр.: 8 назв. — англ. 0556-171X https://nasplib.isofts.kiev.ua/handle/123456789/47063 539.4 For strength evaluation of reactor pressure vessels, large-scale benchmark tests have been conducted on determination of fracture toughness of specimens with internal defects. The obtained results provide a good check on the transferability of data on standard materials to the assessment of real and postulated flaws in reactor pressure vessels. The Master Curve method makes it possible to get more reliable predictions of ductile-brittle transition, in comparison with approaches using KIc as function of RTndt. С целью оценки прочности корпусов реакторов АЭС проведены натурные стендовые испытания на трещиностойкость образцов с внутренними дефектами. Представленные результаты свидетельствуют о возможности использования в расчетах полученных стандартных данных для обнаруженных и предполагаемых дефектов корпусных сталей. Показано, что применение метода "Master Curve” обеспечивает более точное прогнозирование условий вязкохрупкого перехода, чем подходы, использующие характеристику KIc в зависимости от RTndt. Із метою оцінки міцності корпусів реакторів AEC проведено натурні стендові випробування на тріщиностійкість зразків із внутрішніми дефектами. Представлені результати свідчать про можливість використання в розрахунках отриманих стандартних даних для виявлених дефектів корпусних сталей та прогнозованих. Показано, що метод “Master Curve” забезпечує більш точне прогнозування умов в ’язкокрихкого переходу, ніж підходи, в яких використовується характеристика KІс в залежності від RTndt. The authors greatly acknowledge the contributions from all the Network partners, in particular, Steering Committee members and their organizations, as well as the support from the Joint Research Center of the European Commission. en Інститут проблем міцності ім. Г.С. Писаренко НАН України Проблемы прочности Научно-технический раздел Transferability of Laboratory Fracture Data to Safety Assessment of Postulated Defects in Critical NPP Components Применимость результатов экспериментальных исследований на трещиностойкость для оценки прочности ответственных узлов АЭС Article published earlier |
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| title |
Transferability of Laboratory Fracture Data to Safety Assessment of Postulated Defects in Critical NPP Components |
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Transferability of Laboratory Fracture Data to Safety Assessment of Postulated Defects in Critical NPP Components Taylor, N. Nilsson, K.F. Научно-технический раздел |
| title_short |
Transferability of Laboratory Fracture Data to Safety Assessment of Postulated Defects in Critical NPP Components |
| title_full |
Transferability of Laboratory Fracture Data to Safety Assessment of Postulated Defects in Critical NPP Components |
| title_fullStr |
Transferability of Laboratory Fracture Data to Safety Assessment of Postulated Defects in Critical NPP Components |
| title_full_unstemmed |
Transferability of Laboratory Fracture Data to Safety Assessment of Postulated Defects in Critical NPP Components |
| title_sort |
transferability of laboratory fracture data to safety assessment of postulated defects in critical npp components |
| author |
Taylor, N. Nilsson, K.F. |
| author_facet |
Taylor, N. Nilsson, K.F. |
| topic |
Научно-технический раздел |
| topic_facet |
Научно-технический раздел |
| publishDate |
2004 |
| language |
English |
| container_title |
Проблемы прочности |
| publisher |
Інститут проблем міцності ім. Г.С. Писаренко НАН України |
| format |
Article |
| title_alt |
Применимость результатов экспериментальных исследований на трещиностойкость для оценки прочности ответственных узлов АЭС |
| description |
For strength evaluation of reactor pressure vessels, large-scale benchmark tests have been conducted on determination of fracture toughness of specimens with internal defects. The obtained results provide a good check on the transferability of data on standard materials to the assessment of real and postulated flaws in reactor pressure vessels. The Master Curve method makes it possible to get more reliable predictions of ductile-brittle transition, in comparison with approaches using KIc as function of RTndt.
С целью оценки прочности корпусов реакторов АЭС проведены натурные стендовые испытания на трещиностойкость образцов с внутренними дефектами. Представленные результаты свидетельствуют о возможности использования в расчетах полученных стандартных данных для обнаруженных и предполагаемых дефектов корпусных сталей. Показано, что применение метода "Master Curve” обеспечивает более точное прогнозирование условий вязкохрупкого перехода, чем подходы, использующие характеристику KIc в зависимости от RTndt.
Із метою оцінки міцності корпусів реакторів AEC проведено натурні стендові випробування на тріщиностійкість зразків із внутрішніми дефектами. Представлені результати свідчать про можливість використання в розрахунках отриманих стандартних даних для виявлених дефектів корпусних сталей та прогнозованих. Показано, що метод “Master Curve” забезпечує більш точне прогнозування умов в ’язкокрихкого переходу, ніж підходи, в яких використовується характеристика KІс в залежності від RTndt.
|
| issn |
0556-171X |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/47063 |
| citation_txt |
Transferability of Laboratory Fracture Data to Safety Assessment of Postulated Defects in Critical NPP Components / N. Taylor, K.F. Nilsson // Проблемы прочности. — 2004. — № 1. — С. 61-67. — Бібліогр.: 8 назв. — англ. |
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| fulltext |
UDC 539.4
Transferability of Laboratory Fracture Data to Safety Assessment of
Postulated Defects in Critical NPP Components
N. Taylor and K. F. Nilsson
Joint Research Centre of the European Commission, Institute for Energy, Petten, the
Netherlands
УДК 539.4
Применимость результатов экспериментальных исследований на
трещиностойкость для оценки прочности ответственных узлов
АЭС
Н. Тейлор, К. Ф. Нилссон
Объединенный исследовательский центр Европейской комиссии, Энергетический
институт, Петтен, Нидерланды
С целью оценки прочности корпусов реакторов АЭС проведены натурные стендовые испы
тания на трещиностойкость образцов с внутренними дефектами. Представленные резуль
таты свидетельствуют о возможности использования в расчетах полученных стандарт
ных данных для обнаруженных и предполагаемых дефектов корпусных сталей. Показано,
что применение метода "Master Curve” обеспечивает более точное прогнозирование усло
вий вязкохрупкого перехода, чем подходы, использующие характеристику KIc в зависимости
от RTndt.
Клю чевые слова : конструкционная целостность, механика разрушения,
стендовое испытание, “Master Curve”.
Introduction. Dealing with real or postulated cracks in large structures is a
classical engineering problem, which takes on special significance for
components in nuclear power plants. While regulators, utilities, and plant
manufacturers have developed effective procedures to assess structural integrity, a
policy of continuous development is required to ensure that safety margins are
maintained as the plants accumulate many years of service. Measurement of
fracture toughness parameters on compact laboratory specimens plays a key role
in this process, and there is now wide acceptance of methods such as Master
Curve to assess shifts in the transition behavior from as-received to end-of-life
states. However, there remain obstacles to the application of such
laboratory-generated representations of the fracture transition behavior for
assessing postulated defects in critical components such as reactor pressure
vessels, the so-called transferability factors:
♦ Constraint due to geometry
♦ Constraint due to loading
♦ Crack front size (cleavage site sampling effect)
♦ Representativeness of materials data and fracture mode consistency
© N. TAYLOR, K. F. NILSSON, 2004
ISSN 0556-171X. Проблемы прочности, 2004, № 1 61
N. Taylor and K. F. Nilsson
♦ Local variations and/or gradients in material properties
♦ Environment.
Transferability has been addressed by several projects organized by the
Network for Evaluation of Structural Components (NESC). The NESC is an
international network operated by the European Commission’s Joint Research
Center to verify the overall structural integrity assessment process using
large-scale experimental projects, designed as benchmarks. The NESC has four
major projects: NESC-I, the spinning cylinder pressurized thermal shock (PTS)
test was completed in 2000. In NESC-II, two PTS tests on cylinders with shallow
cracks were completed in 1999. NESC-III concerns a large-scale test on a
dissimilar weld pipe assembly performed in February 2003. The NESC-IV project
completed a test series on defect-containing beams in 2002. This paper focuses on
the consideration given to transferability issues for RPV flaws in the NESC-I, II
and IV projects.
Pressurized Therm al Shock (PTS) Tests. A characteristic feature of PTS
simulations is that the highest probability for defect extension is in the
near-surface region, where its assessment is complicated by the variation of the
material and fracture toughness properties, the HAZ size, the loss of the defect-tip
constraint, warm prestressing effects, and clad residual stresses. The first NESC
project [1] centered on the spinning cylinder experiment conducted at the AEA
Technology, UK, and was designed to simulate the conditions associated with an
aging flawed RPV. The test piece was an internally clad 7-ton steel cylinder into
which a total of 18 defects, differing widely in fabrication method, size, and
location, were introduced. During the test, it was subject to mechanical loads due
to high-speed rotation and thermal shock loads resulting from a cold-water
quench directed at the inner surface of the heated cylinder (Fig. 1). For the
fracture analysts it posed a special challenge both a) in the test design stage, to
ensure a combination of defect size, material condition, and loading that would
produce crack initiation and b) in the post-test analysis phase, to assess the
observed behavior of the defects in greater detail. The test was successfully
performed in March 1997 and realized the planned cleavage run-and-arrest event
at one end of a large through-clad defect. Although the defect growth also
occurred at the large sub-clad crack as planned, the expected cleavage event
failed to materialize.
In the post-test analysis, the Master Curve approach (which had been
calibrated in an extensive material characterization program) provided a useful
tool for assessing the sensitivity to the analysis parameters via comparison of the
probability of cleavage for the given values of the crack-tip driving force and
temperature (Table 1). The best estimate of 0.97 was obtained using a 3-D
cracked body analysis to determine K j and applying the Master Curve function
derived from a deep notch, i.e., high constraint specimens. The use of low
constraint shallow notch data reduces the failure probability. However, the
detailed analysis [2] of the local constraint conditions indicates that, at this depth,
small-scale yielding conditions should prevail (Fig. 2). Hence, the higher
constraint data are appropriate for the assessment of the aforementioned crack
front region. This 0.97 level is above that, at which a cleavage event could be
reasonably expected in a single test, implying that this analysis technique is still
62 ISSN 0556-171X. npoÖÄeubi npounocmu, 2004, № 1
Transferability o f Laboratory Fracture Data
somewhat pessimistic. Simpler engineering methods provided even more
pessimistic predictions of the crack driving force and hence even higher
probabilities of cleavage. A coupled cleavage-ductile local approach formulation
with a 3-D elastic-plastic finite-element model was also applied [3]. The observed
time of the cleavage event corresponded well to the maximum predicted cleavage
probability, although the extent of ductile tearing was underestimated.
T a b l e 1
NESC-I Spinning Cylinder Tests: Probability of Cleavage in the Near-Surface Base
Material Region for a Large Through-Clad Defect at the 217th s of the PTS Transient
No. Analysis Type KJ at 217 s
MPaVm
Probability
of cleavage
1 PD6493 420 1.0
2 2-D FE + weight function 350 0.999996
3 2-D FE + weight function + reduced residual stress 320 0.9999
4 R6 Appendix 4 310 0.9996
5 3-D elastoplastic FE analysis: high constraint
(a/W = 0.5) data, standard crack-front normalizing
parameter (25 mm)
260 0.97
6 3-D FE analysis with low constraint toughness data 260 0.23
7 3-D FE analysis with a reduced Master Curve crack
front normalizing parameter (10 mm)
260 0.77
Front View
Fig. 1. NESC-I component and the principal defects considered in fracture assessment.
The NESC-II project considered two large-scale PTS tests performed at MPA
Stuttgart [4]. The test pieces were thick-walled cylinders with an outer diameter
of 800 mm and wall thickness of 190 mm, fabricated of 17MoV 8 4 mod steel
ISSN 0556-171X. npoôëeubi npounocmu, 2004, № 1 63
N. Taylor and K. F. Nilsson
with a two-layer austenitic cladding on the internal surface. The NP2 test piece,
containing a fully circumferential sub-clad defect with a depth of 8 mm, produced
an intergranular crack growth event; with a maximum extension of approximately
15 mm. In the case of the NP1 test piece containing two shallow semi-elliptical
through-clad defects of depth 21 mm and length 60 mm, the planned loading
transient was achieved but no growth occurred. While the results of the two tests
underline the inherent conservatism of the existing defect assessment procedures
for shallow RPV flaws, the absence of the desired cleavage event shows that the
transferability issues (interpretation of material test data and quantifying the
effects of crack front length and constraint loss effects) have not been sufficiently
resolved.
HAZ- i G
^ -
' 1 Clad
------ H _
■ f
" 1 SSY solution ■
, -— ------------- -
f ,
'</. 1 Surface crack, t = 240 s '
I I : . !
" • • • i i • • i 1 ■ i • - i • •
0 15 30 45 60 75 90
6,0
Fig. 2. Variation of constraint parameters H and Q along the boundary of a large through-clad
defect, showing the area of loss of constraint close to the surface.
Isotherm al Tests on Biaxially-Loaded Beams. The NESC-IV project is a
focussed experimental/analytical program for the assessment of an RPV shallow
flaw. Two series of benchmark tests [5] have been performed at the ORNL facility
in the US. In Part A, six clad cruciform specimens (Fig. 3) containing shallow
surface-breaking flaws located in the weld material were successfully tested. For
Part B, further four beam tests were performed using an innovative test piece
design with a simulated embedded flaw. The material used was removed from a
production-quality RPV that had never been put in service, hence representing a
start-of-life condition (T0 for the weld is — 88°C). The European partners have
contributed with extensive material characterization testing using standard C(T)
25 mm specimen geometry and 10x10 mm SE(B) bend bars, as well as pre-test
calculations to select an appropriate test temperature in the lower transition
region. Figure 4 shows provisional results together with the Master Curve from
standard fracture deep-notch testing. The data for both test series have been size
corrected to a 25 mm crack front as per ASTM E1921.
Detailed FE analysis has shown that, in the uniaxially loaded beams, the
near-surface crack tip encounters a low constraint situation (Table 2), and this
correlates with the fact that in the transition regime the data points lie above the
Master Curve. Table 2 also indicates that biaxially loaded cruciform beams
present a high constraint situation. Although the standard Master Curve should be
appropriate, the tests appear to have produced lower stress intensities than
expected at fracture.
64 ISSN 0556-171X. npoÖÄeubi npounocmu, 2004, № 1
Transferability o f Laboratory Fracture Data
T a b l e 2
g-Values Evaluated at r/(Y/ aY) = 2.0 for a NESC-IV Cruciform Specimen (Biaxial
Bending) and an Embedded Flaw Specimen (Uniaxial Bending)
Specimen type Case Q
Single-edge notch bend beam specimen SE(B) a/W = 0.50 + 0.05
a/W = 0.20 -0.34
a/W = 0.10 -0.68
Semi-elliptical defect under biaxial-loading deepest point -0.20
HAZ -0.05
Embedded flaw under uniaxial bending deep tip -0.35
shallow tip -1.05
Fig. 3. The ORNL cruciform beam design used for the NESC-IV tests.
Normalized temperature, T —T0 (°C)
Fig. 4. Data (crack front size-corrected to 25 mm) from the biaxial and uniaxial bend tests compared
with the Master Curve for deep notch specimens.
Detailed post-test fracture analyses are now in progress, including the
application of a Weibull stress model, which was used successfully for predicted
biaxial effects in previous ORNL test series [6]. This work is being done in close
ISSN 0556-171X. npoôëeubi npounocmu, 2004, № 1 65
N. Taylor and K. F. Nilsson
collaboration with the VOCALIST shared cost action project (Validation of
Constraint-Based Methodology in Structural Integrity) [7]. In particular, it is
intended that the results will contribute to the VOCALIST “Best Practice
Handbook for Application of Constraint-Based Procedures in Structural Integrity
Assessment” [8].
C o n c l u s i o n s
1. Large-scale experimental benchmarks have a key role to play in assessing
transferability issues. The NESC network projects aim to address this need.
2. The Master Curve approach provides reliable and less conservative
estimates of fracture events than taking K ic as a function of R T ndt. Local
approaches show promise, but are complex to calibrate and apply.
3. Treatment of biaxial loading effects and crack front size adjustment is still
under investigation, to provide a better basis for deciding if additional safety
margins exist from constraint-loss effects.
Acknowledgements. The authors greatly acknowledge the contributions from
all the Network partners, in particular, Steering Committee members and their
organizations, as well as the support from the Joint Research Center of the
European Commission.
Р е з ю м е
Із метою оцінки міцності корпусів реакторів AEC проведено натурні
стендові випробування на тріщиностійкість зразків із внутрішніми дефек
тами. Представлені результати свідчать про можливість використання в
розрахунках отриманих стандартних даних для виявлених дефектів корпус
них сталей та прогнозованих. Показано, що метод “Master Curve” забезпечує
більш точне прогнозування умов в ’язкокрихкого переходу, ніж підходи, в
яких використовується характеристика K Іс в залежності від RTndt-
1. B. R. Bass, J. Wintle, R. Hurst, and N. Taylor, NESC-1 Project Overview,
European Commission, EN 19051.
2. I. Sattari-Far et al., A Sensitivity Study on the Fracture Analysis o f the
NESC-1 Spinning Cylinder Experiment, NESCDOC TG3 (99) 09.
3. A. H. Sherry et al., “The application of Local Approach to predict the
outcome of the NESC experiment,” ASME PVP, 365, 75-84 (1998).
4. L. Stumpfrock, D. I. Swan, D. Siegele, et al., NESC-IIFinal Report - Brittle
Crack Initiation, Propagation and Arrest o f Shallow Cracks in a Clad Vessel
under PTS Loading, NESCMAN (02) 07, Report EUR 20696 EN (2003).
5. N. Taylor and B. R. Bass, “Overview of NESC-IV cruciform specimen test
results,” in: ASME PVP Conf. (August 2002), Vancouver, Canada (2002).
6. B. R. Bass, W. J. McAfee, P. T. Williams, and W. E. Pennell, “Fracture
assessment of shallow-flaw cruciform beams tested under uniaxial and
biaxial loading conditions,” Nucl. Eng. Design, 188, No. 3, 259-288 (1999).
66 ISSN 0556-171X. Проблеми прочности, 2004, № 1
Transferability o f Laboratory Fracture Data
7. D. Lidbury et al., “VOCALIST - an international program for the validation
of constraint-based methodology in structural integrity,” in: Proc. ICONE 9,
Paper 722.
8. K. F. Nilsson et al., “Validation of constraint-based methodology in
structural integrity: best practice handbook,” in: ASME PVP Conf. (August
2002), Vancouver, Canada (2002).
Received 26. 05. 2003
ISSN 0556-171X. npoôëeuu npouuocmu, 2004, № 1 67
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