Post on 04-Apr-2018
Introduction to RPV Structural Integrity Procedures
William Server ATI Consulting
Assessment of Degradation Mechanisms of Primary Components in Water Cooled Nuclear Reactors: Current Issues and
Future Challenges 29 September – 2 October 2014
CIEMAT, Madrid (Spain)
Main Topics
• Fracture mechanics as applied to structural integrity of RPVs
• Material property requirements for operating RPVs (including surveillance programs)
2
1
What is Fracture Mechanics?
Fracture is a deformation process whereby regions of a
material body separate and load-carrying capacity decreases
significantly, approaching zero for brittle materials
Fracture can be viewed at different levels: macro to nano
(atomistic) scale
Fracture is defined when the applied loading of a cracked
body (crack driving force) exceeds the material’s resistance to
failure (fracture toughness)
Fracture toughness is a material property for a given material
condition (including environmental exposure)
Fracture mechanics is an engineering discipline which
evaluates the behavior of crack-like defects in structures or
components and their effect on integrity
3
1
Brief Overview of Fracture Mechanics
Fracture mechanics was conceived by A.A. Griffith during
World War I
Early applications were limited to fracture in very brittle cracked bodies,
such as glass
Interest intensified when approximately 25% of the all-welded U.S.
Liberty ships in WW II experienced brittle fracture, exposing the urgent
need to understand failure in ferritic structural steels and weldments
First use of fracture mechanics in mid-1950’s for missile and
rocket motor cases
Brittle fracture in these high strength materials
U.S. Sec of Defense requested assistance from the American Society
for Testing and Materials (ASTM)
4
1
Brief Overview of Fracture Mechanics
ASTM formed a special committee in 1959 to develop
technology and test methods for brittle fracture
Linear Elastic Fracture Mechanics (LEFM) technology
developed rapidly
LEFM first applied to fatigue crack growth 1963 and to stress
corrosion crack growth in 1965
In 1965, the special committee changed to standing Committee
E24 on Fracture Testing of Materials (now part of E08
Committee on Fatigue and Fracture)
Power generation equipment manufacturers and utilities also
became interested since brittle fracture was unacceptable
5
1
Brief Overview of Fracture Mechanics
Fracture mechanics has been used as the basis for ASME
Boiler & Pressure Vessel Code, Section III, Appendix G, fracture
prevention criteria dating back to about 1970 – provides the
methodology for development of pressure-temperature (P-T)
operating curves, ASME Section XI flaw evaluation, etc.
For LEFM, EPFM, and HTTDFM, the crack driving force is a
function of the applied stresses, the size of the crack in the
subject body, and body geometry factors
The basic concept: if a material's resistance to failure in the
presence of a sharp crack in a structure is less than the crack-
tip stress-strain conditions imposed by the loading and
geometry conditions, failure will occur
To avoid failure:
Material Resistance (MR) > Crack Driving Force (CDF)
6
1
Elements of Fracture Mechanics
7
1
Variables Affecting Material Fracture Toughness
External and mechanical variables
Temperature
Loading rate
Environment (neutron irradiation, corrosive, etc.)
Material variables
Chemical composition/impurities
Heat treatment
Microstructure
Strength level
Fabrication (welding method, rolling practice, etc.)
Time-temperature metallurgical changes (temper embrittlement)
8
1
Generalized Categories of Fracture Mechanics
9
1
Linear Elastic Fracture Mechanics (LEFM)
LEFM is based on elastic stress analysis of relatively brittle
materials containing infinitely sharp cracks
The intensity of the localized, elastic, stress-strain field in the
vicinity near the crack-tip is described in terms of a singular
term called the stress intensity factor, K
K parameter usually includes a subscript (I, II, or III) which
refers to the 3 different modes of loading a cracked body
Mode I: the opening mode where the cracked body is loaded by normal
stresses (technically the most important loading mode)
Mode II: sliding or in-plane shearing mode
Mode III: tearing mode caused by out-of-plane shear
10
1
Modes of Crack Opening
11
1
Modes of Crack Opening
12
1
Modes of Crack Opening
13
1
Stress Intensity Factor, K
14
KI is the LEFM parameter of concern for RPV integrity
1
Stress Intensity Factor, K
15
1
Stress Intensity Factor, K
Continuum mechanics solutions for prescribed applied
loadings and geometries permit characterization of stress (and
deformation) fields near a crack tip
Functional form of the local asymptotic field includes a scalar
amplitude value of K that can be expressed in Mode I loading
for crack opening in the yy-direction as
16
1
Stress Intensity Factor, K
17
1
Stress Intensity Factor, K
The variables of which K is a function are given in the following
equation:
where KI is the applied fracture driving force, or the
fracture toughness,
is the stress,
a is the flaw size, and
F(a/W) is the geometry factor
18
1
Plane-Strain Fracture Toughness, KIc
KIc is a measure of the plane-strain, brittle fracture resistance
of a material and is commonly referred to as plane strain
fracture toughness
Unstable, rapid crack extension is predicted to occur when the
applied structural K reaches KIc
KIc is a unique material property for a given material condition,
temperature, and loading rate
KIc can be measured in the laboratory using the specimen(s) and test
procedures described in ASTM Standard Test Method E 399 for “Plane-
Strain Fracture Toughness of Metallic Materials” or using the unified
ASTM Standard Test Method E 1820 for “Measurement of Fracture
Toughness” which covers both potential LEFM and EPFM conditions
KIc can be used to evaluate the brittle fracture conditions in any other
linear-elastic loaded cracked body of practical interest so long as it
contains the same material and environmental condition, is loaded at
the same rate, and is at the same temperature as the laboratory test
19
1
Plane Strain vs. Plane Stress
20
Transverse contractions in z-direction are opposed by unyielding faces of crack area resulting in transverse stresses xx and zz ahead of the crack
Plane strain occurs when ezz = 0
Plane stress occurs when zz = 0
Generally, most cracks in structures/components are loaded somewhere in between plane strain and plane stress
1
Static Fracture Toughness of RPV (bcc) Steel
21
1
Crack Arrest Fracture Toughness, KIa
KIa is a measure of the plane-strain crack-arrest toughness of
ferritic steels and represents the level of K at which a rapidly
running crack can be arrested
A crack can initiate in a local region of a structure due to a combination
of factors (e.g., low temp., high stresses, embrittled material, and high
loading rates)
The fast running crack may eventually run into a region of higher
temperature, lower stress, or greater toughness. The crack will arrest
only when the applied K (crack driving force) is less than KIa
KIa is a material property that has a unique value for a given
material at a given temperature – ASTM Standard Test Method
E 1221 for “Determining Plane-Strain Crack-Arrest Fracture
Toughness, KIa, of Ferritic Steels”
22
1 ASME Code KIC Curve
23
1
Normalized KIa and Dynamic KIC Fracture
Toughness Data – ASME Code KIR = KIa Curve
24
1
Comparison of KIa and KIc for RPV Steels
25
1
Material Resistance to Crack Growth
Termination of the life of a component may be based on the
applied stress intensity factor reaching a critical value (KIc),
representing the material's brittle fracture resistance
Useful life of a component depends on the rate of growth of
flaws (cracks) from some subcritical size to the critical size
when K reaches KIc
Recall that K is a function of the applied stress (σ), the cracked
body geometry, and the crack size (a)
In order to utilize LEFM concepts it is necessary to characterize
the material resistance to crack growth in terms of K under
cyclic loading (e.g., fatigue) and/or static loading (e.g., stress
corrosion) conditions
26
1
Fatigue Crack Growth
The rate of crack growth expressed as da/dN (change in crack
size, a, with respect to elapsed cycles of loading, N) depends
primarily on the cyclic range of the applied stress intensity
factor, ΔK, above a threshold ΔKth
ΔK is analogous to Δσ as commonly used in conventional
fatigue analyses
Select a K expression applicable to the geometry and loading
method of practical interest and substitute Δσ for σ in order to
compute ΔK rather than K
For Mode I loading (simple tensile opening of a crack):
KI = σ(πa)½
For fatigue analysis: ΔKI = Δσ(πa)½
27
1
Schematic of Fatigue Crack Growth Behavior
28
1
Stress Corrosion Crack Growth
For some combinations of materials and environments, it is
possible for cracks to grow under sustained (constant) loading
conditions
Two characterization parameters:
Material behavior under constant loading conditions in an aggressive
environment can be characterized in terms of KIscc, a threshold level of K
below which a crack will not grow for a given material and environment
if the applied K level is greater than the KIscc threshold, the material crack
growth rate behavior can be characterized in terms of da/dt versus K
(crack growth rate per unit time as a function of the applied K)
29
1
Stress Corrosion Crack Growth Behavior
30
1
LEFM Material Parameters and Test Methods
31
Parameter Characterizes Comments ASTM Test Method
KIc Plane strain, brittle fracture toughness
Material property, static & dynamic
E 399-09
E 1820-09 (unified)
KIa Plane strain, crack arrest toughness
KI when running crack is arrested
E 1221-06
KIscc Threshold for SCC propagation
Sustained loading and environment
E 1681-03 (2008)
da/dt vs. K Growth rate for SCC Sustained loading and environment
Under development
Kth Fatigue crack growth threshold
Region I crack growth Under development
da/dn vs. K Fatigue crack growth rates Region II crack growth E 647-08
1
General Approach for Applying LEFM
32
1
Limitation of LEFM Concepts
The entire concept of LEFM is based on elastic stress analysis
of cracked bodies and is therefore limited to small-scale local
yielding at the crack tip
The most significant limitation of LEFM is that the amount of
local crack-tip plasticity prior to and/or during the crack growth
or fracture processes must be small by comparison to the K
zone in which the elastic-stress field equations apply
33
1
Limitations of LEFM Led to EPFM
Since LEFM is applicable only when the plastic zone is small, it
is commonly used for some limited situations involving:
High strength
Relatively brittle materials
Mechanical restraint
Heavy section thicknesses
Low temperatures
Extremely high rates of loading
Factors that cause embrittlement of materials (i.e., irradiation damage
and temper embrittlement)
When there is too much local crack-tip plasticity, EPFM is used
34
1
Elastic Plastic Fracture Mechanics (EPFM)
EPFM was developed to address common situations in nuclear
systems:
Sections not sufficiently thick to satisfy very conservative LEFM size
requirements
Temperature range sufficiently high, resulting in much higher levels of
toughness than at lower temperatures
Test specimen size limited as in surveillance capsules (specimen sizes
large enough to meet LEFM requirements would impractical)
The fracture mode of structural steels change from a brittle to a ductile
fracture type with increasing temperature
Austenitic stainless steels that are much too tough for LEFM to be
applicable
35
1
Elastic Plastic Fracture Mechanics (EPFM)
J-integral approach to EPFM is most popular although others
can be utilized, such as crack tip opening displacement (CTOD)
J is a field parameter that defines the plastic stress and strain
intensity in the region around a crack tip
J is a function of stress, strain, crack size, and geometry of the
crack and body
J is directly analogous to K used in LEFM
36
1
Elastic Plastic Fracture Mechanics (EPFM)
JIc defines the level of applied J at the onset of ductile, stable
crack extension during monotonic loading of a precracked
specimen at a temperature within the ductile behavior regime
(i.e., the ductile upper shelf for ferritic materials)
JIc is a basic material property representing a lower-bound
measure of ductile fracture toughness in the presence of an
initial sharp crack (fatigue precrack)
Tearing modulus, T, accounts for sustained stable crack growth
and is treated as a material parameter:
T = [E / o2] (dJ/da)
37
1
Stages of Ductile J-Resistance Curve
38
1
EPFM Material Parameters and Test Methods
39
Parameter Characterizes Comments ASTM Test Method
JIc or Jc Initiation J for ductile
crack extension Material property, static & dynamic
Old E 813, now E 1820-09 (unified); also E 1921-09ce2 for ferritic
steels
J-R Curve Resistance to stable, ductile crack growth
J-a under monotonic loading
Old E 1152, now E 1820-09 (unified)
T0 Ductile-cleavage
transition temperature for ferritic steels
Master Curve application
E 1921-09ce2
da/dn vs. J Fatigue crack growth
rates
Crack extension per
cycle of J Under consideration
HTTDM: da/dt vs. C* Creep crack growth
rate High temperature, time-dependent
E 1457-07e2
1
Two Common Test Specimens for J Testing
40
1
Fracture Behavior of Ferritic RPV Steels
41
1
Master Curve for Ferritic Steels
Master Curve technology is a direct method of determining
cleavage fracture toughness of ferritic steels, as opposed to
the traditional methods of inferring toughness from RTNDT,
reference curves, and Charpy shifts
“Master Curve methodology” refers to a method of
characterizing the cleavage fracture toughness of all ferritic
steels using a universal curve shape
For a ferritic steel, the spread of fracture toughness values
around the median value at any temperature in the transition
region follows a consistent statistical pattern that can be
described using a Weibull three parameter equation
The statistical consistency of a Weibull distribution provides a
means to calculate confidence bounds on cleavage data
Ferritic steels also exhibit a common variation of cleavage
fracture toughness with temperature
42
1
The Master Curve Is Defined Using T0
43
-80 -60 -40 -20 0 20 40 60 800
50
100
150
200
KJc [
MP
am
]
T - T0 [
oC]
Master Curve
KJc
= 30 + 70 exp [0.019 (T-T0)]
1
Master Curve Methodology
Master Curve technology was first incorporated into ASTM Test
Method in 1997
ASTM Standard Test Method E 1921 describes how to measure
the index temperature, T0, for the Master Curve
T0 positions the Master Curve on the temperature scale in
terms of (T – T0) for the steel of interest
ASTM E 1921 permits determination of T0 using specimens as
small as a pre-cracked Charpy V-notch geometry single-edged
notched bend, SE(B) – i.e., irradiated surveillance specimens
44
1
ASTM E 1921 Test Method for Determining T0
45
VG 3
Relation Between KJc Measurements and To
Test 6-10 toughness
specimens at one
temperature
Convert to 1T
equivalence
Calculate median 1T
equivalent KJc from
data
Using the Master
Curve, extrapolate till
median 1T equivalent
KJc is 91 ksi*in0.5
This temperature is To
To
1
2 5
4
2
1
3
4
5
3
0
50
100
150
200
-100 -50 0 50 100
T - T o [oF]
1T
Eq
uiv
. K
Jc
[k
si*
in0.5
]
1
Effect of Specimen Size Adjustment
46
1
Application to Irradiated RPVs
ASME Boiler and Pressure Vessel Code Cases N-629 and N-631
published in 1998 permit use of a Master Curve-based index
temperature as an alternative to RTNDT:
RTTo = T0 + 35ºF [19.4ºC]
Code Case N-629 is for Section XI applications for both
irradiated and non-irradiated RPV steels; Code Case N-631 is
essentially the same for Section III design applications for only
non-irradiated RPV steels
RTTo is a direct measure of reference temperature for irradiated
materials, but application often requires some normalization to
project to slightly different fluences
Other uncertainties and margins can be applied as needed for
the specific application – eg., see IAEA TRS 429 (2005),
Guidelines for Application of The Master Curve Approach to
Reactor Pressure Vessel Integrity in Nuclear Power Plants
47
1
Normalized Fracture Toughness Data
48
No Index RTNDT RTTo
0
100
200
300
400
500
-300 -150 0 150 300
T [oF]
KJ
c
[ksi*
in0
.5]
CVN
1/2T
1T
1.25T
2T
3T
4T
6T
8T
9T
10T
11T
0
100
200
300
400
500
-300 -150 0 150 300
T - RT NDT [oF]
KJ
c
[ksi*
in0
.5]
0
100
200
300
400
500
-300 -150 0 150 300
T - RT To [oF]
KJ
c
[ksi*
in0
.5]
1
Probabilistic/Deterministic Fracture Mechanics
Fracture mechanics analyses can be performed either
deterministically or probabilistically
Deterministic methodology is used when all the input
information to the analysis is considered to be known with
certainty or when conservative estimates provide acceptable
results
Probabilistic fracture mechanics (PFM) has evolved from the
need to provide results more representative of actual situations
rather than conservative lower bound analyses
PFM has been used for reliability analyses of components such
as reactor pressure vessels, piping, steam turbine generator
rotors and gas turbine disks and blades
49
1
Probabilistic/Deterministic Fracture Mechanics
Purpose of PFM is to estimate or bound the reliability [1 -
(probability of failure)] of a component subject to cracking, and
to quantify the influence of engineering and management
decisions on component reliability
The probability of failure is typically defined as the number of
simulations that resulted in failure divided by the total number
of simulations
Deterministic methods assume inputs are known or use
conservative or “worst-case,” estimates which can lead to
multiple, compounded conservatisms giving overly pessimistic
results
PFM combines conventional fracture mechanics calculations
with appropriate statistical methods to minimize stacking of
conservatisms
Monte Carlo sampling techniques are widely used in PFM
50
1
FAVOR Computer Code
The FAVOR computer code (Fracture Analysis of Vessels – Oak
Ridge) was developed at Oak Ridge National Laboratory
(ORNL) under NRC Research funding and has been used for
the reassessment of pressurized thermal shock (PTS) and for
risk-informed approach for RPV operating curves in the USA
The FAVOR PFM model uses Monte Carlo techniques
Deterministic fracture analyses are performed on a large
number of stochastically generated RPV trials or realizations
Each trial considers the uncertainties of the vessel’s properties and the
postulated flaw population, which are described by statistical
distributions
Trials propagate the input uncertainties (with associated interactions)
through the model and determine probability of crack initiation and
through-wall cracking
51
USA Reactor Vessel Integrity Rules
Low Upper Shelf
Toughness
10CFR50 Appendix G
Reg. Guide 1.99 Rev. 2 (for calculating embrittlement
effects)
ASME Section XI Appendix K
Pressure-Temperature
Limits
10CFR50 Appendix G
Reg. Guide 1.99 Rev. 2 (for calculating embrittlement
effects)
ASME Section XI Appendices
G & E
Reactor Vessel Material
Surveillance
10CFR50 Appendix H
ASTM E 185 (-73, -79, or -82; testing &
reporting procedures should
meet E 185-82)
Pressurized Thermal Shock
10CFR50.61 •(use Reg. Guide 1.99 Rev. 2
methods to calculate embrittlement effects)
10CFR50.61a •(specifies own methods to
calculate embrittlement effects)
52
Monitoring of Vessel Irradiation Effects
• Variables that can influence the rate and degree of vessel embrittlement: – Number and energy level of neutrons impacting the
vessel wall – Temperature of the vessel wall during irradiation
(essentially, coolant inlet temperature during power operations)
– Correlation of neutron impact rate to material damage
• Surveillance programs obtain data on these variables by placing capsules near the pressure vessel wall
53
Surveillance Capsules Placed Near RPV Wall to Monitor Embrittlement
54
Typical Placement of Capsules in a PWR
55
Monitoring of Vessel Irradiation Effects
• RPV surveillance programs are administered to accurately assess actual material embrittlement levels and to provide a physical correlation to predictive techniques
• Capsules contain material specimens, temperature monitors and dosimetry to measure the level of neutron bombardment
• Periodic withdrawal and testing of capsules allows for verification of analytical predictions of vessel embrittlement
• In some cases - depending on the quality of the data – surveillance data may be used to change the estimate/prediction of vessel embrittlement
56
Monitoring of Vessel Irradiation Effects
• In July 1973, 10 CFR Part 50, Appendix H, “Reactor Vessel Material Surveillance Program Requirements,” established the first legal requirements for a comprehensive surveillance program – Plants already licensed had generally installed irradiation
test samples (usually Charpy V-notch specimens) of RPV material per ASTM E 185, “Surveillance Tests on Structural Materials in Nuclear Reactors”
• ASTM E 185 (1961, 1966, 1970, 1973, 1979, 1982, 1994, 1998….2010) plus Standard E 2215 (2002….2010)
57
Appendix H to 10 CFR 50
• Design of surveillance program and capsule withdrawal schedule must meet requirements of ASTM E 185, “Standard Practice for Conducting Surveillance Tests for Light-Water Cooled Nuclear Power Reactor Vessels” – ASTM E 185-73, -79, or -82, depending on which was in
effect on date the RPV was purchased
• Proposed capsule withdrawal schedule must be approved by NRC
• Capsule test report must be submitted to NRC within one year of capsule withdrawal
• Appendix H also provides the requirements for integrated surveillance programs (e.g., B&W, BWRVIP)
58
Monitoring of Vessel Irradiation Effects
• Specific requirements for surveillance program design have evolved over time and are very detailed – Each vessel program is designed to “...the edition of ASTM
E 185 that is current on the issue date of the ASME Code to which the reactor vessel was purchased.”
• Later editions may be used, but only those editions through 1982
• RPVs which have peak neutron fluence greater than 1017 n/cm2 (E > 1 MeV) at end of design life must have the beltline materials (base metal and weld metal) monitored
59
Requirements for Materials in RVSP
• Actual material used in the construction of the beltline • Include at least one heat of base metal, weld metal, and
heat-affected-zone (HAZ) material – current status is that it is not recommended to test HAZ
• Selection of materials based on materials predicted to be most limiting, with regard to setting P-T limits at end-of-license (e.g., those with highest projected ART); any material projected to be < 68 J (50 ft-lb) at ¼-T also included
• Fabrication history of surveillance specimens is to be fully representative of fabrication history of the vessel materials (e.g., thermal annealing, austenitizing treatment, quenching and tempering, and PWHT)
60
Test Specimen Requirements
• CVN and tension specimens • Fracture toughness specimens included if surveillance
materials are predicted to exhibit marginal properties • Tension and CVN specimens for base metal & HAZ shall be
from ¼- thickness (¼-T) locations; weld specimens may be from any thickness except near root or surface of weld
• Base metal tension & CVN specimens: major axis normal to principal rolling direction of the plate, normal to the major working direction for forgings – Charpy specimens should have the transverse (T-L) orientation
(weak direction) – Older plants may have L-T or both orientation specimens
61
Specimen Requirements
62
Specimen Orientation Issues
• Prior to 1973, the specification required Charpy specimens to be longitudinal (L-T), so older plants will have L-T, not T-L, specimens in their capsules
• Branch Technical Position 5-3 provides conservative guidance (but now being questioned):
“If transversely-oriented Charpy V-notch specimens were not tested, the temperature at which 68 J (50 ft-lbs) and 0.89 mm (35 mils) LE would have been obtained on transverse specimens may be estimated by one of the following criteria:
(a) Test results from longitudinally-oriented specimens reduced to 65% of their value to provide conservative estimates of values expected from transversely oriented specimens.
(b) Temperatures at which 68 J (50 ft-lbs) and 0.89 mm (35 mils) LE were obtained on longitudinally-oriented specimens increased 11°C (20°F) to provide a conservative estimate of the temperature that would have been necessary to obtain the same values on transversely-oriented specimens.”
63
Minimum Number of Test Specimens Required by ASTM E185-82
1Number of test specimens per exposure set (capsule) 2A minimum of 15 shall be tested 3Later versions of ASTM E185 (e.g., ASTM E185-92) require 6, not 3, unirradiated tension specimens
64
Capsule Dosimetry
• Dosimeters are placed in the surveillance capsules when the capsules are fabricated during reactor construction – Because each dosimeter reacts to neutrons of a
particular energy in the spectrum, a set of dosimeters are used in each capsule to provide adequate spectrum coverage
– Most are thin circular activation foils, although other shapes are available
– In addition to fast-neutron threshold monitors, a thermal monitor such as 59Co is typically included to determine thermal neutron fluence
65
Coiled Dosimetry Wire
66
Temperature Monitors
• Mechanical and impact properties of irradiated specimens depend on the temperature at which the material is irradiated
• ASTM E 185 requires temperature monitors in capsules • Low-melting alloys or pure metals are inserted inside
of the capsules to monitor peak temperature – Including different detectors of varying composition of the
alloys allows for a range of temperatures to be monitored – Provide indication of the maximum temperature
experienced by the capsule specimens (and RPV wall) – Value is limited since they do not give temperature history,
especially at lower operating temperatures
67
Other Material Requirements
• Chemical Analysis – available chemical composition information for the surveillance materials should documented for at least P, S, Cu, V, Si, Mn, Ni
• Archive materials
– Full-thickness sections of the original materials (plates, forgings and welds) should be retained
– Enough material to fill six additional capsules should be available
– Heat-affected-zone (HAZ) associated with archive weld material should also be retained
68
ASTM E 185-82 Reporting Requirements
• Administrative: Both conventional and SI units reported • Surveillance Program Description • Surveillance Material Selection & Characterization • Test results
– Tension Tests – Charpy Tests – Hardness tests (optional) – Other Fracture Toughness Tests (if performed) – Temperature & neutron radiation environment
• Application of Test Results (compare with predicted) • Deviations
69
Charpy V-notch Testing: Measured Parameters
• Impact energy (CVE) - the energy needed to fracture the test specimen – Determined directly from the impact test machine scale
(corrected for windage and friction losses)
• Lateral expansion (LE) - the amount of deformation caused by the pendulum striking the specimen resulting in the expansion of the specimen thickness
• Shear fracture (% Shear) - the percent area on the face of the fractured specimen that is attributed to brittle failure – Not commonly used as a specification – Plays important role in determination of Upper Shelf Energy
(USE)
70
Set of Tested CVN Specimens (broken faces)
GF 009
GF 004
GF 005
GF 006
GF 001
GF 003
GF 010
GF 008
GF 007
GF 002
Ductile-to-Brittle Transition Behavior
72
CVN Lateral Expansion
CVN Specimen Fracture Appearance
Illustration of digital optical measurement of shear fracture area. First,
the brittle fracture area is outlined within green line. Next, the outer
ductile fracture area is outlined in red. Software integrates the areas and
calculates the percent shear fracture area.
Summary – Output of Charpy Test Results
Index temperature shifts and USE
decreases for a surveillance material
irradiated in four capsules; each capsule has
a unique fluence (not listed here) 75
Charpy Data Curve Fitting Technical Description
• ASTM E 185-82 requires that irradiation effects be determined from Charpy data by measuring the differences in the 30 ft-lb (41 J) energy, 50 ft-lb (68 J) energy, and 35 mil (0.89 mm) lateral expansion index temperatures before and after irradiation
• “The index temperatures shall be obtained from the average curves.” – The guide does not specify how the average
curves are to be determined
76
Charpy Data Curve Fitting Technical Description • The general shape of Charpy test data (energy
versus temperature, or lateral expansion versus temperature) is that of an "S”
• The hyperbolic tangent (tanh) function is used as a simple statistical curve-fit tool to describe the "S"-shaped response – The tanh curve fit parameters defining the "S" shape
have physical meaning relative to what is generally evaluated from the test results
– The tanh curve fitting of Charpy V-notch data is a standard practice within the industry
• Mager, T. R., Server, W. L., and Beaudoin, B. F., “Use of the Hyperbolic Tangent Function for Fitting Transition Temperature Toughness Data,” WCAP-14370, Westinghouse Electric Corporation, May 1995.
77
Charpy Data Curve Fitting Technical Description
• The general tanh model often used for modeling Charpy curves: Cv = A + B tanh [(T – To) / C] where
Cv = Charpy impact energy, lateral expansion, or fracture appearance
T = test temperature
A = the mean energy level between the upper and lower shelves
B = the + or – deviation from the mean
To = a parameter that represents the mid-energy transition temperature
C = the + or – deviation of the intercepts of the tangent to the transition of To and the upper and lower shelves
78
Mathematical Interpretation of the Tanh Curve
79
Tanh Fit of Actual Charpy Test Data using CVGRAPH
80