2067-1c
Joint ICTP/IAEA Workshop on Irradiation-induced Embrittlement ofPressure Vessel Steels
Enrico Lucon
23 - 27 November 2009
Italy*
Small Specimen Test Technologies for measuring mechanical properties of reactor pressure vessel steels
Enrico Lucon
Joint ICTP/IAEA Workshop Trieste, 23‐27 November 2009
OutlineOutline
Introduction – General concepts
Miniature tensile specimens
Instrumented indentation testsInstrumented indentation tests
Reconstitution of Charpy specimens
I b i Ch iImpact tests on sub‐size Charpy specimens
Fracture toughness testing on sub‐size specimens• transition region (Master Curve)
• upper shelf regime (miniature C(T) specimens)
Standardization of SSTT
Small Punch Testingg
IntroductionIntroductionWhy small specimen testing?
Evaluating mechanical properties is needed for i i d lif di iintegrity assessments and life predictions
Materials are subject to degradation due to high temperatures, aggressive environment and/or neutron irradiation
Evaluation of mechanical properties is by definition a destructive techniqueq
If specimen size is small enough, it can become “semi‐destructive” (easy repair or even no repair is needed)destructive (easy repair or even no repair is needed)
IntroductionDevelopment of SSTT
Small Specimen Test Techniques have been developed, qualified and applied for the mechanical characterization of reactor pressure vessel steels (unirradiated and irradiated)
Mechanical properties addressed:tensile strength (miniature specimens instrumentedtensile strength (miniature specimens, instrumented indentation)
impact toughness (reconstitution of Charpy specimens, KLSTp g ( py p ,sub‐size specimens)
fracture toughness (transition and upper shelf regimes, various sub‐size specimen geometries)
Several options when only broken Ch i il blCharpy specimens are available
bMiniatureC(T)
Sub‐sizetensilesamples
Charpyreconstitution Small
crackedround bars
Sub sizeSub‐sizeCharpy(precracked)
Miniature flat tensile specimens (1)Miniature flat tensile specimens (1)
The specific problem
Miniature flat tensile specimens (2)Miniature flat tensile specimens (2)
Miniature flat tensile specimens (3)Miniature flat tensile specimens (3)1000
Standard
900
Standard
Miniature
800
Pa)
700
Stre
ss (M
P
600
UTS
500
yield
400-200 -100 0 100 200 300
Test temperature (°C)
Miniature flat tensile specimens (4)‐ Basic “facts” ‐
Results from miniature specimens are in good agreementResults from miniature specimens are in good agreement with standard sample data, within a few %
Th t iti l tThe most critical aspects are:Misalignments and extraneous displacements during gripping and mounting operations have to be carefully avoided (a special “specimen holder” was developed)
The significant influence of the test setup compliance has to be accounted for when determining the elastic portion of the test record (only the last part should be considered)
Since data scatter tends to increase for decreasing specimen size, a minimum number of 3 tests per temperature is recommended (preferably 5)
Instrumented indentation tests (1)Instrumented indentation tests (1)
d d ( )Instrumented indentation tests (2)
bl h l lFavourable comparison with tensile test results
Yield strength estimation
Tensile strength estimationg
Reconstitution of Charpy specimens (1)Reconstitution of Charpy specimens (1)
Reconstitution of Charpy specimens (2)‐ Basic “facts” ‐
If insert length is greater than 15 mm, no influence of reconstitution can be appreciated
For inserts of 10‐12 mm length, a decrease in Upper Shelf Energy (Charpy) and upper shelf toughness (PCCv)Shelf Energy (Charpy) and upper shelf toughness (PCCv) can be observed
No influence of reconstitution in case of toughness tests in the transition regime (Master Curve analysis)
The shortest inserts (10 mm) allow changing the sample orientation (e.g. from LT to TL)orientation (e.g. from LT to TL)
Impact tests on sub‐size specimensKLST type (1)
f l f f ll
0 2378USEss
250
Estimation of USE values for full‐size specimens
USEfs = 29.398e0.2378USEss
R2 = 0.962200
)
150
ize
USE
(J)
50
100
Full-
s
0
50σ = ±11.3 J
2 3 4 5 6 7 8 9
Sub-size USE (J)
Impact tests on sub‐size specimens KLST type (2)
fEstimation of transition temperatures200
C) 41J/1.9J
T T 59 3 °C
100
150
ex te
mp.
(°C
68J/3.1J
0.89mm/0.3mm
FATT50 -2σ
+2σTfs = Tss + 59.3 °C
Tfs = 1.124 Tss + 58.3 °C50
%SF
A in
de
± 2 = ± 42 2 °C
-50
0
/0.3
mm
/50% ± 2σ = ± 42.2 °C
-100
1.9J
/3.1
J/
Tfs = Tss + 65 °C
-150-100 -50 0 50 100 150 200 250 300
41J/68J/0.9mm/50%SFA index temperatures (°C)
Fracture toughness testingVarious geometries investigated (1)
Ductile‐to‐brittle transition regime (Master Curve analysis)y )
precracked sub‐size Charpy specimens, P‐KLST
sub‐size cracked round bars, CRB
miniature Compact Tension specimens, MC(T)p p ( )
Fully ductile regime (JIc values, crack resistance curves)
k d b i Ch i P KLSTprecracked sub‐size Charpy specimens, P‐KLST
miniature Compact Tension specimens, MC(T)
Fracture toughness testingVarious geometries investigated (2)
Precracked KLSTMini C(T)
Sub‐size CRB
Fracture toughness testing (transition)Master Curve analysis (1)
250 0.18-SE(B)0.16-C(T)0 20D CRB
95% t.b
MCB = 4.15 mmB = 4.6 mm
D 5
150
200
a √m
)
0.20D-CRB D = 5 mm
100
150
T,SS
Y (M
Pa
5% t.b
50
100
KJc
,1T
0
50
0-150 -130 -110 -90 -70 -50 -30
Temperature (°C)
Fracture toughness testing (transition)Master Curve analysis (2) – Basic “facts”
Irrespective of the specimen geometry chosen referenceIrrespective of the specimen geometry chosen, reference temperatures measured from small specimens are in good agreement with those measured from larger samples (within statistical uncertainties)
The main issue is the limited test temperature validity domain, determined by:
the lower limit of applicability for the Master Curve method(T – 50 °C)(To 50 C)
the specimen measuring capacity (inversely proportional to the specimen ligament length)
From this point of view, MC(T) are preferable to P‐KLST (longer ligament larger validity domain)The sub‐size CRB results can be corrected for loss‐of‐constraint using a factor derived from FEM analyses
Fracture toughness testingUpper Shelf (fully ductile) regime (1)
1000
800
1000Jlim - 1TC(T)
600
800
/m²)
400
600
nteg
ral (
kJ/ MC(T)
1TC(T)
PCCv B=4.2 mm
200
400
J-in
Jli - PCCv B=4 2mm / MC(T)"Effective" limit forsize-independence
0
200 Jlim - PCCv B=4.2mm / MC(T)size independence
00 0.5 1 1.5 2 2.5 3 3.5 4
Ductile crack extension (mm)
Fracture toughness testingUpper Shelf (fully ductile) regime (2)
900+2σ
750
900BESTFITUC + NDR+MS
J1T = JMCT + 0.0013 JMCT2.0586
600
T) (k
J/m
²)
-2σ
450
from
1TC
(T
1 0
300
J Q o
r JIc
Unloading ComplianceNormalization
0
150 NormalizationMulti-specimen
0 100 200 300 400 500
JQ or JIc from MC(T) (kJ/m²)
Fracture toughness testingUpper shelf regime (3) – Basic “facts”
Miniature specimens (of bend or C(T)‐type) clearly underestimate the ductile fracture toughness measured from standard 1TC(T) samples
An empirical correlation can be established which allows estimating the actual JIc with an uncertainty of 34% at the 95% confidence level
The role of work hardening in lowering the tearing resistance of small samples has been confirmed
The use of alternative fracture toughness parameters (CTOD, CTOA, Enrst’s modified J‐integral) seems to improve the agreement
ConclusionsMost critical aspects related to SSTT
Significance of experimental data
Transferability of measurements obtained from smallTransferability of measurements obtained from small specimens to actual components under investigation
Analytical techniques which can be:Analytical techniques, which can be:equivalent to the “conventional” ones (e.g. Master Curve analysis)
specific to small specimen geometries, i.e. based on correlation approachesspecific to small specimen geometries, i.e. based on correlation approaches (e.g. KLST versus full‐size Charpy specimens)
Accuracy of test methods, accounting for the characteristics of the available instrumentation and the magnitude of the signals involved (force, displacement etc.)
Standardization of SSTTPresent Status & Perspectives
Mi t t l id ti di t t th t l iMicrostructural considerations dictate that only specimens with cross sectional dimensions sufficient to ensure a
t ti l f t i l i t t d h ld b drepresentative volume of material is tested should be used
In order to satisfy this requirement, the size scale and mean separation distance of inhomogeneities that exist in the material must be known
The cross sectional dimension of the miniature/subsizespecimens should be at least 3‐5 times greater than the largest inhomogeneity
Therefore, the recommended SS size depends on theTherefore, the recommended SS size depends on the microstructure of the investigated material
Standardization of SSTTPresent Status & Perspectives
Mi t t l id ti di t t th t l iMicrostructural considerations dictate that only specimens with cross sectional dimensions sufficient to ensure a
t ti l f t i l i t t d h ld b drepresentative volume of material is tested should be used
In order to satisfy this requirement, the size scale and mean separation distance of inhomogeneities that exist in the material must be known
The cross sectional dimension of the miniature/subsizespecimens should be at least 3‐5 times greater than the largest inhomogeneity
Therefore, the recommended SS size depends on theTherefore, the recommended SS size depends on the microstructure of the investigated material
Worldwide Standardization Forumsfor mechanical tests
ISO International Standards OrganisationISO – International Standards OrganisationTC164 ‐Mechanical Tests
SC1 (U i i l T t )– SC1 (Uniaxial Tests)
– SC4/P (Pendulum)
SC4/F (F )– SC4/F (Fracture)
– SC5 (Fatigue)
/Meets once a year (September/October)
ASTM (American Society for Testing and Materials)Technical Committee E08 (Fracture and Fatigue)
Technical Committee E28 (Mechanical Tests)
Meet twice a year (May and November)
T il T iTensile Testing
B th ASTM E8/E8M d ISO 6892 1998 d t li itlBoth ASTM E8/E8M and ISO 6892:1998 do not explicitly limit the minimum size of the specimen
Both include “subsize” specimens (an example: round specimen with 2.5 mm diameter)
Any alternative miniature/subsize tensile specimen has to be validated and qualified against standard specimens
If microstructural considerations do not come into play, SS should deliver equivalent results to larger samplesSS should deliver equivalent results to larger samples
⇒ Standardization is not needed, but robustqualification (unirradiated condition) is requiredqualification (unirradiated condition) is required
Charpy Impact TestingCharpy Impact Testing
Subsize Charpy specimens (KLST‐type, 3 × 4 × 27 mm) are commonly used in the Fusion community for DBTTmeasurement and materials’ qualification
Neither ASTM E 23 nor ISO 148 include subsizespecimens as suchp
However:ASTM E 2248 on miniaturised Charpy specimens has beenASTM E 2248 on miniaturised Charpy specimens has been issued in April 2009 (geometries: 5 × 5 × 27.5 mm and KLST)ISO 14556:2000 (instrumented tests) includes KLST specimens ( ) p(Annex D)
⇒ Standardization is already happening; correlations⇒ Standardization is already happening; correlationswith standard specimens should be validated
Fracture Toughness TestingFracture Toughness Testing(linear elastic regime)
In order to obtain valid fracture toughness measurements in case of fully brittle behaviour, largemeasurements in case of fully brittle behaviour, large specimens are required
Small specimens are generally not applicable to fractureSmall specimens are generally not applicable to fracture toughness testing in the linear elastic regime
F th l h lf diti h t b id dFurthermore, lower shelf conditions have to be avoided throughout the operation of any structure or component
⇒ This fracture regime is not relevant for RPV integrityassessments
Fracture Toughness TestingFracture Toughness Testing(ductile‐to‐brittle transition)
Fracture toughness properties in the ductile‐to‐brittle transition region are of primary importance for assessing the integrity of a structure or componentcomponent
ASTM E 1921 (Master Curve) does not restrict the minimum size of a specimenp
However, validity requirements related to specimen size have to be fulfilled for the results to be valid
The most commonly SS used are the precracked KLST and the miniature C(T) (thickness 4‐5 mm)
Mi i C(T) i h l lidit d i th KLST d h ldMini C(T) specimens have a larger validity domain than KLST, and should be given higher priority
⇒Standardization is not needed; existing standards (basically E 1921) can⇒Standardization is not needed; existing standards (basically E 1921) can and should be used
Fracture Toughness Testing(fully plastic regime)
U h lf f t t h ti i t i th i iti tiUpper shelf fracture toughness properties consist in the initiation value and the crack resistance curve (R‐curve)
ASTM E 1820 and ISO 12135:2002 do not restrict the minimumASTM E 1820 and ISO 12135:2002 do not restrict the minimum size of the specimen
However validity requirements related to specimen size areHowever, validity requirements related to specimen size are imposed
SS appear to underestimate the actual fracture toughness of theSS appear to underestimate the actual fracture toughness of the materials
Correlations with larger specimens should be established and g pvalidated
⇒Standardization is not necessary, but correlations with larger specimens should be qualified
Small Punch Testingg(a really miniature specimen!)
It’s the smallest specimen ever (typically, TEM disc with 3 mm diameter and 0.25 thickness)
Can be used for estimating:tensile properties (using empirical correlations or FEM analyses)
DBTT values (using empirical correlations)DBTT values (using empirical correlations)
fracture toughness (using FEM analyses; reliability is doubtful)
creep properties
Correlations are strongly material‐dependent and need to be carefully validated
Standards do not exist nor are in preparation (to my knowledge)
⇒Standardization can be pursued, preferably in the ISO framework (A i t t k thi )(Americans are not too keen on this)