small specimen test technologies for fine-grained nuclear graphite

Small Specimen Test Technologies for Fine-Grained Nuclear Graphite

Prepared byYutai KatohWith contributions fromC. Phillip Shih, Mary A. Fechter, Lance L. Snead, and Timothy D. BurchellFor presentation atASTM Symposium on Graphite Testing for Nuclear Applications: The Significance of Test Specimen Volume and Geometry and the Statistical Significance of Test Specimen Population

September 19-20, 2013Seattle, WA

2 Managed by UT-Battellefor the U.S. Department of Energy ASTM Graphite Symposium, 19-20 September 2013, Seattle

Contents of Presentation

• Introduction• Bulk density• Dynamic elastic modulus• Thermal conductivity• Flexural strength• Compressive strength• Tensile strength• Conclusions and recommendations


Motivation• Small specimens are attractive for neutron irradiation

study and qualification• Fine-grained graphite anticipatedly allows the use

smaller specimens than larger grain graphite does• ASTM graphite test standards have historically been

written assuming use use of medium to large grain graphite; definitions of acceptable specimen dimensions should be revised for qualification of fine-grained materials

• It is important to understand the applicability and limitations of small specimens in relations with properties to be measured and materials’ microstructures.


Specimens

Capsule Housing

Specimen Holder

SiC Springs

SiC Temperature

Monitors

5 cm


Grain Sizes for Selected Nuclear Grade Graphite• Cost of fast neutrons (HFIR):

– ~9 k$ / rabbit / cycle = ~1 k$ / cm3 / 1025 n/m2 = ~1.4 k$ / cm3 / dpa– Typical specimen loading ~25% = ~5.6 k$ / cm3 / dpa

• Typical qualification program– Hundreds specimens for irradiation– ~20 dpa average

• This does not include cost for capsule design,construction, safety analysis, PIE, etc.

• PIE cost largely driven by amount of radioactivity


Grain Sizes for Selected Nuclear Grade Graphite

H-451

NBG-18NBG-17

PCEA ATR-2E

IG-110IG-430

ETU-10

G458AG357A

Fine Grained(Near-) Isotropic Medium Grained

(Near-) IsotropicIGS743NH

• Metals: mechanical property tests typically requires the minimum dimension of test specimen >10 times grain diameter.

• Graphite test standards often specifies the minimum dimension >5 times grain size.

• For medium grained graphite, grain size dictates the minimum dimension of test specimens.

• This may not be the case for super-fine grained graphite.


Property Method Discussed ASTM Standard (Graphite)

ASTM Standard (Adv. Ceramics)

Bulk Density Mass and dimensions C838, C559

Elastic constants Impulse excitation and vibration C1259

CTE Push rod dilatometry E228 E228

Thermal diffusivity Flash t1/2 E1461 E1461

Tensile strength Dumbbelltension C749 C1273

Compressive strength Round rod compression C695 C1424, C773

Flexural strength4P-1/3

Equibiaxial

C651 C1161

C1499

Key Design Properties for Nuclear Graphite


1) Bulk Density• Applicable Test Standard:

– ASTM C781-08 refers to ASTM C559 for determination of bulk density of machined graphite samples

– ASTM C559-90: Bulk Density by Physical Measurements of Manufactured Carbon and Graphite Articles• “Measure each dimension of the test specimen to an accuracy of 0.05 %”.

• Test Method, Accuracy, and Anticipated Size Effect Issues

– No size effect issue is anticipated (?)

Typical Accuracy of measurement

Minimum Dimension or Mass for 0.05% Accuracy

Dimension with Micrometer, Measurement Microscope, Keyence Devices 1 micron 2 mm

Dimension with Caliper 10 micron 20 mm

Mass with Digital Balance 0.1 mg 0.2 g


Measured bulk density vs. specimen volume

• Open symbols do not satisfy the <0.05% measurement accuracy requirement.

• Greater standard deviations for smaller specimen volumes (as expected)

• Slightly lower density for smaller specimen volume

• Effect is minor with ~0.5% discrepancy across 2 orders of magnitude change in volume


• DD appears to be correlated with the surface-to-volume ratio of specimen

• Mean recession depth model:

• Comparison of among different graphite grades will be interesting

Mean recession depth model

Envelopesurface

𝑥

Physicalsurface

May add optical micrograph showing loss of surface particles


2) Elastic Constants (here limited to E)• Applicable Test Standard:

– ASTM C781-08 refers to ASTM C747 for determination of elastic modulus– ASTM C747-93: Moduli of Elasticity and Fundamental Frequencies of Carbon

and Graphite Materials by Sonic Resonance• Recommended specimen aspect ratio: L/t ratio must be between 5 and 20

– ASTM C1198: Dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio for Advanced Ceramics by Sonic Resonance• Defines more detailed standard practice• Recommended specimen dimensions

– ~10 < L/t < ~25, w/t > 5 for shear modulus determination– 75(L) x 15(w) x 3(t) for example

• Test Method, Accuracy, and Anticipated Size Effect Issues– Resonance frequency measurement is explicit (as far as correctly excited)

– Modulus determination will obviously be affected by edge and surface conditions

– Increased size effect is anticipated for smaller specimens

𝐸=0.9465 𝑚 𝑓 2

𝑏 ( 𝐿𝑡 )3

𝑇 1


Sonic modulus vs. specimen dimensions

ASTM C125975 x 15 x 3

50 x 4 x 2

48 x 6 x 130 x 3.8 x 3

• Correlation of dynamic Young’s modulus with specimen volume is apparent

• Decrease in measured dynamic Young’s modulus with specimen volume below ~300 (?) mm3 ; more pronounced with thin specimens

ASTM C125975 x 15 x 350 x 4 x 2

30 x 3 x 2.5

24 x 5 x 1


Effective surface recession approach (again) to size scaling for dynamic Young’s modulus

• Dimensional correlation of -0.02 mm was applied to all dimensions in these examples

• Need for more sophisticated approach is obvious…

• Premise: cracks exposed to surface and loss of surface-exposed grains contribute to reduced dynamic modulus


3) Thermal Conductivity• Applicable Test Standard:

– ASTM C781-08 refers to ASTM E1461 for determination of thermal diffusivity– ASTM E1461: Thermal Diffusivity by the Flash Method

• Applicable to homogeneous solid materials• Recommended dimensions: D = 6 to 18 mm, t = 1 to 6 mm

• Accuracy, and Anticipated Size Effect Issues– Accuracy is determined by various factors including time resolution of

measurement and lateral heat flaw within specimen– Larger D/t is preferred– Minimum t is limited by travel time depending on pulse shape and

measurement resolution


Thermal diffusivity vs. specimen dimensions

• Specimen dimensions within certain ranges impose only minimal effect on flash thermal diffusivity measurement

• Very thin specimen challenges the minimum travel time limit for the instrument


Factors limiting reduced specimen size for flash thermal diffusivity measurement

• Heat loss– Caused by deviation from 1-D heat

transport assumption– Aperture size and alignment in relation

with stray light propagation (to detector) matters

– Appeared to not be a significant factor in current examples

• Insufficient half-rise time– Minimum required half-rise time a

function of pulse width, detector time response, system noise, software, etc.

– Netzsch LFA457 requires >~2.5 ms half-rise time for reliable diffusivity measurement

Laser pulse mapLFA457

Detector response1mm-t graphite at 50°CLFA457


4) Flexural Strength• Nuclear Graphite Test Standard:

– ASTM C781-08 refers to ASTM C651 for determination of flexural strength– ASTM C651: Flexural Strength of Manufactured Carbon and Graphite Articles

Using Four-Point Loading at Room Temperature• “The size of the test specimen shall be selected such that the minimum dimension of

the specimen is greater than 5 times the largest particle dimension”. • “The test specimen shall have a length to thickness ratio of at least 8, and a width to

thickness ratio not greater than 2”.• “The load span is at least two times the sample thickness, and the support span three

times the load span, but not less than 11⁄2 in. (38.1 mm)”.

• Equibiaxial Test Standard for Advanced Ceramics– ASTM C1499-05: Monotonic Equibiaxial Flexural Strength of Advanced Ceramics

at Ambient Temperature• “This test method is intended primarily for use with advanced ceramics that

macroscopically exhibit isotropic, homogeneous, continuous behavior”.• No absolute minimum specimen size specified.


Specimen size effect on 4Pt flexural strength• IG-110 tested in 4 point -1/3 point flexural configuration

Normal averages and standard deviations Weibull 95% confidence ratio rings

Axia

l, B1

2 x

H6 x

LS12

.8

Axia

l, B8

x H

4 x

LS9.

8

Axia

l, B2

.9 x

H2.

8 x

LS6.

6

Tran

sver

sal,

B12

x H6

x LS

12.8

Tran

sver

sal,

B8 x

H4

x LS

9.8

Tran

sver

sal,

B2.9

x H

2.8

x LS

6.6


Weibull scaling for 4pt. flexural strength of IG-110

• Effect of specimen size is unclear.

• When Weibull scaling law is assumed, data suggest that flexural strength starts to deviate from law when:– Specimen thickness < ? mm– Effective volume < ? mm3


Specimen size effect on 4Pt flexural strength of G347A

• Effect of specimen size is obvious.

• FS (full size ) specimen– ASTM C1161 Config. B– L45 x W4 x H3

• ½ (half size) specimen– ASTM C1161 Config. A– L25 x W2 x H1.5– ~20% reduction in

apparent strength is noted.


Deviation from Weibull scaling for 4pt. flexural strength

• When Weibull scaling law is assumed, data suggest that flexural strength starts to deviate from law when:– Specimen thickness < ~3 mm– Effective volume < ~100 mm3

• Grain size does not dictate the deviation.– t = ~3 mm >> Dg = ~0.02 mm

• Why deviation?– Increased relative contribution from

surface / edge effects, including those arising from machining flaws

– Increasing contact load in shorter load span


Equibiaxail flexural test for brittle materials• ASTM C1499 – 09

– Standard Test Method for Monotonic Equibiaxial Flexural Strength of Advanced Ceramics at Ambient Temperature

Loading Ring

Support Ring

• Specimen may be round disc or rectangular coupon


Here switch to C1499 PDF


Specimen Deflection Causes an Issue for Equibiaxial Flexure Tests• When specimen experience excessive

deflection– True stress – load relationship deviate from

linearity– Stress state in specimen changes– Stress and strain concentrate at the loading ring– Friction between specimen and rings contibutes

• Becomes an issue when– High fracture stress– Low Young’s modulus– Deformation is significantly elasto-plastic

F

sP

lowtE

F 2

1s

r

sP

hightE

F 2

1s


Finite Element Analysis• Assumed graphite properties

– E = 10.5 GPa– n = ~0.15

• Other conditions– Specimen thickness 100 to 350

micron– Loading ring diameter 2.5 mm

and 1.16 mm– Maximum principal stress up to

x2 reported flexural strength

• Results indicate– Specimen thickness <350

micron inadequate with DL <= 2.5 mm

0.0 0.5 1.0 1.5 2.0 2.5 3.00

30

60

90

120

150at s

p,max=135MPa D

L=2.51mm

Graphite: E=10.5GPa, n=0.15

Prin

cipa

l stre

ss, s p [M

Pa]

Distance from center of disc, rd[mm]

t=100m t=150m t=200m t=350m

DL=2.5mm

0.0 0.5 1.0 1.5 2.0 2.5 3.00

30

60

90

120

150at s

p,max=135MPa

DL=1.16mm

Graphite: E=10.5GPa, n=0.15

Prin

cipa

l stre

ss, s p [M

Pa]

Distance from center of disc, rd[mm]

t=100m t=150m t=200m t=350m

DL=1.16mm


DL=1.16 mmt = 100 m

DL=2.5 mmt = 100 m

DL=2.5 mmt = 350 m

DL=1.16 mmt = 350 m

Likely origin

Fracture patterns / POCO AXF-5Q

Load ring trace

Likely origin

Likely origin

Likely origin

1mm

Experimental Verification of FEA Result

• t = 100 m specimens: fracture initiates clearly at the loading ring locations

• t = 350 m / DL=2.5 mm specimen: fracture initiates inside the loading ring but crack propagates along the ring indicating limited stress concentration

• t = 350 m / DL=1.16 mm specimen: fracture initiates at the center of disc; cracking pattern indicates no influence of stress concentration


Fracture Patterns Indicate Lack of Significant Stress Concentration at Load-Transfer Locations


Specimen size effect on equibiaxal flexural strength• ETU-10 tested in ring-on-ring

equibiaxial flexural configuration using round disc and square coupon specimens.

(size in mm) Ds D or L h

Size L, Disc 40 50 4

Size M, Disc 20 25 2

Size S, Disc 10 12 1

Size SS, Disc 5 6 0.5

Size M, Coupon 20 25 2

Size SS, Coupon 5 6 0.5

Bad data


Weibull scaling for equibiaxial flexural strength

• Present data for ETU-10 suggest that equibiaxial flexural strength may follow Weibull scaling law down to:– Specimen thickness 0.5 mm– Effective volume ~1.5 mm3

• Grain size consideration:– t (0.5 mm) = ~12 x Dg (0.04 mm)

• Why different from 4pt flexure size effect?– Lack of contribution from

machined edge?– Reduced effect of loading fixture?


5) Compressive Strength• Applicable ASTM Test Standard:

– ASTM C781-08 refers to ASTM C695 for determination of compressive strength

– ASTM C695-91: Compressive Strength of Carbon and Graphite• “The diameter of the test specimen shall be greater than ten times the

maximum particle size of the carbon or graphite”.• “The ratio of height to diameter may vary between 1.9 and 2.1”.• “The recommended minimum test specimen size is 3⁄8 in. (9.5 mm) diameter

by 3⁄4 in. (19 mm) high”.


Comparison of compressive strength of IG-110 in various rod specimen dimensions


Compressive Strength

D10 x L20m95 = 35 - 64

D6 x L8m95 = 21 - 37

D6 x L8Perforatedm95 = 26 - 46

D10 x L13.3m95 = 30 - 53

D6 x L12m95 = 26 - 45

• Data fit two-parameter Weibull okay. (however with small n = 30)

• Weibull modulus (mean MLE m = 41) appears reasonably consistent across all specimen sizes.


Effect of Test Specimen Volume on Compressive Strength

• Compressive strength appear to be insensitive to specimen volume.

• Also insensitive to:– Specimen diameter– L/D ratio– Surface-to-volume ratio– Presence of center hollow

• Weibull scaling does not seem to apply

m = 41


6) Tensile Strength• Applicable Test Standard:

– ASTM C781-08 refers to ASTM C749-08 for determination of tensile strength

– ASTM C749-08: Tensile Stress-Strain of Carbon and Graphite• “the gauge diameter should not be reduced to less than three to five times

the maximum particles size in the material” • Requirement to gauge length-to-diameter ratio is not defined. However,

standard specimen dimensions typically have the gauge length-to-diameter ratio close to 2.

– ASTM C781-08 adds the following recommendations.• “The recommended test specimen size is 6.5 mm (0.256 in.) diameter”.• “The recommended height to diameter ratio for the specimen gage section

is 4”.• Note that the diameter recommendation assumes medium to large grain

graphite as the materials subjected to the tests.


Tensile Strength Test Matrix in ORNL Program for NTC

Type Gauge Dimensions

[mm]

Grips # of Tests Note

TS6.5U DG6.5 x LG26+ Unibody 30 each Ax/Tr

Full ASTM

TS5U DG5 x LG20+ Unibody 30 Ax Intermediate

TS4U DG4 x LG14+ Unibody 30 Ax Irradiation size gauge

TS4 DG4 x LG14+ Epoxy-Glued 30 Ax Irradiation specimen

TS3U DG3 x LG12+ Unibody 30 Ax Smaller than irrad. specimen

(180) (Total)

LG = length of straight gauge section (actual gauge lengths per ASTM definition are longer)

• Proposed test matrix is designed to provide systematic information on – Effect of specimen size (primarily gauge diameter)– Effect of specimen orientation– Effect of epoxy-glued ends


R= 6.63

Tensile Test Specimens

ASTM C749-08Cylindrical

whole piece

TS6.5UTS5U

16.13

12.95 Dia

3.175

16.195

12.39

26

9.91 Dia

R= 25.4

6.5 Dia

12.41

9.96 Dia

12.46

9.53

20R= 19.54

5 Dia

TS4U

9.93

7.97 Dia

9.97

7.62

14

R= 15.63

4 Dia

25

Extender

144 Dia

TS4E

7.45

5.98 Dia

7.45

5.72

12

R= 11.72

3 Dia TS3U


Tensile Strength: Specimen Size Effect Validation

ID Gauge Diameter Type Orientation Quantity

Full ASTM 6.5 mm Whole piece Axial 30Full ASTM 6.5 mm Whole piece Trans 30

Intermediate 5 mm Whole piece Axial 30

Irradiation size with gauge 4 mm Whole piece Axial 30

Irradiation specimen 4 mm Epoxy glued Axial 30

Smaller than irradiation specimen 3 mm Whole piece Axial 30


TS4 with Steel Extenders and Alignment Block Setup

Alignment block top

Steel extensor

Alignment block base

V notch Push screw


Epoxy Extension for Graphite Tensile Test

• Epon 828 epoxy was used with Jeffamine T403 hardener.

• Area of bonding D6+side, >2 times greater than the gauge cross-section D4.

• Tensile strength of material ~35 MPa.

• 16 valid tests out of 30 attempts with most invalid tests due to bond failure; considered inadequate for use in PIE.

• Clam shell type end tabs extending to the transitional section will be needed.

D4

D6

Epoxy Epoxy


Tensile Strength – Preliminary Results

0 5 10 15 20 25 30 35 400

0.1

0.2

0.3

0.4

0.5

0.6

Tensile Stress (MPa)

Stra

in (%

)

0 5 10 15 20 25 30 35 40

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0

Stra

in (%

)

TS6.5U Axial SpecimenX-Y Strain Gauge Reading

Poisson’s ratio: 0.13

• No significant effects of gauge diameter and epoxy-extension.

– More discussion in Katoh et al ASTM paper later this week.

Epoxy-extended Specimens

Unibody Specimens


Two-parameter Weibull Analysis

TS6.5UTS5U

TS3UTS4U

TS4E

Same x-y scales for all plots


Weibull Statistics


Weibull Parameters 95% Confidence Bounds

• No evidence for significant specimen size effect on statistical tensile strength properties.


Tensile Strength – Weibull Scaling

• Weibull scaling does not appear to apply.

• Effect of reduced Young’s modulus for smaller dimensions?

• Bending moment (misalignment effect) relative to tensile load?

m = 20


Conclusions and Recommendations• Use of small test specimens is considered valid when

– Absolute data value and data scatter are consistent with those determined in fully standard-conformant tests, or

– Absolute data (and scatter) are scalable to those determined in fully standard-conformant tests

• Examples of small test specimens that appeared valid for superfine grained graphite evaluated in studies presented:– Bulk density

• Per ASTM C559– Young’s modulus

• Beam specimen volume down to ~300 mm3 in standard proportions for determination of absolute constants

– Flash thermal diffusivity• Highly dependent on instrument and setup used• Disc specimen diameter down to 6 mm and thickness down to 2 mm in

studies presented• Minimum thickness limited by transport time


Conclusions and Recommendations (2)• Examples of small test specimens that appeared valid for

superfine grained graphite evaluated in studies presented (continued):– Four point flexural strength

• Rectangular beam specimen effective volume down to ~100 mm3 and height to ~3 mm

– Equibiaxial flexural strength• Disc or coupon specimens thickness down to 0.5 mm and effective volume

~1.5 mm3

– Compressive strength• Round rod specimens with diameter down to 3 mm and height-to-diameter

ratio down to 1.– Tensile strength

• Round cross-section straight gauge tensile specimen with gauge diameter down to 3 mm and gauge volume to ~85 mm3.


Conclusions and Recommendations (3)• ASTM C28 standards for advanced ceramics appears generally

appropriate for properties determination of fine grained graphite.

• Equibiaxial flexural test appears particularly useful and reliable for determination of flexural strength of fine grained graphite using very small test specimens.


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Grain Sizes for Selected Nuclear Grade Graphite

H-451 NBG-17

NBG-18 PCEA IGS74

3NHIG-110

IG-430

G347A

G458A

ETU-10

Ext.

Density 1.75 1.87 1.80 1.77 1.82 1.85 1.86 1.75Flexural Strength 20/24 ~30 54 ~39 54 49 54 59

Tensile Strength 15/13 ~20 35 ~25 37 31 35 34

Isotropy Factor ~1.3 <1.1 <1.1 ~1.1 <1.1 1.06 1.1 ~1.15

Filler Coke Petro Pitch Pitch Petro Pitch Petro Pitch Pitch Pitch Pitch

Grain Size <1.6 mm

<0.8 mm

<1.6 mm

<0.8 mm

<0.05 mm

<0.02 mm

<0.02 mm

<0.05 mm

<0.05 mm

<0.04 mm

small specimen test technologies for fine-grained nuclear graphite

Documents

graphite testing

medium grained graphite

superfine grained graphite

times grain size

use smaller specimens

times grain diameter

limitations of small

nuclear applications