Small Specimen Test Technologies for Fine-Grained Nuclear Graphite Prepared by Yutai Katoh With contributions from C. Phillip Shih, Mary A. Fechter, Lance L. Snead, and Timothy D. Burchell For presentation at ASTM 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, 2013 Seattle, WA

2Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Contents of Presentation Introduction Bulk density Dynamic elastic modulus Thermal conductivity Flexural strength Compressive strength Tensile strength Conclusions and recommendations

3Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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.

4Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Specimens Capsule Housing Specimen Holder SiC Springs SiC Temperature Monitors 5 cm

5Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Grain Sizes for Selected Nuclear Grade Graphite Cost of fast neutrons (HFIR): – ~9 k$ / rabbit / cycle = ~1 k$ / cm 3 / n/m 2 = ~1.4 k$ / cm 3 / dpa – Typical specimen loading ~25% = ~5.6 k$ / cm 3 / 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

6Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Grain Sizes for Selected Nuclear Grade Graphite H-451 NBG-18 NBG-17 PCEAATR-2E IG-110 IG-430 ETU-10 G458A G357A Fine Grained (Near-) Isotropic Medium Grained (Near-) Isotropic IGS743NH 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.

7Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle PropertyMethod 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 Thermal diffusivityFlash t1/2E1461 Tensile strength Dumbbell tension C749C1273 Compressive strength Round rod compression C695C1424, C773 Flexural strength 4P-1/3 Equibiaxial C651C1161 C1499 Key Design Properties for Nuclear Graphite

8Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 1) Bulk Density Applicable Test Standard: – ASTM C 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 micron2 mm Dimension with Caliper10 micron20 mm Mass with Digital Balance0.1 mg0.2 g

9Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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

10Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Mean recession depth model Envelope surface Physical surface May add optical micrograph showing loss of surface particles

11Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 2) Elastic Constants (here limited to E) Applicable Test Standard: – ASTM C 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 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

12Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Sonic modulus vs. specimen dimensions ASTM C x 15 x 3 50 x 4 x 2 48 x 6 x 1 30 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 (?) mm 3 ; more pronounced with thin specimens ASTM C x 15 x 3 50 x 4 x 2 30 x 3 x x 5 x 1

13Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Effective surface recession approach (again) to size scaling for dynamic Young’s modulus Dimensional correlation of 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

14Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 3) Thermal Conductivity Applicable Test Standard: – ASTM C 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

15Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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

16Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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 map LFA457 Detector response 1mm-t graphite at 50°C LFA457

17Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 4) Flexural Strength Nuclear Graphite Test Standard: – ASTM C 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 C : 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.

18Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Specimen size effect on 4Pt flexural strength IG-110 tested in 4 point -1/3 point flexural configuration Normal averages and standard deviationsWeibull 95% confidence ratio rings Axial, B12 x H6 x LS12.8 Axial, B8 x H4 x LS9.8 Axial, B2.9 x H2.8 x LS6.6 Transversal, B12 x H6 x LS12.8 Transversal, B8 x H4 x LS9.8 Transversal, B2.9 x H2.8 x LS6.6

19Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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 < ? mm 3

20Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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.

21Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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 mm 3 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

22Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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

23Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Here switch to C1499 PDF

24Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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 PP r PP

25Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Finite Element Analysis Assumed graphite properties – E = 10.5 GPa – = ~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 D L <= 2.5 mm

26Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle D L =1.16 mm t = 100  m D L =2.5 mm t = 100  m D L =2.5 mm t = 350  m D L =1.16 mm t = 350  m Likely origin Fracture patterns / POCO AXF-5Q Load ring trace Likely origin 1mm Experimental Verification of FEA Result t = 100  m specimens: fracture initiates clearly at the loading ring locations t = 350  m / D L =2.5 mm specimen: fracture initiates inside the loading ring but crack propagates along the ring indicating limited stress concentration t = 350  m / D L =1.16 mm specimen: fracture initiates at the center of disc; cracking pattern indicates no influence of stress concentration

27Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Fracture Patterns Indicate Lack of Significant Stress Concentration at Load- Transfer Locations

28Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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)DsD or Lh Size L, Disc40504 Size M, Disc20252 Size S, Disc10121 Size SS, Disc560.5 Size M, Coupon20252 Size SS, Coupon560.5 Bad data

29Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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 mm 3 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?

30Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 5) Compressive Strength Applicable ASTM Test Standard: – ASTM C 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”.

31Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Comparison of compressive strength of IG- 110 in various rod specimen dimensions

32Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Compressive Strength D10 x L20 m 95 = D6 x L8 m 95 = D6 x L8 Perforated m 95 = D10 x L13.3 m 95 = D6 x L12 m 95 = Data fit two-parameter Weibull okay. (however with small n = 30) Weibull modulus (mean MLE m = 41) appears reasonably consistent across all specimen sizes.

33Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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

34Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 6) Tensile Strength Applicable Test Standard: – ASTM C refers to ASTM C 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 C 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.

35Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Tensile Strength Test Matrix in ORNL Program for NTC TypeGauge Dimensions [mm] Grips# of TestsNote TS6.5UD G 6.5 x L G 26+Unibody30 each Ax/Tr Full ASTM TS5UD G 5 x L G 20+Unibody30 AxIntermediate TS4UD G 4 x L G 14+Unibody30 AxIrradiation size gauge TS4D G 4 x L G 14+Epoxy-Glued30 AxIrradiation specimen TS3UD G 3 x L G 12+Unibody30 AxSmaller than irrad. specimen (180)(Total) L G = 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

36Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle R= 6.63 Tensile Test Specimens ASTM C Cylindrical whole piece TS6.5U TS5U Dia Dia R= Dia Dia R= Dia TS4U Dia R= Dia 25 Extender 14 4 Dia TS4E Dia R= Dia TS3U

37Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Tensile Strength: Specimen Size Effect Validation ID Gauge Diameter TypeOrientationQuantity Full ASTM6.5 mmWhole pieceAxial30 Full ASTM6.5 mmWhole pieceTrans30 Intermediate5 mmWhole pieceAxial30 Irradiation size with gauge4 mmWhole pieceAxial30 Irradiation specimen4 mmEpoxy gluedAxial30 Smaller than irradiation specimen3 mmWhole pieceAxial30

38Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle TS4 with Steel Extenders and Alignment Block Setup Alignment block top Steel extensor Alignment block base V notch Push screw

39Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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

40Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Tensile Strength – Preliminary Results TS6.5U Axial Specimen X-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

41Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Two-parameter Weibull Analysis TS6.5U TS5U TS3U TS4U TS4E Same x-y scales for all plots

42Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Weibull Statistics

43Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Weibull Parameters 95% Confidence Bounds No evidence for significant specimen size effect on statistical tensile strength properties.

44Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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

45Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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 mm 3 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

46Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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 mm 3 and height to ~3 mm – Equibiaxial flexural strength Disc or coupon specimens thickness down to 0.5 mm and effective volume ~1.5 mm 3 – 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 mm 3.

47Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle 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.

48Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Bookmark Here

49Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle

50Managed by UT-Battelle for the U.S. Department of Energy ASTM Graphite Symposium, September 2013, Seattle Grain Sizes for Selected Nuclear Grade Graphite H-451 NBG- 17 NBG- 18 PCEA IGS74 3NH IG- 110 IG- 430 G347 A G458 A ETU- 10 Ext. Density Flexural Strength 20/24~3054~ Tensile Strength 15/13~2035~ Isotropy Factor ~1.3<1.1 ~1.1< ~1.15 Filler CokePetroPitch PetroPitchPetroPitch Grain Size <1.6 mm <0.8 mm <1.6 mm <0.8 mm <0.05 mm <0.02 mm <0.05 mm <0.04 mm