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Small Specimen Test Technologies for Fine-Grained Nuclear Graphite

Small Specimen Test Technologies for Fine-Grained Nuclear Graphite. Prepared by Y utai Katoh With contributions from C. Phillip Shih, Mary A. Fechter, Lance L. Snead, and Timothy D. B urchell

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Small Specimen Test Technologies for Fine-Grained Nuclear Graphite

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  1. Small Specimen Test Technologies for Fine-Grained Nuclear Graphite Prepared byYutai Katoh With contributions fromC. Phillip Shih, Mary A. Fechter, Lance L. Snead, and Timothy D. Burchell For 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. Contents of Presentation • Introduction • Bulk density • Dynamic elastic modulus • Thermal conductivity • Flexural strength • Compressive strength • Tensile strength • Conclusions and recommendations

  3. 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.

  4. SiC Springs Capsule Housing 5 cm SiC Temperature Monitors Specimen Holder Specimens

  5. 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

  6. Grain Sizes for Selected Nuclear Grade Graphite • 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. H-451 Fine Grained (Near-) Isotropic Medium Grained (Near-) Isotropic ETU-10 IGS743NH PCEA ATR-2E G458A G357A IG-110 IG-430 NBG-18 NBG-17

  7. Key Design Properties for Nuclear Graphite

  8. 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 (?)

  9. 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

  10. Mean recession depth model • 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 Physicalsurface Envelopesurface  May add optical micrograph showing loss of surface particles

  11. 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

  12. Sonic modulus vs. specimen dimensions • 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 C1259 75 x 15 x 3 50 x 4 x 2 ASTM C1259 75 x 15 x 3 50 x 4 x 2 48 x 6 x 1 30 x 3 x 2.5 30 x 3.8 x 3 24 x 5 x 1

  13. Effective surface recession approach (again) to size scaling for dynamic Young’s modulus • Premise: cracks exposed to surface and loss of surface-exposed grains contribute to reduced dynamic modulus • Dimensional correlation of -0.02 mm was applied to all dimensions in these examples • Need for more sophisticated approach is obvious…

  14. 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

  15. 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

  16. Factors limiting reduced specimen size for flash thermal diffusivity measurement Laser pulse map LFA457 • 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 Detector response 1mm-t graphite at 50°C LFA457

  17. 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.

  18. 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 Transversal, B12 x H6 x LS12.8 Transversal, B8 x H4 x LS9.8 Transversal, B2.9 x H2.8 x LS6.6 Axial, B12 x H6 x LS12.8 Axial, B8 x H4 x LS9.8 Axial, B2.9 x H2.8 x LS6.6

  19. 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

  20. 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.

  21. Deviation from Weibullscaling 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

  22. Equibiaxail flexural test for brittle materials • ASTM C1499 – 09 • Standard Test Method for Monotonic Equibiaxial Flexural Strength of Advanced Ceramics at Ambient Temperature • Specimen may be round disc or rectangular coupon Loading Ring Support Ring

  23. Here switch to C1499 PDF

  24. 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 sP F sP r

  25. 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

  26. Experimental Verification of FEA Result Fracture patterns / POCO AXF-5Q • t = 100 mm specimens: fracture initiates clearly at the loading ring locations • t = 350 mm / DL=2.5 mm specimen: fracture initiates inside the loading ring but crack propagates along the ring indicating limited stress concentration • t = 350 mm / DL=1.16 mm specimen: fracture initiates at the center of disc; cracking pattern indicates no influence of stress concentration Likely origin Likely origin Likely origin Likely origin Load ring trace 1mm DL=1.16 mm t = 100 mm DL=2.5 mm t = 100 mm DL=1.16 mm t = 350 mm DL=2.5 mm t = 350 mm

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

  28. Specimen size effect on equibiaxal flexural strength • ETU-10 tested in ring-on-ring equibiaxial flexural configuration using round disc and square coupon specimens. Bad data

  29. 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?

  30. 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”.

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

  32. Compressive Strength D10 x L20 m95 = 35 - 64 D6 x L12 m95 = 26 - 45 D10 x L13.3 m95 = 30 - 53 • Data fit two-parameter Weibull okay. (however with small n = 30) • Weibull modulus (mean MLE m = 41) appears reasonably consistent across all specimen sizes. D6 x L8 Perforated m95 = 26 - 46 D6 x L8 m95 = 21 - 37

  33. 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

  34. 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.

  35. Tensile Strength Test Matrix in ORNL Program for NTC • 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 LG = length of straight gauge section (actual gauge lengths per ASTM definition are longer)

  36. Tensile Test Specimens 5.98 Dia 7.97 Dia 9.96 Dia 12.95 Dia Extender TS3U 7.45 16.13 12.41 9.93 9.91 Dia 3.175 7.45 16.195 9.97 12.46 R= 11.72 R= 15.63 12.39 5.72 7.62 9.53 25 R= 25.4 R= 6.63 R= 19.54 14 12 6.5 Dia 26 5 Dia 4Dia 3Dia 20 14 4Dia TS6.5U TS5U ASTM C749-08 Cylindrical whole piece TS4E TS4U

  37. Tensile Strength: Specimen Size Effect Validation

  38. TS4 with Steel Extenders and Alignment Block Setup Alignment block top Steel extensor V notch Push screw Alignment block base

  39. 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. Epoxy Epoxy D6 D4

  40. Tensile Strength – Preliminary Results TS6.5U Axial Specimen X-Y Strain Gauge Reading Poisson’s ratio: 0.13 Unibody Specimens • No significant effects of gauge diameter and epoxy-extension. • More discussion in Katoh et al ASTM paper later this week. Epoxy-extended Specimens

  41. Two-parameter Weibull Analysis TS5U TS6.5U TS4E TS3U TS4U Same x-y scales for all plots

  42. Weibull Statistics

  43. Weibull Parameters 95% Confidence Bounds • No evidence for significant specimen size effect on statistical tensile strength properties.

  44. 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

  45. 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

  46. 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.

  47. 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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  49. Grain Sizes for Selected Nuclear Grade Graphite

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