Presentation on theme: "Evaluation of Fracture toughness of fine- and coarse-grain graphite"— Presentation transcript:
1Evaluation of Fracture toughness of fine- and coarse-grain graphite J. Sumita1, T. Shibata1, Y. Tachibana1, M. Kuroda21 : Nuclear Hydrogen and Application Research Center , JAEA2 : Kumamoto University14th International Nuclear Graphite Specialist Meeting15-18th September, 2013, Seattle , USA11
3Non-destructive inspection Methodology to confirm the integrity of graphite component in HTTR -Acceptance inspection-In order to remove the components with harmful defects, the non-destructive inspection UT and ET are carried out for the components in acceptance inspection.StructureNon-destructive inspectionMaterialAfter machiningControl rod guide blockUTETHot plenum block-Support post/seatUT : Ultra sonic test, ET : Eddy current testComponents without harmful defects do not fracture through the in-service period on the basis of fracture mechanics.
4Methodology to confirm the integrity of graphite component in HTTR -In-service inspection- In order to confirm the integrity of the graphite components in in-service inspection,Surveillance testMandatory or non-mandatory (depending on necessity)Visual inspection by TV camera etc.Mandatory (for HTTR)If a defect ( > design assumption) is found in graphite components by in-service inspection,Causes shall be investigatedPropriety of continuous use shall be judged by application of fracture mechanics etc.It is necessary to understand the fracture mechanism of graphite.
5ObjectivesPrevious studies of evaluation on fracture features of graphiteEffect of notch sharpness and size of specimenEffect of oxidation, etcExperimental methodology to determine fracture toughness of graphite (ASTM D7779)Some qualitative theories for fracture of graphiteTo characterize fracture features of fine- and coarse-grained graphitesThe three-point-bending test is carried out for two kinds of specimens, a fine- and coarse-grained graphite.The fracture mechanism of both fine- and coarse-grained graphites is investigated.
7Typical properties of G347 and FE250 MaterialFine-grained isotropic graphiteG347 (Manufactured by TOKAI CARBON)Coarse-grained extruded graphiteFE250 (Manufactured by TOKAI CARBON)Typical properties of G347 and FE250GradeBulk density(g/cm3)Bending strength(MPa)Tensile strengthThermal expansion*(10-6/K)Thermal conductivity(W/m・K)G3471.8549.031.45.5116FE2501.7524.5-3.3162*RT – 1000oC
8Specimen preparation Single edge notched beam specimen Straight-through notchSpecimen geometryIntroduce of notch: razor bladeNotch angle: approximately 15oDepth and width of notch: profile projectorCleaning: ethanol and acetone in the ultrasound bathRoot radius and notch angle: laser microscope
9Fracture toughness test Three-point-bending testThree-point-bending test setupOuter support span: 160 mmCrosshead speed: 0.1 mm/minSampling rate to record load and displacement: 200 Hz
10Calculation of fracture toughness Calculation of value of fracture toughnessFracture toughness KC is given by linear fracture mechanicsKC : Fracture toughness (MPa/m1/2)Pmax : Maximum force (N)S : Support span (m)B : Specimen breadth (m)W : Specimen width (m)a : Notch depth (m)The value of the fracture toughness was calculated using the maximum force on the load-displacement curve obtained by the three-point-bending test.
12Load-displacement curves A typical load-displacement curveDisplacement (mm)Load (N)A : across grain<FE250AG347SlopeMaximum load PmaxSlopeMaximum load Pmax
13Value of fracture toughness (Obtained in this study) GradePmax(N)Kc(MPa/m0.5)(Obtained in this study)G347148.31.15FE250A125.00.96FE250W132.41.02A : across grain, W : with grainPrevious study for G347*1Fracture toughness obtained by CT specimen (1.06 MPa/ m0.5)Fracture toughness obtained by SENB specimen (1.09 MPa/ m0.5)*1 Ekinaga, at. El., INGSM 9Nearly equal
14Fracture toughness of fine-grained graphite GradePmax(N)Kc(MPa/m0.5)(ASTM)(Previous study)IG110115.30.89*10.85 *2IG430136.01.04*10.91*2ETU10121.00.93*3－*1 Yamada, et. al., HTR*2 Kurumada, et. al., Transaction of the Japanese society of the mechanical engineeras, A,63(608), , (1997)*3 Matsushima, et. al., M&M 2012 (in Japanese)The fracture toughness of fine-grained graphite obtained in accordance with ASTM D7779 is almost the same as that reported in the previous studies.The value of fracture toughness of the fine-grain graphite is not different from that of the coarse-grain graphite.
15Application to HTGR components Fracture stressσc : Fracture stress (MPa)KIC : Fracture toughness (MPa/m1/2)a : Harmful defect size(m)α : Shape factor𝜎 𝑐 = 𝐾 𝐼𝐶 𝛼 𝜋∙𝑎The fracture stress depends on the harmful defect size.Fine-grained graphiteSmall harmful defectHigh fracture stressThe employment of fine-grained graphite for core components of the HTGR has some advantages.
16Fracture mechanismGrainBinder (pitch)PoreCrackGrain sizeLargeSmallThe fracture of graphtie is influenced by pre-existing defects or weak region.The crack would basically propagate in pore.If higher stress applied, the crack would propagate in binder.The crack in both fine-grained and coarse-grained graphite would propagate in pores and binders along with grain boundary.If the direction of crack growth corresponds to the grain with proper orientation, the crack would propagate inside grain.In order to confirm this mechanism, the cracks propagation are observed.
18Crack observation by stereomicroscope （G347） 1000um×CrackThe crack seems to propagate straight.
19Crack observation by stereomicroscope （FE250W） ×CrackInside grain?×The crack seems to propagate in pores and binders.Some cracks seem to propagate inside grain.1000um
20Crack observation by stereomicroscope （FE250A） ×1000umCrackInside grain?The crack seems to propagate in pores and binders.Some cracks seem to propagate inside grain.Future worksPolarization microscope , EBSD (electron back scattering diffraction).
22SummaryThe fracture toughness of nuclear grade graphite obtained in accordance with ASTM D7779 is almost the same as that reported in the previous studies.The value of fracture toughness of the fine-grained graphite is not different from that of the coarse-grained graphite.The crack in coarse-grained graphite seems to propagate along with grain boundary and some cracks seem to propagate inside grain.It is planned that the crack propagation in graphite is observed using the polarization microscope and EBSD to confirm the direction of crack growth.