Presentation on theme: "1 Evaluation of Fracture toughness of fine- and coarse-grain graphite J. Sumita 1, T. Shibata 1, Y. Tachibana 1, M. Kuroda 2 1 : Nuclear Hydrogen and Application."— Presentation transcript:
1 Evaluation of Fracture toughness of fine- and coarse-grain graphite J. Sumita 1, T. Shibata 1, Y. Tachibana 1, M. Kuroda 2 1 : Nuclear Hydrogen and Application Research Center, JAEA 2 : Kumamoto University 14 th International Nuclear Graphite Specialist Meeting th September, 2013, Seattle, USA
3 Methodology to confirm the integrity of graphite component in HTTR -Acceptance inspection- UT : Ultra sonic test, ET : Eddy current test Structure Non-destructive inspection MaterialAfter machining Control rod guide blockUTET Hot plenum block-ET Support post/seatUTET 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. Components without harmful defects do not fracture through the in-service period on the basis of fracture mechanics.
4 Surveillance test Mandatory or non-mandatory (depending on necessity) Visual inspection by TV camera etc. Mandatory (for HTTR) Causes shall be investigated Propriety of continuous use shall be judged by application of fracture mechanics etc. If a defect ( > design assumption) is found in graphite components by in-service inspection, In order to confirm the integrity of the graphite components in in-service inspection, It is necessary to understand the fracture mechanism of graphite. Methodology to confirm the integrity of graphite component in HTTR -In-service inspection-
5 Previous studies of evaluation on fracture features of graphite Effect of notch sharpness and size of specimen Effect of oxidation, etc Experimental methodology to determine fracture toughness of graphite (ASTM D7779) Some qualitative theories for fracture of graphite Objectives To characterize fracture features of fine- and coarse- grained graphites The 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.
Material 7 Typical properties of G347 and FE250 Fine-grained isotropic graphite GradeBulk density (g/cm 3 ) Bending strength (MPa) Tensile strength (MPa) Thermal expansion* (10 -6 /K) Thermal conductivity (W/m K) G FE G347 (Manufactured by TOKAI CARBON) Coarse-grained extruded graphite FE250 (Manufactured by TOKAI CARBON) *RT – 1000 o C
Specimen preparation Introduce of notch: razor blade Notch angle: approximately 15 o Depth and width of notch: profile projector Cleaning: ethanol and acetone in the ultrasound bath Root radius and notch angle: laser microscope 8 Single edge notched beam specimen Specimen geometry Straight-through notch
Fracture toughness test Outer support span: 160 mm Crosshead speed: 0.1 mm/min Sampling rate to record load and displacement: 200 Hz Three-point-bending test setup 9 Three-point-bending test
Calculation of fracture toughness Fracture toughness K C is given by linear fracture mechanics 10 K C : Fracture toughness (MPa/m 1/2 ) P max : Maximum force (N) S: Support span (m) B: Specimen breadth (m) W : Specimen width (m) a : Notch depth (m) Calculation of value of fracture toughness The value of the fracture toughness was calculated using the maximum force on the load-displacement curve obtained by the three-point-bending test.
Load-displacement curves 12 Displacement (mm) Load (N) Slope Maximum load P max Slope Maximum load P max FE250A G347 < A typical load-displacement curve A : across grain
13 GradeP max (N)Kc(MPa/m 0.5 ) (Obtained in this study) G FE250A FE250W Value of fracture toughness *1 Ekinaga, at. El., INGSM 9 Previous study for G347 *1 Fracture toughness obtained by CT specimen (1.06 MPa/ m 0.5 ) Fracture toughness obtained by SENB specimen (1.09 MPa/ m 0.5 ) Nearly equal A : across grain, W : with grain
14 GradeP max (N)Kc(MPa/m 0.5 ) (ASTM) Kc(MPa/m 0.5 ) (Previous study) IG * *2 IG * *2 ETU *3 Fracture toughness of fine-grained graphite *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.
15 Application to HTGR components Fracture stress σ c : Fracture stress (MPa) K IC : Fracture toughness (MPa/m 1/2 ) a : Harmful defect size(m) α: Shape factor The fracture stress depends on the harmful defect size. Small harmful defect High fracture stress Fine-grained graphite The employment of fine-grained graphite for core components of the HTGR has some advantages.
16 Fracture mechanism The 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. GrainBinder (pitch)PoreCrack Grain size LargeSmall In order to confirm this mechanism, the cracks propagation are observed.
Method Notch Crack Observed area Stereomicroscope OLYMPUS SZX7 Material:G347, FE250W, FE250A Observation of crack propagation 17
18 Crack observation by stereomicroscope G um × × Crack The crack seems to propagate straight.
19 Crack observation by stereomicroscope FE250W × × Crack 1000um Inside grain? The crack seems to propagate in pores and binders. Some cracks seem to propagate inside grain.
20 Crack observation by stereomicroscope FE250A × × 1000um Crack Inside grain? The crack seems to propagate in pores and binders. Some cracks seem to propagate inside grain. Future works Polarization microscope, EBSD (electron back scattering diffraction).
22 Summary The 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.