Presentation on theme: "14 th International Nuclear Graphite Specialist Meeting (INGSM ̵ 14 ) September 16-18, 2013, Hilton Seattle, Washington State, USA Load relaxation and."— Presentation transcript:
14 th International Nuclear Graphite Specialist Meeting (INGSM ̵ 14 ) September 16-18, 2013, Hilton Seattle, Washington State, USA Load relaxation and damage formation behavior of selected nuclear graphite grades under controlled compressive loading- unloading cycles Se-Hwan Chi, Min-Hwan Kim Nuclear Hydrogen Development and Demonstration Project, Korea Atomic Energy Research Institute (KAERI), Dae Deok-daero , Yuseong, Daejeon Rep. of Korea ) 1
3 Large differences in the fracture characteristics of IG-110 and NBG-18 of different forming method and ingredient Fracture toughness of IG-110 is enhanced by crack bridging, deflection and microcracking. In case of NBG-18, big pores are important. IG-110 The main mechanisms for fracture toughness enhancement Crack bridging Deflection Microcracking NBG-18 No crack bridging Large crack deflection Big pores When primary crack meets pores in matrix, crack tip is blunted. These pores absorb crack propagation energy. IG-110 NBG-18
IG ㎛ Role of pores/grains during crack extension Grain Size = 20 ㎛
5 NBG18-a 400 ㎛ Large crack deflection Ave. Grain Size: 300 ㎛ Role of pores/grains during crack extension
PCEA-a 200 ㎛ Grain Size: 360 ㎛ Role of pores/grains during crack extension Large crack deflection
Fracture Toughness and Strain Energy Release Rate NBG-18 > IG-110
Purpose of Study Based on these observation (crack initiation and extension behavior), differences in the mechanical damage processes far before main crack formation in the IG-110, NBG-18, and PCEA were investigated and compared based on the load relaxation and pore microstructure change owing to compressive cyclic loading-unloading.
GradeManufacture Forming Method Coke Type Grain Size (ave. ㎛ ) Density (g/cm 3 ) NBG-18SGLVibr.Pitch PCEAGraf.Extr.Pet IG-110Toyo Tan.Iso-MolPet Material * For NBG and PCEA, the forming (molding) direction was considered as NBG-18-(a), - (c), PCEA- (a), - (c), where (a) and ( c ) refer to the notch direction machined to the “molding (extrusion) direction” and “ perpendicular to the molding (extrusion) direction, respectively.
4-1/3 flexure loading (ASTM C ) Specimen Dimension : 16 (W) x 18 (T) x 64 (L) mm Loading rate: 0.5 mm/min in compression to 0.13 mm (10 cycles) mm corresponds to 0.81 (IG-110, NBG) and 0.87 (PCEA) of the displacement to fracture, and produces 0.65 (NBG-18-c) ̶ 0.97 (IG-110) of the fracture load (0.68 (NBG-18-a, PCEA-c), 0.75 (PCEA-a)). ASTM C (Reapproved 2005) Standard Test Method for Flexural Strength of Manufactured Carbon and Graphite Articles Using Four-Point Loading at Room Temperature Instron Load Cell: 5 KN
11 After 10 loading ̶ unloading cycles under the displacement control, specimens for X-Ray Tomography (3 x 3 x 15mm) were machined from the notch-root area using a diamond saw.
WALISCHMILLER RAY SCAN 250 (No. of Pixel : 1024 x 1024) Voxel size: 9.7 ㎛ at 90 KeV, 110 ㎂ (IG-110), 11.0 ㎛ at 100 KeV, 130 ㎂ (NBG, PCEA) Scanning: 50 min. per specimen Detector: Digital X-Ray detector ( 16 " a-silicon sensor. Model: Perkin Elmer XRD 1641AN) Scanned data were processed by VX3D (www.3Dii.kr)).www.3Dii.kr) X-Ray Tomography Estimation of the changes in the number of pores, and pore volume owing to compressive cyclic loadings.
Contents Loading-Unloading Behavior No cyclic hardening or softening observed. After the first loading-unloading cycle, next 9 cycles were the same with the first unloading curve.
Differences owing to crack orientation observed. Cyclic softening in C direction. (microcracks ?)
15 Differences owing to crack orientation observed. Cyclic softening in C direction may be attributed to the formation of microcracks.
Comparison of the relaxation load after 10 loading-unloading cycles Grade Initial load before unload- ing (Kg) Relaxation load after 10 cycles (Kg) Relaxation load (%) IG NBG-18 (a ) NBG-18 (c ) PCEA (a) PCEA (c) Small cyclic softening ( 0.26 – 1.28 %) and anisotropy in the relaxation load (c > a) were observed during the cycle. PCEA(a) and NBG-18(a) showed a small load relaxation (0.26, 0.40 %). Small load relaxation = high toughness ?
Large pores are seen to be formed below v-notch. IG-110 No crack observed Number of Pores: 9,953, Total Volume of Pores: 3.59 mm 3 NBG-18-a Number of Pores: 1,753, Total Volume of Pores:3.00 mm 3 Results from X-Ray Tomography (After 10 cycles)
PCEA-a Number of Pores: 8,930, Total Volume of Pores: 1.59 mm 3
GradeConditionNumber of poresTotal volume of pores (mm 3 ) IG-110 Before (As-received) 15, After (Damaged) 9, 953 (36% decreased) 3.59 (146% increased) NBG-a Before2, After 1,753 (40% decreased) 3.00 (150% increased) PCEA-a Before12, After 8,930 (29% decreased) 1.59 (46% increased) Number of pores decreased 29 – 40 %, and the total volume of pores increased 47 – 150 % after 10 compressive loading-unloading cycles. These changes in the pore structure may reflect the main crack formation process in graphite.
20 Mechanical damages on the crack tip area of the notched 4- 1/3 loading graphite specimens before crack formation under the displacement controlled compressive ten loading- unloading cycles (displacement=1.3 mm) appeared as a grade-specific small load relaxation ( %) and a large pore microstructure change in the number of pores and total pore of volume: Thus, decrease in the number of pore ( %) and an increase in the total pore volume (46 – 150 %). Crack-initiation mechanism in the notched graphite structure under the compressive loading–unloading cycles may be understood based on the present observation on the pore microstructure change during the cycles. Conclusion