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

Contents - Introduction - background/purpose - Experiment- specimens, compressive cyclic loading condition, X-ray tomography - Results - loading-unloading behaviors under compressive loading- unloading cycles - pore microstructure change during load relaxation (before crack initiation) - Conclusion Contents

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 ( 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

21 Thank you for your attention !