Three point bending tests of single edge notched beam (SENB) specimens were carried out to evaluate the fracture behavior of the fine-grain isotropic nuclear grade graphite, IG-11. To measure the crack initiation point and the subsequent crack growth, the direct current potential drop (DCPD) method ...
Three point bending tests of single edge notched beam (SENB) specimens were carried out to evaluate the fracture behavior of the fine-grain isotropic nuclear grade graphite, IG-11. To measure the crack initiation point and the subsequent crack growth, the direct current potential drop (DCPD) method and a traveling microscope were used. The effects of test variables like initial crack length, specimen thickness, notch type and loading rate on the measured fracture toughness, $K_Q$, were investigated. Based on the test results, the ranges of the test variables to measure the reliable fracture toughness value were proposed. During the crack growth, the rising R-curve behavior was observed in IG-11 graphite when the superficial crack length measured on the specimen surface was used. The increase of crack growth resistance was discussed in terms of crack bridging, crack meandering, crack branching, microcracking and crack deflection, which increase the surface energy and friction force.
Three point bending tests of single edge notched beam (SENB) specimens were carried out to evaluate the fracture behavior of the fine-grain isotropic nuclear grade graphite, IG-11. To measure the crack initiation point and the subsequent crack growth, the direct current potential drop (DCPD) method and a traveling microscope were used. The effects of test variables like initial crack length, specimen thickness, notch type and loading rate on the measured fracture toughness, $K_Q$, were investigated. Based on the test results, the ranges of the test variables to measure the reliable fracture toughness value were proposed. During the crack growth, the rising R-curve behavior was observed in IG-11 graphite when the superficial crack length measured on the specimen surface was used. The increase of crack growth resistance was discussed in terms of crack bridging, crack meandering, crack branching, microcracking and crack deflection, which increase the surface energy and friction force.
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문제 정의
Previously, a rising R-curve behavior was reported and explained by crack bridging and microcracking mechanisms [6]. In this paper, additional toughening mechanisms of the isotropic graphite are studied through the microstructural observations.
가설 설정
1. Schematic diagram of the DCPD test system; (a) SENB specimen for DCPD test, (b) DC current circuit.
1. To obtain reliable fracture toughness value, the specimen thickness should be larger than 10 mm and a0/W within the range of 0.1 and 0.5. The loading rate has a minor effect on KQ below 1 mm/min.
2. Traveling microscope measures the superficial crack length, while DCPD measures the effective crack length, which excludes the bridged area. DCPD method is beneficial in determining the KQ of the nuclear grade graphite.
3. The decrease in KQ was observed for a0/W larger than 0.6. This tendency might be caused by the boundary effect which is related to the process zone toughening.
4. The rising R-curve behavior was observed in IG-11 graphite. The main mechanisms for the rising R-curve behavior of the nuclear grade graphite were considered to be crack bridging and microcracking.
제안 방법
After the tests, the specimens were polished and observed using optical microscope (OM) to investigate the microcracks, branch cracks and others features produced during the fracture test. Some of the cracked specimens were oxidized at 900℃ for 10 min to expose the coke particles which show relatively high oxidation resistance than binder phase of graphite.
Using the above mentioned test conditions, the fracture toughness tests were performed with 30 specimens at a0/W of 0.4, B/W of 0.75 and a loading rate of 0.05 mm/min. The mean KQ was 0.
이론/모형
Direct current potential drop (DCPD) method was used to measure the crack initiation point and crack length. Schematic diagram of the DCPD system is shown in Fig.
성능/효과
Weibull parameter in this study was considerably high, which means the narrow distribution of measured KQ. Therefore, the reliability of test method used in this study was considered sufficiently high.
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