Radiation Effects on the Fracture Stress of Coated SiC Layer

Full text of publication follows: The effects of irradiation on the fracture stress of coated SiC layers have been investigated by using tubular specimens. Thin-walled tubular SiC specimens of 1.22 mm outer diameter, 0.1 mm wall thickness, and 5.8 mm length were produced by a serial process of chemical vapor deposition (CVD), grinding, and carbon burning. The specimens were irradiated in the high flux isotope reactor (HFIR) to 1.9 - 9.3 dpa at 600 - 1300 deg. C. Both the irradiated and non-irradiated specimens were fractured by internal pressurization and diametrical loading methods. In the internal pressurization tensile hoop stress was induced in the wall of tubular specimens by compressively loading polyurethane insert, while in the diametrical loading the tensile hoop stress was given at the inner surface below loading contact. In the results of internal pressurization tests, the baseline data for non-irradiated specimens showed that the mean fracture stress before irradiation was about 297 MPa and the Weibull modulus was 6.9. After irradiation to 1.9 dpa at 1020 deg. C the mean fracture stress increased to 337 MPa and the Weibull modulus decreased to 3.9. The mean fracture stresses and Weibull moduli at 4.2 dpa were similar to those at 1.9 dpa. The results for 4.2 dpa cases also indicated that the increase in irradiation temperature from 1020 deg. C to 1280 deg. C did not cause any discernable change in fracture stress distribution. It was observed that the neutron irradiation increased the range of fracture stress distribution without significant change in the lowest fracture stress values, about 200 MPa. However, the irradiation increased the highest fracture stress from ∼350 MPa to 400 - 600 MPa. This widened range resulted in the increase of mean fracture stress and in the decrease of Weibull modulus. Similar but wider range of fracture stresses were measured from the diametrical loading method. Results from this study were compared with the data on fracture and statistics taken from other specimen types. (authors)