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Conference Article: Elastic-plastic fracture toughness of HTGR graphites

Oku, T.; Ishiyama, S.; Eto, M. (Japan Atomic Energy Research Inst., Tokai, Ibaraki. Tokai Research Establishment); Inagaki, M.; Hashida, T.; Takahashi, H

Abstract

A fracture mechanics approach is important for evaluating the possibility of extension of cracks in graphite components in an HTGR. The effects of various factors on J-integral values which correspond to the initiation of crack extension were examined for the purpose of establishing a test method for obtaining the valid toughness value. Materials tested are five grades of HTGR candidate graphites. The compact tension specimen was used for evaluating elastic-plastic fracture toughness values. As the size of the notch root radius of CT specimens increases, J-integral values increase. Jin values which corresponds to the crack initiation point are nearly constant when the notch root radius is below 0.2 mm. Jin values are not affected by the crosshead speed between 0.005 mm/min and 0.05 mm/min and by a ratio of crack length to width between 0.4 and 0.55. Jin values depend on the thickness of specimen and become constant above a value of thickness which depends on the graphite grade. From these results the conditions for obtaining valid fracture toughness values were made clear. Jin values and J-R curves showed anisotropy which is reflected by the preferred orientation of the basal plane in the graphite blocks. Frequency spectra of acoustic emission associated with crack extension could be classified into some types which depend on the graphite grade.

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key words: acoustic emission testing; anisotropy; cracks; elasticity; fracture properties; graphite; htgr type reactors; isotropy; notches; plasticity; acoustic testing; carbon; gas cooled reactors; graphite moderated reactors; materials testing; mechanical properties; nondestructive testing
Reference:
Specialists' meeting on graphite component structural design, JAERI Tokai (Japan), September 8-11, 1986
International Atomic Energy Agency, Vienna (Austria). International Working Group on Gas-Cooled Reactors
IWGGCR--11, pp:217-221