Concerning the fracture of graphite under different test conditions

Abstract

Results from different fracture tests on near-isotropic polycrystalline graphites of the type used in nuclear reactors are presented in terms of the maximum elastic tensile stress at failure in uniform tensile, bend, internal pressure and diametral compression tests. These show that the calculated stress at failure exceeds the tensile strength in uniform tension and increases with the degree of severity in the stress gradient. Various theoretical approaches offering qualitative explanations are explored quantitatively in an attempt to inter-relate the results from the different laboratory tests, viz. the effect of the non-linear stress-strain curve for graphite, the statistical theory of Weibull, and empirical correlations in terms of energy density and stress gradient. The limitations of these approaches are discussed, and it is concluded that in a general case none of them offers a consistent method of treating failure under different geometric conditions and improving providing on a pessimistic criterion of equating the maximum calculated elastic stress to the tensile strength. It is possible that specific cases in service may be related to a particular laboratory test with some better definitions of the failure conditions.