A fracture mechanics approach to the failure of graphite in laboratory tests

Abstract

Laboratory tests show that for some graphites the calculated stress at failure exceeds the tensile strength in uniform tension by an amount which depends on the test considered and increases with the severity of the stress gradient. Fracture mechanics has been applied to bend, internal pressure and diametral compression tests to investigate whether it can provide a consistent failure criterion for graphite in situations where stress gradients are important. A finite element method of analysis has been used to compute the stress distributions in specimens containing cracks of various lengths, from which energy release rates and stress intensity factors have been derived. The results indicate that there is not constant effective inherent crack size which can be employed with a constant critical stress intensity factor KIC to predict the failure conditions in the three tests considered. However, assuming a constant KIC, a relationship has been found between the effective inherent crack size responsible for failure and the maximum stress gradient in a specimen, which may be of value in developing a fracture criterion for practical purposes.