Effect of stress gradient on the fracture of graphite

In an attempt to formulate consistent fracture criteria for reactor moderator graphites when stress gradients are present in the specimen, the explicit dependence of the stress intensity on stress gradient in such specimens has been examined and the possibility of formulating a simple fracture criteria involving the gradient, considered. Using the expression obtained for the stress intensity factor, data on five graphite beams have been analysed. From the results obtained it is concluded that expressions for the critical stress intensity factor of the form under consideration are incapable of explaining the data for graphite, even in situations where the stress gradient is uniform, and consequently no simple fracture criterion involving the stress gradient can be formulated. (U.K.)