Fracture criterion of reactor graphite under multiaxial stresses

New fracture criteria for graphite under multiaxial stresses are presented for designing core and support materials of a high temperature gas cooled reactor. Different kinds of fracture strength tests are carried out for a near isotropic graphite IG-11. Results show that, under the stress state in which tensile stresses are predominant, the maximum principal stress theory is seen as applicable for brittle fracture. Under the stress state in which compressive stresses are predominant, there may be two fracture modes for brittle fracture, namely, slipping fracture and mode II fracture. For the former fracture mode the maximum shear stress criterion is suitable, but for the latter fracture mode the following mode II fracture criterion including a restraint effect for cracks is verified to be applicable, σ3 = {σc/σt(KIc/KIIc) - 1}σ1 - σc where σ1 and σ3 are the maximum and minimum principal stresses, σt and σc are the tensile and compressive strengths and KIc and KIIc are the mode I and II fracture toughness values, respectively. The above equation is similar in form to the Coulomb-Mohr criterion. Also a statistical correction for brittle fracture criteria under multiaxial stresses is discussed. By considering the allowable stress values for a safe design, the specified minimum ultimate strengths corresponding to a survival probability of 99 % at the 95 % confidence level are presented. (author)