A study of crack growth in nuclear graphites using R-curve analysis

During service, moderator graphites are subject to neutron irradiation that induces dimensional changes and generates internal stresses. Historically, the fracture behaviour of poly-granular graphites has been characterized by linear-elastic fracture mechanics, but graphites exhibit marked non-linearity in stress-strain curves and irreversible deformation mechanisms like crack branching and bridging which are characteristic of elastic-plastic materials. The internal stresses generated eventually lead to cracking and ultimately failure of graphite components by fast fracture. In this paper, the crack growth resistance, R, is considered and is defined as the energy required per unit crack area increase and therefore includes both the elastic and plastic energy components of the fracture process. Two independent graphical methods to determine R have been evaluated. The first method requires the comparison of two adjacent load cycles (to zero load) to determine the envelope of energy consumed in propagating the crack whilst the second method compares the load displacement curve (without the need for load cycles) with that of an ideal linear elastic material. Both methods show similar results which differ from other carbon materials in that a relatively high resistance to crack initiation is observed followed by a slightly decreasing crack propagation and crack termination. (authors)