Influence of oxidation on toughness parameters for two nuclear grade graphites

Abstract

Single edge notched beams of two nuclear graphites were oxidized in CO2 at 900°C up to 49% burn-off for IM1-24 graphite and up to 12% burn-off for PGA graphite, and fracture properties KIc, γwof and GIc were measured as a function of oxidation. Property decrements as a function of burn-off were fitted to an exponential decay curve (the Knudsen equation) characterized by a decay parameter, b. For both graphites oxidized to the same extent, the decrements in γwof and GIc are less than for the stress-based parameter KIc. The more severe decrements for KIc are probably because critical flaws of high aspect ratio influence this toughness parameter, whereas the energy-based toughness parameters are influenced by a spectrum of pores of different sizes and shapes. For IM1-24 graphite, crack propagation makes an increasing contribution to the total work of fracture upon oxidation due to growth in the process zone size ahead of the crack tip. The anisotropy of PGA graphite is reflected in the values of Q0 for the three properties parallel and perpendicular to the extrusion direction, but there is no clear evidence for anisotropy in the exponential decay parameter, b. This may indicate that processes that occur in the more isotropic binder phase rather than the anisotropic filler particles dominate fracture.