Effect of oxidation on fracture mechanical properties of isotropic graphite

The mechanical properties and fracture behavior of isotropic graphite were investigated after oxidation at 400 or 500degC in air. The bending strength after oxidation was affected significantly by the surface roughness, which was depended on open pores estimated by kerosine. The bending strength decreased with increasing porosity, given by σ=σ0exp(-bp), where σ0 and b were constants. The fracture energy of isotropic graphite was obtained from the load-displacement curve by using a CT specimen. The total fracture energy and elastic fracture energy decreased with the extended oxidation, accompanied with weight loss and increase of porosity. Pores were not localized at the fracture surface after oxidation, but the number of pores increased with oxidation time. The modified fracture energy could be determined by the calculated fracture area A', which was estimated from the area A for non-oxidized specimen and the porosity p for the oxidized specimen, as A'=A(1-p2/3). The modified fracture energy of isotropic graphite after oxidation was constant and independent of the degree of oxidation. (author)