EFFECTIVE SURFACE ENERGY OF POROUS SOLIDS.

The effective surface energy for fracture initiation and for fracture propagation has been determined by three-point bending on notched bars of a series of porous polycrystalline graphites, having nearly identical solid matrix but a different pore sub-structure owing to impregnation of some samples with pitch followed by graphitization. The pure effect of pore volume and pore shape could be thus studied, not obscured by modifications of other textural characteristics usually accompanying changes of porosity in other ceramic materials. Discussion of the data, based on analysis of variance and regression, has made it possible to account for the effect not only of the volume per unit area but also of the shape of pores on the effective energies and thus on the fracture strength vs. porosity relationships.