  The geometry of post mortem rough fracture surfaces of porous glass ceramics
made of sintered glass beads is shown experimentally to be self-affine with an
exponent zeta=0.40 (0.04) remarkably lower than the 'universal' value zeta=0.8
frequently measured for many materials. This low value of zeta is similar to
that found for sandstone samples of similar micro structure and is also
practically independent on the porosity phi in the range investigated (3% < phi
< 26%) as well as on the bead diameter d and of the crack growth velocity. In
contrast, the roughness amplitude normalized by d increases linearly with phi
while it is still independent, within experimental error, of d and of the crack
propagation velocity. An interpretation of this variation is suggested in terms
of a transition from transgranular to intergranular fracture propagation with
no influence, however, on the exponent zeta.
