For two porous components *A* and *B* in welded contact, the volume fractions
of the components *v*_{A}, *v*_{B} satisfy

v_A + v_B = 1, while the overall porosity is given by

= v_A_A + v_B_B,
where are respectively the porosities of the porous constituents
*A* and *B*.
The fractions of the total volume occupied by the solid components are
and , respectively.
The solid components in *A* and *B* are individually pure, so that each
porous constituent may be thought of as a microhomogeneous (or Gassmann) material.

11/12/1997