Exact results concerning multifield moduli of two-phase composites

Chen has recently shown how the response matrix of a two-phase composite can be written as linear combinations of products of the component matrices. We elaborate on Chen's expansions by deriving them in a different way, which a. shows them in a different light, and b. permits us to generalize them further. As an application of our results we find exact microstructure-independent relations between the moduli of the two components and those of any composite. The body of these relations is equivalent to the compatibility relations of Milgrom and Shtrikman (1989), but they are cast in a rather different form, which has certain advantages. As an example, we show how any modulus of an arbitrary two-phase composite can be written in closed form as a linear combination of any other $n$ of its moduli, with coefficients that depend only on the component moduli, but not on the volume fractions, or the microstructure.