The normal stresses on each face of the deformed solid are

(28.10)

Hence equation 28.9 can be written as,

(28.11)

The above expressions suggest that an isotropic stress produces no strain and that the superposition of an isotropic stress on existing stresses generates no incremental deformation. The incremental stress-strain relations are obtained by taking differentials of the finite stresses

(28.12)

Differentiating the stress-strain relations in equation 28.11 we obtain,

(28.13)

which yields the following expressions for the incremental strains,

(28.14)

Or

(28.15)

For which the coefficients  and  are written as,

(28.16)