The Voigt and Reuss approximations can be interpreted as the ratio of average stress and average strain within the microstructure. The stress and strain are generally unknown and expected to be non-uniform.

The upper bound (Voigt) assumes that the strain is uniform everywhere. The lower bound (Reuss) assumes that the stress is uniform everywhere. The Mori-Tanaka approximation works for Ellipsoids and circular fibers and needs the input of analytic data (*.gad files).