We do not characterize materials by their E mnpq The Emnpa are useful in doing transformations, manipulations, etc. We characterize materials by their \ENGINEERING CONSTANTS\ (or, Elastic Constants)
Let's first review a bit... from Unified, saw that there are basic considerations in elasticity: 1. Equilibrium 2. Strain-Displacement 3. Stress- Strain Relations (Constitutive Relations)