Extreme conditions of elastic constants and principal axes of anisotropy

This paper describes the derivation of extremality conditions of each elasticity coefficient (Young's modulus, shear modulus, et al.,) for the general case of linear-elastic anisotropic materials. The stationarity conditions are obtained, and they determine the orthogonal coordinate systems being the main anisotropy axes, where the number of independent elasticity constants decreases from 21 to 18 and, in some cases of anisotropy, to 15 or lower. An example of a material with cubic symmetry is given.