Extreme values of Poisson's ratio and other engineering moduli in   anisotropic materials
  Conditions for a maximum or minimum of Poisson's ratio of anisotropic elastic materials are derived. For a uniaxial stress in the 1-direction and Poisson's ratio $\nu$ defined by the contraction in the 2-direction, the following three quantities vanish at a stationary value: $s_{14}$, $[2\nu s_{15} + s_{25}]$ and $[(2 \nu -1)s_{16} + s_{26}]$, where $s_{IJ}$ are the components of the compliance tensor. Analogous conditions for stationary values of Young's modulus and the shear modulus are obtained, along with second derivatives of the three engineering moduli at the stationary values. The stationary conditions and the hessian matrices are presented in forms that are independent of the coordinates, which lead to simple search algorithms for extreme values. In each case the global extremes can be found by a simple search over the stretch direction $\bf n$ only. Simplifications for stretch directions in a plane of orthotropic symmetry are also presented, along with numerical examples for the extreme values of the three engineering constants in crystals of monoclinic symmetry. 
Andrew N. Norris