Solid Mechanics: Hooke's Law 
Orthotropic Definition 
Some engineering materials, including certain
piezoelectric materials (e.g. Rochelle
salt) and 2ply fiberreinforced composites, are orthotropic.
By definition, an orthotropic material has at least 2 orthogonal planes of symmetry, where material properties are independent of direction within each plane. Such materials require 9 independent variables (i.e. elastic constants) in their constitutive matrices. In contrast, a material without any planes of symmetry is fully anisotropic and requires 21 elastic constants, whereas a material with an infinite number of symmetry planes (i.e. every plane is a plane of symmetry) is isotropic, and requires only 2 elastic constants. 
Hooke's Law in Compliance Form 
By convention, the 9 elastic constants in
orthotropic constitutive equations are comprised of
3_{ } Young's modulii E_{x},
E_{y}, E_{z}, the 3 Poisson's ratios
n_{yz}, n_{zx}, n_{xy}, and the 3 shear modulii
G_{yz}, G_{zx},
G_{xy}.
The compliance matrix takes the form,
where . Note that, in orthotropic materials, there is no interaction between the normal stresses s_{x}, s_{y}, s_{z} and the shear strains e_{yz}, e_{zx}, e_{xy} The factor 2 multiplying the shear modulii in the compliance matrix results from the difference between shear strain and engineering shear strain, where , etc. 
Hooke's Law in Stiffness Form 
The stiffness
matrix for orthotropic materials, found from the inverse
of the compliance matrix, is given by,
where,
The fact that the stiffness matrix is symmetric requires that the following statements hold,
The factor of 2 multiplying the shear modulii in the stiffness matrix results from the difference between shear strain and engineering shear strain, where , etc. 
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