Elastic Constants of Plane Orthotropic Elasticity

The four independent material parameters of plane orthotropic elasti city are introduced as the effective stiffness, the effective Poisson ratio, the stiffness ratio and the shear parameter. It is proved that stress boundary value problems with zero resulting force on internal contours lead to stress fields that are independent of the effective stiffness and the effective Poisson ratio, and a general transformation is described which is equivalent to a change of the stiffness ratio. These properties suggest the importance of the remaining shear parameter, that has the interesting property of being invariant with respect to a 90 degree rotation of the principal axes of the material.