## Chapter 2: Principal Directions in Autodyn

An orthotropic material has properties that are different in three mutually perpendicular directions and has three mutually perpendicular planes of material symmetry. Thus the properties of such a material are a function of orientation of the material with respect to the global co-ordinate system. The orthotropic constitutive models to be described in 3 are done so in terms of the principal material directions. It is therefore required to define the initial orientation of these principal material directions in an Autodyn model with respect to the global co-ordinates. Definition of the principal material direction is described in Autodyn-2D for Autodyn-2D and in Autodyn-3D Lagrange and ALE Parts and Autodyn-3D Shell Parts for Autodyn-3D.

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