$\newcommand{\Dev}{\operatorname{Dev}}$ $\newcommand{\dev}{\operatorname{dev}}$ $\newcommand{\grad}{\operatorname{grad}}$ $\newcommand{\divg}{\operatorname{div}}$ $\newcommand{\Ei}{\operatorname{Ei}}$ $\newcommand{\tr}{\operatorname{tr}}$

Some of the materials are transversely isotropic, requiring the specification of an initial material direction, which is called a fiber direction in FEBio. Other materials are orthotropic, requiring the specification of initial material axes that define the three planes of symmetry for those materials. Only one of these specifications should be provided. By default, material axes are aligned with the global Cartesian basis $\left\{ \mathbf{e}_{1},\mathbf{e}_{2},\mathbf{e}_{3}\right\}$ and the fiber direction is along $\mathbf{e}_{1}$ . Local fiber or material axes orientation may be specified in several ways. FEBio gives the option to automatically generate the orientation, based on some user-specified parameters. However, the user can override this feature and specify the fiber or axes directions for each element manually in the ElementData section. See Section 3.8↑ for more details on how to do this.