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

The free format organizes the FEBio input data into hierarchical XML elements. The root element is called febio_spec. This root element also defines the format version number (Note that FEBio and the input format specification follow different version numberings). This document describes version 3.0 of the FEBio specification [F]  [F] FEBio continues to read some older formats, but they are considered to be obsolete. (see Section 3.1.1↓ below for more details on the different input specification formats). The root element will therefore be defined as follows:

<febio_spec version="3.0">
<!-- contents of file -->
</febio_spec>

The different sections introduced in this chapter are child elements of this root element. The following sections are currently defined:

The current format specification expects the different sections of the input file to be listed in the same order as given above. Not all sections are required. Empty sections can be omitted and some are optional. A minimal file must contain at least the Control, Material, Mesh, and MeshDomains sections. The rest of this chapter describes each of these sections in more detail.