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

A finite element mesh is required to solve a problem with FEBio. The mesh defines a discretization of the problem domain in nodes and connected elements. FEBio only supports certain elements and thus the mesh must be composed of elements from this set. See Section 3.6.2↑ for a discussion of the supported elements. In addition, FEBio assumes a specific ordering of the nodes of an element. FEBio cannot discover if the nodes are in the correct order, but if they are not, FEBio will most likely have trouble converging or throw negative jacobians. If FEBio discovers negative jacobians before the first time step, it is likely that the nodes of the elements are not defined in the proper order.