  If a finite element mesh contains concave elements, it is said to tangled.
Tangled meshes can occur during mesh generation, mesh optimization, and large
deformation simulations, and will lead to erroneous results during finite
element analysis. Recently, the authors introduced the tangled finite element
method (TFEM) to accurately handle tangled 2D concave quadrilateral elements.
In this paper, TFEM is extended to 3D hexahedral elements. We demonstrate that
TFEM leads to accurate results over tangled hexahedral elements, requiring
minimal changes to existing FEM framework.
