FEBio implements the finite element discretized balance of linear momentum equation of continuum mechanics. The nonlinear equations are solved by an iterative Newton-based solver that requires the linearization of the discretized equations. The BFGS nonlinear iteration method with line search is the primary and preferred method for the nonlinear solution, but a full Newton nonlinear iteration method can be used as well.

FEBio is designed for 3D applications so it features 3D (solid) elements and 2.5D (surface) elements. The available element types are hexahedral, pentahedral and tetrahedral for the solid elements and triangle and quadrilateral for the surface elements. For nearly-incompressible materials, FEBio uses a three-field element that does not lock. As part of our recent work on poroelasticity, we have augmented the element models to yield a u-p formulation of poroelasticity.

Several standard isotropic hyperelastic constitutive models are available, including St-Venant Kirchhoff, Neo-Hookean and Mooney-Rivlin. A number of biologically relevant constitutive models are available as well such as a transversely isotropic hyperelastic model. This constitutive model can be used to model materials consisting of a solid matrix and a (local) preferred fiber direction such as muscles, ligaments and tendons. An active contraction model is also available for use with the anisotropic constitutive models. This can be used to model active contraction of skeletal and cardiac muscle. When using the poroelastic material model, the user can choose from any of the solid phase constitutive models mentioned above.

In FEBio rigid bodies are represented using active constraints (in stead of using very stiff elements) on the nodes that compose the rigid body such that the system of linear equations for the overall model can be drastically reduced and a much more computationally efficient algorithm is obtained. Deformable and rigid bodies can be connected easily. Rigid bodies can be linked together using kinematic joints.

Prescribed displacements, nodal forces and pressure forces (follower forces) are available as boundary conditions. In addition, body forces to model base acceleration or gravity are also available.