1.1 Overview of FEBio
FEBio is a nonlinear finite element solver that is specifically designed for biomechanical applications. It offers modeling scenarios, constitutive models and boundary conditions that are relevant to many research areas in biomechanics, thus offering a powerful tool for solving 3D problems in computational biomechanics. The software is open-source and the source code, as well as pre-compiled executables for Windows, OS-X and Linux platforms are available for download at https://febio.org. This chapter presents a brief overview of the available features of FEBio.
FEBio can solve different kinds of physics. It can solve problems in structural mechanics, biphasic and multiphasic physics, fluid mechanics, and fluid-solid interaction (FSI). Both (quasi-) static (or steady-state) and dynamic (or transient) analyses can be performed in each of the different physics modules. For instance, in the structural mechanics module, the (quasi-) static response of the system is sought in a quasi-static analysis and the effects of inertia are ignored. In a dynamic analysis, the inertial effects are included in the governing equations to calculate the time dependent response of the system. In the biphasic module, a coupled solid-fluid problem is solved. In a transient biphasic analysis the time dependent response of both the solid and the fluid phase is determined. For the steady-state analysis the final relaxed state is recovered. Similarly, for multiphasic problems, both the time dependent transient response as well as the steady-state response can be determined. For fluid analyses, dynamic and steady-state responses may be specified.
Many nonlinear constitutive models are available, allowing the user to model the often complicated biological tissue behavior. Several isotropic constitutive models are supported such as Neo-Hookean, Mooney-Rivlin, Ogden, Arruda-Boyce and Veronda-Westmann. All these models have a nonlinear stress-strain response and are objective for large deformations. In addition to the isotropic models there are several transversely isotropic and orthotropic constitutive models available. These models exhibit anisotropic behavior in a single or multiple preferred directions and are useful for representing biological tissues such as tendons, muscles, cartilage and other tissues that contain fibers. FEBio also contains a rigid body constitutive model. This model can be used to represent materials or structures whose deformation is negligible compared to that of other materials in the overall model. Several constitutive models are available for representing the solid phase of biphasic and multiphasic materials, which are materials that contain both a solid phase and a fluid phase. For incompressible materials FEBio employs special algorithms for enforcing the incompressibility constraint. A three-field formulation is used for tri-linear hexahedral and wedge elements. This algorithm allows the user to capture the accurate response of highly incompressible materials.
FEBio can now also solve first-order computational homogenization problems. In such problems, the response of the macro-model is determined by the averaged local response of a representative volume element (RVE). The deformation of the macro-model, and more specifically the local deformation gradient, is applied to a RVE model which in turns determines the stress (and tangent) of the macro-model.
FEBio supports a wide range of boundary conditions and loads to model interactions between materials that are relevant to problems in biomechanics. Deformable models can be connected to rigid bodies. With this feature, the user can model prescribed rotations and torques for rigid segments, thereby allowing the coupling of rigid body mechanics with deformable continuum mechanics. FEBio provides the ability to represent frictionless and frictional contact between rigid and/or deformable materials using sliding interfaces. A sliding surface is defined between two surfaces that are allowed to separate and slide across each other but are not allowed to penetrate. Variations of the sliding interface, such as tied interfaces, tied-sliding (tension-compression) and rigid walls, are available as well. As of version 1.2 it is also possible to model the fluid flow across two contacting biphasic materials. Finally, the user may specify a body force to model the effects such as, gravity, base acceleration or centripetal acceleration.
FEBio has a large library of element formulations. These include linear and quadratic tetrahedral, hexahedral and pentahedral (wedge) elements. FEBio also supports triangular quadrilateral shell elements, with linear and quadratic interpolations.
FEBio is a nonlinear implicit FE solver and does not have mesh generation capabilities. The finite element mesh, as well as all constitutive parameters and loading is defined in an input file, the format of which is described in detail in this document. This input file needs to be generated by preprocessing software. The preferred preprocessor for FEBio is called FEBioStudio. FEBioStudio can convert some other formats to the FEBio input specification. For instance, NIKE3D  and Abaqus input files can be imported in FEBioStudio and can be exported from as a FEBio input file.