FEBio 3.1

• The xplt format 3 is now used, which adds data on rigid bodies and rigid constraints to the plot file.

• The "fiber" property was added to the "tendon" material.

• Some backward compatibility issues were addressed: The deprecated "print_level" parameter is ignored and will not cause FEBio to stop. The deprecated "value" parameter of the prescribed BC is now mapped to the "scale" parameter.

• An issue was fixed with reading mat_axis element data in MeshData section.

• The "solid stress" plot variable was added for biphasic, multiphasic, and fluid-FSI materials.

• The bulk-modulus (k) of uncoupled materials is only taken from the top-level material. This is different from previous FEBio versions, where the specification of the bulk modulus depended on the material. For backward compatibility, febio 2.5 files will still processed in a manner that produces the same behavior as in FEBio 2.10. For the 3.0 files, only top-level bulk moduli are allowed.

• A bug was fixed that caused a crash when a file was using 9-node quadratic surface elements.

• Support was added to reading mat3d data from ElementData section.

• Added support for assigning data maps to F0 parameter of const prestrain gradient.

FEBio 3.0.1

• A bug was fixed with continuous fiber distribution materials that use the Gauss-Kronrod integration rule.

• A bug was fixed in reading surfaces with quad9 elements.

FEBio 3.0

• FEBio 3.0 is the first version for which the source code is available on github (https://github.com/febiosoftware). The source code is distributed under the MIT license.

• FEBio3 defines a new file structure. One of the major changes is that the Geometry section was replaced with a Mesh section, which defines the mesh independent of any physics attributes (i.e. materials), and a MeshDomains section, which maps materials and other parameters to element sets. Please see the FEBio3 user's manual for more information on the new file format. Despite this new format, FEBio3 will continue to read format 2.5 and format 2.0 with few exceptions. Also see the backward compatibility notes below.

• FEBio3 introduces a new mechanism for defining heterogeneous model parameters. Many parameters can now be made dependent on position and other model parameters via mathematical expressions and parameter maps. This new mechanism is very flexible and works with many material parameters, boundary conditions, and loads.

• Support for iterative linear solvers was significantly expanded, including some solvers based on Algebraic Multigrid Method (AMG). In addition, a Jacobian-Free-Krylov-Newton method is available as well. Some of these new solvers have shown great performance compared to the default direct solver. However, it must be emphasised that performance is highly problem-dependent and may require tuning of solver parameters. Please see the user manual for details on how to use and configure the iterative solvers.

• FEBio3 introduces a generalization of "load curves", namely "load controllers". Load controllers essentially control the values of model parameters as a function of time and even as a function of other model parameters. This allows users to use load controllers to create feedback systems (e.g. PID control system).

• FEBio3 also contains a framework for doing adaptive mesh refinement. The framework is currently still somewhat bare bones, but integrates with the plugin framework and users can develop their own refinement strategies as a plugin.

• Plot variables for "fluid pressure", "fixed charge density" and "osmolarity", which were previously available only for biphasic and multiphasic materials, now provide output for the "Donnan equilibrium" material in a "solid" analysis.

• A plot variable "surface area" has been added to calculate the surface area of a named surface.

• A bug has been fixed in biphasic and multiphasic contact interfaces to properly set free-draining boundary conditions for the portions of contact surfaces which are not currently contacting.

• A "natural neo-Hookean" material has been introduced which uses the natural (Hencky) strain.

• To allow modeling shell domains that form a T-connection, a "shell_normal_nodal" flag has been added to the Control section to use face normals instead of nodal normals. Set "shell_normal_nodal" to 0 (false) when modeling T-connections.

• Added option to evaluate the tangent stiffness matrix of a solid material using a secant method. Set the parameter "secant" to 1 in the material model. This can be useful when developing plugins for new materials and trying to debug (or avoid deriving) the analytical tangent stiffness.

• fluid-FSI domains may now interface with shell domains. Set "shell_bottom" to 1 in "fluid-FSI traction" if the fluid-FSI domain attaches to the bottom of a shell domain.

• Added a new hydraulic permeability material called "perm-exp-iso".

• Added log variables to save relative motion (translation and rotation) of rigid connectors.

• Updated elastodynamics capabilities to use generalize-alpha time integration scheme

• Updated the "fluid-FSI" solver to allow running "fluid" problems.

• Implemented method to save rigid connector forces/moments and relative motion (translation/rotation) to plot file.

• Added "update_penalty" flag in contact interfaces to update the auto-penalty calculation at each iteration.

• Added plot variable "fluid surface pressure" to evaluate the fluid pressure from the nodal dilatation in "fluid" and "fluid-FSI" analyses, on name surfaces.

• Implemented 4-node finite deformation shell elements that employ the EAS (enhanced assumed strain) and ANS (assumed natural strain) methods.