Include("Specifications/FebioFeatures", from="In Situ Strain"), to="progress")

An algorithm for enforcing a user-defined fiber stretch has been implemented in FEBio2. The implementation is based on the paper by Weiss et. al [1] and uses an iterative method for enforcing the prescribed in-situ fiber stretch while maintaining stress equilibrium. The current implementation only works with transversely isotropic Mooney-Rivlin materials but can easily be expanded to other constitutive models. The examples below show the current capabilities.

Example 1: A constant in-situ fiber stretch of 50% was enforced on a cubical block. The example shows that a constant fiber strain can be achieved while maintaining stress equilibrium.

ImageLink(insitu_block1.png, width=400)

Example 2: A constant in-situ fiber stretch of 3% is prescribed on a cylindrical geometry with the fibers oriented circumferentially (left panel). The first principal stress is shown in the middle panel. A radial cut is introduced which relieves the stresses and introducing the opening angle.

ImageLink(insitu_cylinder1.png, width=600)

[1] Weiss J.A., Gardiner J.C., Ellis B.J., Lujan T.J., Phatak N.S., Three-dimensional finite element modeling of ligaments: Technical aspects, Medical Engineering & Physics 27 (2005) 845-861