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Subsection 8.8.2: Constraint enforcement Up Chapter 8: Troubleshooting Section 8.10: Limitations of FEBio

## 8.9 Guidelines for Using Prestrain

There are a variety of ways in which prestrain can be added to an FEBio model, but the recommended approach – a two-step approach – is outlined here.
In the first step, the prestrain is enforced on the model. This is accomplished by making sure the part that will be prestrained uses the prestrain elastic material (or uncoupled prestrain elastic for uncoupled hyperelastic materials). A suitable prestrain generator must be defined as well. In addition, one of the update rules can be specified as a nonlinear constraint in order to modify the effective prestrain field. The boundary conditions in this step should be minimal but sufficient to ensure a well-defined finite element problem.
After the first step converges, the model is successfully prestrained. However, depending on the application, the reference geometry could have changed and this may complicate the application of further loads (most of FEBio will still use the original geometry as the reference geometry). To eliminate this potential source of confusion, it is recommended to apply the prestrain initial condition (see chapter 4) which essentially fixes the prestrain gradient and resets the reference geometry to the current geometry. FEBio will then apply any subsequent loads to this new reference geometry. One caveat of this approach is that the plotfile still stores the original reference geometry. To prevent inconsistencies in the plotfile it is recommend suppressing the creation of the plot file in the first step. This is done by setting the following control flag in the Control section of the first step.
<plot_level>PLOT_NEVER</plot_level>

Then, in the Control section of the second step, restore the plot level value.
<plot_level>PLOT_DEFAULT</plot_level>

Now, the new reference configuration resulting after the prestrain analysis step, will be the reference configuration that is used in the plot file.
Subsection 8.8.2: Constraint enforcement Up Chapter 8: Troubleshooting Section 8.10: Limitations of FEBio