PreStrain in Membrane element

[MiguelA]

I'm using the PreStrain Plugin in FEBio 2.8.4

My model is a disk held by the edge and pressurized. If I try to run it without the prestrain plugin it converges fine. However, if I try to use it with the prestrain material, even if I set the prestrain to the identity, the problem does not converge. I'm wandering if I'm doing anything wrong. Attached are the two feb files for demonstrating this behavior:

Without prestrain: minidisk_tilt_Equation_Membrane_In_NOPreStrain.feb
With prestrain (but F0=Identity): minidisk_tilt_Equation_Membrane_In_PreStrain.feb

The two feb files are identical except for the following differences:

No Prestrain:

    <Material>
        <material id="1" name="Material1" type="uncoupled solid mixture">
            <solid type="Mooney-Rivlin">
                ...
            </solid>
            <solid type="fiber-exp-pow-uncoupled">
                ...
            </solid>
        </material>
    </Material>

Prestrain:

    <Material>
        <material id="1" name="Material1" type="uncoupled prestrain elastic">
            <prestrain type="prestrain gradient"/>
            <elastic type="uncoupled solid mixture">
                <solid type="Mooney-Rivlin">
                    ...
                </solid>
                <solid type="fiber-exp-pow-uncoupled">
                    ...
                </solid>
            </elastic>
        </material>
    </Material>

    <MeshData>
        <ElementData elem_set="Part1" var="F0">
            <elem lid="1">1.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0</elem>
            ...
            <elem lid="149">1.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0</elem>
        </ElementData>
    </MeshData>

    <Constraints>
        <constraint type="prestrain">
            <tolerance>0.1</tolerance>
            <min_iters>3</min_iters>
        </constraint>
    </Constraints>

[maas]

Hi Miguel,

The problem is that the bulk-modulus (k) needs to be defined in the "uncoupled prestrain elastic" material, not the Mooney-Rivlin material. If you move it up, the model will run.

Cheers,

Steve

[MiguelA]

Thank you Steve,
just one question. Is this in contradiction with the 2.8 user manual?

4.1.2.15 Uncoupled Solid Mixture
This material describes a mixture of quasi-incompressible elastic solids. It is a container for any
combination of the materials described in Section 4.1.2.

<solid> Container tag for compressible material

The mixture may consist of any number of solids. The stress tensor for the solid mixture is
the sum of the stresses for all the solids. The bulk modulus of the uncoupled solid mixture is the
sum of the bulk moduli of the individual <solid> materials. A bulk modulus specified outside of the
<solid> materials will be ignored.

[maas]

Hi Miguel,

Thanks for pointing that out. The manual is indeed not correct and you should always define the bulk modulus at the top-level material. I will update that accordingly.

Cheers,

Steve