Using user-defined material axis in active contraction

[kdwyer]

Hello,
I was wondering the correct way to define the active contraction along user-defined material axis. I am using the trans-iso Mooney Rivlin material model with the active_contraction and defined the material axis using user defined vector, a and d. Ideally, I would want active contraction along these material axis.

I have been using a C++ code from one of your tutorials to define a and d and changed the material in that to Mooney-Rivling then import it into FEBio studio to run the .feb code. I noticed, there is no information under MeshData when I do this. Also I have tried a couple things that I received errors on. First, I tried to define fiber type as "user" which immediately cause an error when trying to run and I also tried to define the material axis in the .feb file before uploading it to FEBio Studio but there is an error while importing and that line of code is ignored. Here is what that looked like:

<ElementData var="mat_axis" elem_set="Part1">
<elem lid="1">
<a>0.788235,0.611278,0.0708841</a>
<d>-0.61041,0.791274,-0.03585</d>
</elem>
<elem lid="2">
<a>0.782976,0.610819,0.117685</a>
<d>-0.608414,0.791378,-0.05961</d>
</elem>
<elem lid="3">
<a>0.775176,0.610136,0.163816</a>
<d>-0.605438,0.791535,-0.0831618</d>
</elem>
.
....
.......

<Material>
<material id="1" name="transisoMV" type="trans iso Mooney-Rivlin">
<density>1</density>
<c1>0.5</c1>
<c2>0</c2>
<c3>0.0005</c3>
<c4>50</c4>
<c5>0</c5>
<lam_max>1.03</lam_max>
<k>100</k>
<mat_axis type = "user"/>
<active_contraction type="active_contraction">
<ascl lc="1">1</ascl>
<ca0>4.35</ca0>
<beta>4.75</beta>
<l0>1.58</l0>
<refl>1.58</refl>
</active_contraction>
</material>
</Material>


Any advice would be greatly appreciated!

[maas]

Hi,

The "user" type is no longer supported. In FEBio3, if you wish to define a custom map, you would do it as follows:

<mat_axis type="map">my_mataxis_map</mat_axis>

where my_mataxis_map is defined in the MeshData section. The older febio2 format is also still supported, where you define the mat_axis variable in the MeshData section. In that case, you would not define the mat_axis in the material definition. That said, I do recall that we recently fixed a bug related to this in febio3, so if this doesn't solve the problem, please reach out to me via email (steve dot maas at utah dot edu) and I can send you our latest development version.

Cheers,

Steve