🐛 Bug report | Issue with fiber map definition in prestrain material

[konris87]

Description:
I am trying to define a custom fiber map for an uncoupled prestrain material using an elastic part of trans iso mooney rivlin

Version:
FEBio 4.12

Steps To Reproduce:
I define the material as:

<material id="13" name="popliteus_test_material" type="uncoupled prestrain elastic">
<density>1.5e-09</density>
<k>500</k>
<elastic type="trans iso Mooney-Rivlin">
<c1>2.75</c1>
<c2>0</c2>
<c3>0.050000001</c3>
<c4>80</c4>
<c5>850</c5>
<lam_max>1.04</lam_max>
<fiber type="user"/>
</elastic>
<prestrain type="in-situ stretch">
<stretch lc="8">1</stretch>
<isochoric>1</isochoric>
</prestrain>
</material>

and i define the meshData element as

<MeshData>
<ElementData type="fiber" elem_set="popliteus_test_elements">
<e lid="1">0.247791,-0.634686,-0.731965</e>
<e lid="2">0.718634,-0.204163,-0.664742</e>
<e lid="3">0.463590,-0.495654,-0.734447</e>​

......

I have played with type "user" and "map" as in the example fi10.feb available in the repository.

Every time I try to run the model I receive the following error:

---

• ERROR *
• tag "ElementData" (line 354373) : invalid value for attribute "type" *
  ************************************************** ***********************​

The issue appears only when I use the prestrain material.

Kind regards,
Kostas

[konris87]

I found the bug!

Problem

In FEBioMeshDataSection4.cpp at line 611, the following code was causing fiber to return as nullptr:
FEProperty* fiber = mat->FindProperty("fiber");

Solution

Adding the true boolean parameter to FindProperty() resolves the issue so it successfully locates the property:
FEProperty* fiber = mat->FindProperty("fiber", true);

I've compiled the code with this change and the model now runs successfully.

[SteveMaas1978]

Hi Kostas,

I looked into this problem today and there was indeed a bug, but the solution you found is not the correct one. By adding true, you are using a different function, which at this point is deprecated. As materials became more complex, it was possible that there are multiple fiber definitions within the material definition. The function you are using cannot distinguish between these different fiber definitions and will typically only return the first one it finds. Therefore, the function FECoreBase::FindProperty(const ParamString& prop) was introduced to overcome this problem. With this function, you could pass, for instance elastic.fiber and it will find the fiber component assigned to the elastic property. However, it turns out there was a bug in this method. This bug should be fixed now, so please update and give it a try. One change you have to make to your input file is that you should use

<ElementData type="elastic.fiber" elem_set="popliteus_test_elements">

for the ElementData definition. With that change, and FEBio updated, the file should run fine. Please give that a try and let me know if that fixed the problem.

Best,

Steve

[konris87]

Hi Steve,

Thanks a lot for the provided solution. I successfully built the new version and everything ran smoothly. However I am not sure if the map is correctly applied.

So my next question is about the definition of the map under the fiber element:

Is the following correct?

<material id="14" name="Tendon_material" type="uncoupled prestrain elastic"> <density>1.5e-09</density> <k>103.31</k> <elastic type="trans iso Mooney-Rivlin"> <c1>2.75</c1> <c2>0</c2> <c3>0.064999998</c3> <c4>115.89</c4> <c5>549.47998</c5> <lam_max>1.0420001</lam_max> <fiber type="map"/> </elastic> <prestrain type="in-situ stretch"> <stretch lc="6">1</stretch> <isochoric>1</isochoric> </prestrain> </material>

Or should I define the map name like:
<fiber type="map">fiber_map</fiber>

Also, what is the difference between map and user mode?

Kind regards,
Kostas

[SteveMaas1978]

Hi Kostas,

The name of the map has to specified explicitly so:

<fiber type="map">fiber_map</fiber>

is the correct syntax.

The user mode is an obsolete mode that is kept around for backward compatibility. I'd recommend not using it and preferring the map option instead.

Best,

Steve