> For the complete documentation index, see [llms.txt](https://tlcfem.gitbook.io/suanpan-manual/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://tlcfem.gitbook.io/suanpan-manual/example/structural/statics/uniform-tension-of-a-rubber-specimen.md).

# uniform tension of a rubber specimen

In this page, a rubber specimen is modelled using solid elements and Mooney-Rivlin material.

Only a quarter of the specimen is modelled considering symmetry. The geometry can be seen as follows. The model can be downloaded. [uniform-tension-of-a-rubber-specimen.zip](https://github.com/TLCFEM/suanPan-manual/tree/dev/docs/Example/Structural/Statics/uniform-tension-of-a-rubber-specimen.zip)

![example one](/files/nsHpIkGhtRDc0hKHNOud)

## Model Setup

The node and element information can be seen in `node.supan` and `element.supan`, the mesh is generated by ABAQUS, it is easy to adapt ABAQUS input file to be used here. `C3D8` element with full integration and nonlinear geometry is used.

```
file node.supan
file element.supan
```

After loading nodes and elements, a `MooneyRivlin` material is defined with $$C\_1=80\~\text{MPa}$$, $$C\_2=20\~\text{MPa}$$ and $$K=10^6\~\text{MPa}$$. This is a regularized version of Mooney-Rivlin model that allows a certain degree of dilatation. The $$D\_1$$ parameter used in ABAQUS is the inverse of bulk modulus so that $$D\_1=0$$ means fully incompressible. The displacement of magnitude of $$10$$ along $$x$$ axis is applied on the free end. A static step with an initial step size of $$0.01$$ is used. With `fixed_step_size` off, the algorithm automatically scale step size based on convergence performance.

```
material MooneyRivlin 1 1E6 80 20
```

To generate the node groups to be used to apply boundary conditions, here the `generatebyplane` command is used.

```
# all nodes on plane x=0
generatebyplane nodegroup 1 1. 0. 0. 0.
# all nodes on plane y=0
generatebyplane nodegroup 2 0. 1. 0. 0.
# all nodes on plane z=0
generatebyplane nodegroup 3 0. 0. 1. 0.
# all nodes on plane x=20
generatebyplane nodegroup 4 1. 0. 0. -20.
```

Now BCs and loads can be applied.

```
grouppenaltybc 1 1 1
grouppenaltybc 2 2 2
grouppenaltybc 3 3 3

groupdisplacement 1 0 10 1 4
```

Setup steps and perform the analysis.

```
step static 1
set fixed_step_size 0
set ini_step_size 1E-2
set symm_mat 0

converger AbsIncreDisp 1 1E-8 10 1

analyze

exit
```

## Result

![animation](/files/wKYrIIoMV27lCbXpR5XB)


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://tlcfem.gitbook.io/suanpan-manual/example/structural/statics/uniform-tension-of-a-rubber-specimen.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.