# HYPERELASTIC MATERIALS INCOMPRESSEBILITY FACTOR D1

**2**

I am currently running simple explicit dynamics impact test where I am impacting a rectangular sandwich on a rigid wall. I am trying to use hyperelastic materials and the data I have currently is only uniaxial. Using Mooney Rvlin 5 parameter curve fit, I am getting an icompressibility factor D1 of 0 which explicit dynamics solver does not accepts. What should I do to make it work?

I tried making it 1e-5, it runs but not sure if it is giving the right analysis results and what implications it poses. I would also appreciate if someone explains what this parameter exactly is.

## Comments

11,054@Yspartan

The incompressibility parameter D1 = 2/K where K = Bulk Modulus.

https://forum.ansys.com/discussion/4867/calculating-incompressibility-parameter-for-ogden-material-model

2@peteroznewman Thank you for your reply. I am using Mooney Rvlin 5 parameter, and this is the material https://docs.carbon3d.com/files/technical-data-sheets/tds_103208-00-e_epu-40.pdf . Will the equation you mentioned above will still work for Mooney Rvlin? Also what exactly is incompressibility factor? Tried to research online couldn't find a satisfying explanation. As you might see the only available properties for this material are elongation and corresponding stress values. However, in an impact test the material will undergo sudden compression. Do you believe these properties can be used still for the impact test ? I am inputting them in uniaxial test data for the material and then applying Mooney rvlin. If you want I can attach a zip of my simple setup.

Regards

Y.S

11,054@Yspartan

Yes, the equation is the same. Here is the Wiki page on Bulk Modulus. https://en.wikipedia.org/wiki/Bulk_modulus

I see in the pdf file you link to there is a Stress-Strain curve for a Tensile Test. You could digitize that data and use it in curve fitting for Money Rivlin.

11,054@Yspartan

Yes, that is the same equation for Money-Rivlin. Here is the Wikipedia entry... https://en.wikipedia.org/wiki/Bulk_modulus

In the PDF link is a graph of the uniaxial test data. You can digitize that graph and do some curve fitting in the material model to come up with coefficients for the material model.

It would be ideal if your simulation involves compression to get compression test data, but if all you have is uniaxial tensile data, that is all you have.

Materials behave differently at high strain rates that are found in impact events. Ideally, you would get rate dependent material properties, but if all you have is quasi-static material properties, that is all you have.

It really comes down to how important is the event you have to simulate and what resources you can bring to bear on the problem. If your resources are limited and the event is not too critical, then you use what you have. If you have the time and money to get proper material test data and the event you are trying to simulate is critical, then you get the proper test data.