# Thread: Mat_simplified_rubber (*181) - Simulation of rubber at high strain rates with LS_Dyna

1. Junior Member
Join Date
2012-03
Posts
7

## Mat_simplified_rubber (*181) - Simulation of rubber at high strain rates with LS_Dyna

Hi,

i am working with a material model, *Mat_181, a simplified rubber model, and I have some questions about the model parameters, MU G and SIGF.

From the manual of material of Ls-Dyna I found:

MU= Damping coefficent
G= Shear modulus for frequency indipendent damping. Frequency indipendent damping is based of a spring and slider in series. The critical stress for the slider mechanism is SIGF. For the best result. the value of G should be 250-1000 times greater than SIGF.
SIGF= Limit stress for frequency indipendent, frictional, damping.
So, I know what the shear modulus is in classical terms, but what is the shear modulus for frequency
independent damping?

Does somebody know what are these parameters and how I can get them?
Has somebody never worked with this material model?

These are some articles about this material model and the material model number 183. The material model 183 is the same of the 181 but with damage.

http://www.dynalook.com/european-con...-of-rubber.pdf
http://www.dynalook.com/internationa...alModeling.pdf

Thank you
Last edited by p.italo; 2012-04-12 at 17:30.

2. I am somewhat familiar with those models, but I much prefer a physically motivated model (such as the Bergstrom-Boyce (BB) model).
[If I had more time I would examine and present a study comparing those simple models and more advanced models such as the BB-model.]

Send me a private message if you would like professional support for the 181 and 183 models.

-Jorgen

3. Junior Member
Join Date
2012-03
Posts
7
Dear Jorgen,

thank you for reply.
Few days ago I sent a e-mail directly to Prof. DuBois (the inventor of material model 181) and he said me that the constant MU,G and SIGF are not physical parameters and they can be set to the default value, but he also said me that the model can't extrapolate the result. That means if I have curves (sig-eps) until strain rate (for example) 200 1/s and I want to do a simulation at strain rate 1000 1/s, the result will be probably wrong.

Since I have to do a simulation at very high strain rates and I can't have the possibility to test rubber at these strain rates, I have to chose another material model, like 76 (general viscoelastic). This model needs only the stress relaxation curve (and I can obtain the bulk relaxation curve so http://polymerfem.com/showthread.php...axation-moduli).

What do you think about, Dr. Bergstrom? Might works?

Thank you

4. Yes, that is a possibility.

Personally I would have used a non-linear viscoelastic material model (such as the Bergstrom-Boyce, BB-model). The BB model is available for LS-DYNA through our PolyUMod library, but is not yet a built-in feature in LS-DYNA.

Alternatively, you can test your rubber at the necessary strain-rates using specialized experimental equipment. We happen to have a Split-Hopkinson pressure bar test machine in our lab, and I would be happy to test your material for you for a fee.

-Jorgen

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