disadvantages of Hill's and Ogden's formula for foams?

[christian]

Hello all,
I try to analyse elastic PUR-Foam. I work with Marc mentat2003 and found out, that the fitting - programm can't fit the experimental data very well. The Experimental curves show a plateau phase, which can't be reproduced by the fitting with the Foam-formula of Ogden and Hill for foam of the early 80'ties.
My question is, is this caused by the mathematical description, which might be insufficient, or is this perhaps a failure of the fitting program of marc/mentat?
Does anybody has any experience with such problems?
The support adviced me to take more experimental data into account, to parameterise the Ogdenformular, but in my experiences, this does not help.
What can I do, if necessary, to change the formula, to come closer to the experimental data?
How could I create my own fitting-Program? Perhaps with gnuplot?

[Jorgen]

Hello Christian,

My experience with Marc is somewhat limited, but it is clear that finding material parameters from experimental data for a non-linear material model is quite challenging, and it is not a big surprise that you are not getting the results that you want.

I have had very good experience with my own specialized fitting-program that I have worked on for a number of years now. If possible I would be quite interested in seeing the experimental data that you are trying to fit. I have been able to fit many different foams in the past and suspect that the problem that you are experiencing is due to the fitting program that you are using. Also, how close are you able to get with the material model that you are using?

In summary, if you are simply interested in the long-term response (i.e. equilibrium response and not the time-dependence) of the foam then there are a number of powerful model that you can use, and you should be able to get pretty good fit of your experimental data.