characterization of foam materials

[zerraldo]

Hello,

This is the first time that I write in this forum so I don't know if you would understand my questions but I will be very gratefull if you could response to them.

I'm doing a project about "crashfoam" and I'm simulating foam materials in Ansys. I've done different simulations with different materials models. The material models that I've used are Mooney_Rivlin, Blatz-ko, Ogden foam... With Mooney_Rivlin model the approximation to my experimental curve is very similar but in the case of the Ogden hyperelastic foam model this approximation is only valid with one of the foams that I want characterized. In the case of the Poliurethane and the EPS the adjusment of this model and the experimental curve is so bad.

How this can be due?

thank you

[Grant]

When you say Ogden doesn't fit your data, are you comparing it to material test data (uniaxial, etc.?) or to a crash simulation? What are the rates, temperatures and strain levels for the test data? Are you sure you're using foam models? The models you listed can't be directly used for foams. They are for solid polymers. To model foam, your model needs to account for relative density.

[zerraldo]

I'm comparing Ogden model with uniaxial data, because I've done compression tests. The test rate is 2.5mm/min and the temperature doesn't matter. I know that the material models that I'm using are not for foams materials but as foams are more or less 100%compressible materials, I want to study how works these material models inserting curves of compression of foams.

But in the case of the Hyperelastic Ogden Foam, this material model could be used with foam and the result that I've obtained are very bad too (with EPS and Poliurethane, bacause with EPP foam the results are very good and they fit to the experimental data perfectly). Do I have to take into account the density of my material to simulate the compression tests? and if I have to do that, how can I do it?

Thank you.

[Grant]

Hyperelastic models are basically for rubbery materials. Those materials don't compress much at all in typical deformations - the bulk modulus is way higher than the shear modulus. Foams compress a lot because of their porous microstructure. In linear elastic terms, a rubbery material has a Poisson's ratio of just under 0.5, while a foam could have a Poisson's ratio of practically zero. They are really two very different classes of materials.

I would suggest looking at elastomeric foam models. The book by Gibson and Ashby is a good place to start. These models have material properties that are functions of the initial relative density.