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characterization of foam materials  


robobobo
Posts: 3
(@robobobo)
New Member
Joined: 13 years ago

Hello,

This is the first time that I write in this forum so I dont know if you would understand my questions but I will be very gratefull if you could response to them.

Im doing a project about crashfoam and Im simulating foam materials in Ansys. Ive done different simulations with different materials models. The material models that Ive 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

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Nestor
Posts: 20
(@Nestor)
Eminent Member
Joined: 13 years ago

When you say Ogden doesnt 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 youre using foam models? The models you listed cant be directly used for foams. They are for solid polymers. To model foam, your model needs to account for relative density.

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robobobo
Posts: 3
(@robobobo)
New Member
Joined: 13 years ago

Im comparing Ogden model with uniaxial data, because Ive done compression tests. The test rate is 2.5mm/min and the temperature doesnt matter. I know that the material models that Im 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 Ive 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.

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Nestor
Posts: 20
(@Nestor)
Eminent Member
Joined: 13 years ago

Hyperelastic models are basically for rubbery materials. Those materials dont 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 Poissons ratio of just under 0.5, while a foam could have a Poissons 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.

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nils98569
Posts: 22
(@nils98569)
Eminent Member
Joined: 10 years ago

Hi,
I would suggest you to consider the Cellular Potts model. CPM is evolved by updating the cell lattice one pixel at a time based on a set of probabilistic rules. In this sense, the CPM can be thought of as a generalized cellular automaton (CA). Although it also closely resembles certain Monte Carlo methods, such as the large-q Potts model, many subtle differences separate the CPM from Potts models and standard spin-based Monte Carlo schemes.
The primary rule base has three components:
1. rules for selecting putative lattice updates
2. a Hamiltonian or effective energy function that is used for calculating the probability of accepting lattice updates.
3. additional rules not included in 1. or 2..

The CPM can also be thought of as an agent based method in which cell agents evolve, interact via behaviors such as adhesion, signalling, volume and surface area control, chemotaxis and proliferation. Over time, the CPM has evolved from a specific model to a general framework with many extensions and even related methods that are entirely or partially off-lattice.

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