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Crushable metal foam modeling - Element distortion

Hi,

I am trying to model Aluminium crushable foam (porosity 8%) in impact analysis problem using RADIOSS block120. I am using LAW 33 visco-elasto-plastic material model and P14_Solid property brick elements (ISOLID =12, ISMSTR = 2, ICPR=2).

I am however getting severe element distortion of foam elements, leading to reduced time step and ultimately resulting in 'Zero or negative volume error'. How can I resolve this. Am I using the correct material model and property? Please help me. This is very very critical for my analysis.

Also, I need help in the interface definition between the soft foam material and rigid aluminum solid part touching it. What parameters to use for defining such a contact?

Thanks.

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Altair Forum User

Hi,

Please refer to below forum post:

http://forum.altairhyperworks.com/index.php?/topic/14527-zero-or-negative-volume-for-3d-element/?hl=%2Bzero+%2Bnegative+%2Bvolume+%2B3d-element

Altair Forum User

To add,

LAW 33 is for low density foam materials.

Can you try with LAW36 elasto-plastic material instead?

You can use TYPE 7 general contact interface to model contact between two components.

Altair Forum User

Hi Prakash,

Thanks for your suggestion. The density of foam I am using is 239 Kg/m3, which is high compared to Polyurethane foams (Density: ~30 Kg/m3) but is much less compared to metals. Are you suggesting that LAW33 is useful only for very low densities like PU materials?

Also, I am not sure if I can use LAW36 to model the foam due to the following differences:

1. The stress-strain behavior in compression (Please find attached) for foam is very different from the mentioned behavior for LAW36.

2. No strain rate effect exhibited by the foam.

I would be grateful if you can suggest the appropriate material model for Al foam, the properties of which are given below:

- Isotropic, open cell foam

- Poisson's ratio = 0

- No strain rate effect

- Density of core material (Al 6061) = 2700 Kg/m3

- Relative density = 8.8%. Therefore, Density of foam = 239 Kg/m3

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Altair Forum User

Hi Prakash,

I have used the TYPE 7 interface for contact between an Aluminum shell and foam. Since the foam is a soft material, I have defined two interfaces - one with foam=master, Al=slave and another with foam=slave and Al=master. The parameters for the foam are as given below:

For foam=master, Al = Slave:

Stfac = 382

Igap = 0

gapmin=1.0

For foam = slave, Al = master:

Stfac: Default value

Igap=0

gapmin = 1.0

I am getting the ZERO OR NEGATIVE SUB-VOLUME : DELETE 3D-ELEMENT error when I used LAW 36 property for the foam. For compression, should I define the plastic strain as negative in the stress-strain input curve for LAW36? Also, I am using a crushable foam with zero poisson's ratio, therefore true stress = engineering stress. I have defined the engineering stress-plastic strain as the input curve. Is it correct?

Please provide your views on the same.

Thanks and Regards,

Sai

Altair Forum User

Hi Sai,

Can you share the model with LAW 33 using the dropbox?

I will try with LAW 70 which is a tabulated visco elastic foam material model.