Soft foam testing and simulation

Hello All,

I have few questions concerning characterization/modeling of soft foams.

Test: stress relaxation (compression)

Sample Geo: Cubic

Type of foam: Visco and PU

1. Compressive strain is ramped @ 7.5% /s to a constant values of strain say 50%, 75% etc.. and then held constant. Is the ramp rate ok? or do I need to slow down to 1%/s ??

2. Initial experiment reveled material response is [U]Non-linear[/U] i.e Normalized stresses for different strain levels do not match! Is this pretty common for soft foams?? or something wrong in my experiments? could it be cause of ramp strain rate?

3. Based on test, foam samples are [U]non-isotropic[/U]. Is this also commonly observed??

4. Abaqus/Standard provides *Hyperfoam model but it is for linear visco material, What material model to use to incorporate non-linear Visco nature?? I heard PRF is an option. But I dont know how to couple it with *hyperfoam model. Please share details...

Please help! Much appreciate your response.

Thanks in advance!

JJosh

Your tests are fine. The responses that you observed are common. If you have not already, you should really get the [URL= https://polymerfem.com/content.php?9-Material-Model-Calibration-MCalibration ]MCalibration [/URL]software. It is the best way to find a calibrate a material model for your foam.

I recommend the [URL= http://polyumod.com/material_models ]Micro Foam Model[/URL] (MFM) from the PolyUMod llibrary.
But the PRF model may also be sufficiently good for your needs.

-Jorgen