Learning Forum

[Mahdi_171]

Hi,

I have done a uniaxial compression test of a polyurethane foam sample, but I can't determine which model I should use for curve fitting. I watched the hyperelasticity course video, and the instructor says that we can use the Ogden foam model for foams with high compressibility. I am not sure if the PUR foam I am testing can be considered compressible or not from the test results.

[1shan]

The poison's ratio of a structure is a measure of its compressibility. Most materials have Poisson's ratio values ranging between 0.0 and 0.5. A perfectly incompressible isotropic material deformed elastically at small strains would have a Poisson's ratio of exactly 0.5 (example is rubber). A metal with a typical poison's ratio of .3 or so is compressible. Interestingly when the material undergoes large plastic deformation, the poisson's ratio transitions to 0.5 and it actually turns to be incompressible. For open-cell polymer foams, Poisson's ratio is near zero( and most have <0.2), since the cells tend to collapse in compression. Given that you can assume the PUV foam to be compressible in most case(there are exceptions of rigid foams and cases like https://www.osti.gov/pages/servlets/purl/1457403 which show low compressibility). You could try using both blatz-ko, ogden foam model and see which better fits your experimental data. Check these links for more info on hyperelastic models, experimental testing and curve fitting- /courses/index.php/courses/hyperelasticity/, https://ansyshelp.ansys.com/account/secured?, https://ansyshelp.ansys.com/account/secured?, https://abaqus-docs.mit.edu/2017/English/SIMACAEMATRefMap/simamat-c-hyperfoam.htm.
Regards Ishan.

[Mahdi_171]

Dear Thank you for your explanation. I have tried the models you suggested, and I don't know why the Ogden-foam model doesn't want to curve fit. Does it require more than the uniaxial that I am providing?
The blatz-ko model worked, but the fit wasn't good with both error criteria. Any ideas why the curve fitting option is not appearing in the image?

Thank you.