fitting data for strain-energy functions

[christian]

I have a problem in understanding:
The Marc/mentat - fittingpogram I use fits the materialmodel of Ogden with the engineer stress. Why is it not fitted with true stress data?
When I built up my own fitting program I startet to fit the uniaxial compression data with true stress data, until I found out, that comercial fitting programs work only with the engineer stress data. I thought, that true stress data are closer to the physical reality, so it seemed to me more realistic to fit the ogden model with true stress data. Why is this wrong?

Thank you for help!

Christian

[Jorgen]

Hello Christian,

You can use either engineering stress or true stress when calibrating the Ogden model (or any other material model for that matter). When you do the calibration you need to make sure that you simply use the type of stress and strain that your software is expecting.

If you write your own fitting program you can use either engineering or true values, either approach will work. There is no stress or strain quantity that is always "the best". It is more of a personal preference.

Best of luck,
Jorgen