Logrithmic Plastic Strain Formula confusion (Hardening Table) Urgent

[moeenhappy]

Hi

I am really confused how to input the hardening values for crushable foam model. Abaqus wants to input the stress - strain values in hardening table as true stress and log plastic strain.
In Abaqus manual the formula for calculating log plastic strain is = ln(1+nominal strain) - true stress/E

But when I use this formule to convert my nominal strains into log plastic strains, the plastic strains decreases as the true stress increases in the densification region, and thus It is impossible to feed these strains as they are no longer in Ascending order.

Do you know how to deal with this situation? please see the attached figure for more clarification

 

[mrgoldthorpe]

I don't know a lot about crushable foam, but I suggest you review the measured material data. I think you really need more measurement points.

Also, if you take the first "leg" of the red strain-strain curve as elastic behaviour: that would suggest an elastic modulus E of about 11/0.12.

However, the last leg has a larger "tangent modulus", suggesting that if you use your initial value of E you will indeed end up with negative plastic strain.

Perhaps the elastic modulus increases with strain increasing beyond about 0.54?

Have you tried some unloading during tests to examine the material behaviour a little more thoroughly?

 

[moeenhappy]

Thanks for your reply.
Actually this is the dummy data I created to understand the concept only. I have a complete data series as well but same is happening for real data as I tried to convert it into log plastic strain using the formula (ln(1+nominal strain) - true stress/E), which means that for the densification curve I will get a negative plastic strain.

I am doing something wrong in the formula. Can you confirm me if my formula is right? I found in Abaqus manual examples that in hardening table, plastic strains are entered which are greater then 1. How can we get a strain greater than 1? Does it means that they used volumetric strains, but then what is the formula for that?

In short, I want to clarify what is the right formula I need to use?