I am having some isses with modelling the uniaxial compression of an aluminium foam using the CRUSHABLE FOAM model in ABAQUS.
I have an nominal stress-strain curve for the foam, which I converted to a true stress-true strain curve using the following formulae:
1. true stress = nom. stress*(1+ nom. strain)
2. true strain = ln(1+ nom. strain)
[NB: I assume nominal strain is negative for uniaxial compression for the above]
I then calculate plastic strain by using the formula:
3. True Plastic Strain = true strain - (true stress/modulus)
After running the analysis (using NLGEOM option ON), I plot S22, NE22, LE22 (stresses and strains in compression direction), and I get the results that i expect (i.e., true stresses, nominal strains, and logarithmic strains that I expect). The problem is that if I try to calculate nominal stress via the sum of reaction forces on the top or bottom row of nodes in my mesh, I get RF's which decrease with displacement (like my true stress-strain curve). I've been trying to figure out where I'm going wrong, but no avail.
I'd like to sort this out because I need to calculate load vs displacement graphs for spherical indentation analysis, but the loads that I am getting out of the model seem to be incorrect. I have a feeling I'm missing out on something fundamental, and I haven't been able to crack it. Any help would be greatly appreciated!
P.S. I've attached my input file so that any errors can be pointed out to me.