Does anyone know how can the plastic strain be greater than unity? Such as in the benchmark manual 3.2.10 Indentation of a crushable foam plate:
*CRUSHABLE FOAM HARDENING
0.2000E6, 0.0000
0.2577E6, 0.0094
0.2760E6, 0.0258
0.3053E6, 0.0452
0.3267E6, 0.0655
0.3623E6, 0.1084
0.3891E6, 0.1540
0.4250E6, 0.2405
0.4568E6, 0.3812
0.4738E6, 0.4600
0.5170E6, 0.6391
0.5862E6, 0.8570
0.6503E6, 0.9857
0.7470E6, 1.1324
0.9820E6, 1.2965
1.4702E6, 1.4808
2.7262E6, 1.6609
5.3911E6, 1.9000
As I Know that the formula for True strain is
True Strain = ln(1+nominal strain)
and if we consider a compression process then nominal strain should not be greater than 1 and thus max true strain will be
True Strain = ln(1+1)=ln(2)= 0.69
I know I am confusing somethings here, Kindly correct me
Be careful with strain signs for compression/tension
Your final equation is correct for a tensile nominal strain of 1. If you had a compressive nominal strain of -0.5 your equation would be
True Strain = ln(1-0.5) = -0.69
If we take the example to the extreme of compressing the material to 1/10th of its original length
True Strain = ln(0.1) = -2.30
Or to 10x the original length
True Strain = ln(10.0) = 2.30
That's how you get logarithmic strains with magnitudes greater than unity.