I've to model the plastic behavior (Yield Stress vs Plastic Strain) of a polyimide material (Elas_Mod=2500MPa) in Abaqus. The engineering stress vs strain plot is shown as the 3 red curves. I've captured the curve corresponding to 23degC in Matlab, as can be seen in the blue plot. The plot starts from yield point stress = 69MPa. Next I convert this eng. stress and eng. strain to true stress and true strain using:

True_Stress = Eng_Stress*(1 + Eng_Strain)

True_Strain = log(1 + Eng_Strain)

The corresponding plot for True Stress vs True Strain is the green plot. Next I convert the true strain to logarithmic plastic strain using:

Log_Plastic_Strain = True_Strain - (True_Stress/Elas_Mod)

The first value of log plastic strain is changed to zero, representing the yield. The plot of true stress vs log plastic strain is the black plot. Here it can be seen that after a point, the plastic strain decreases with increasing true stress. So when assigning the plastic data in Abaqus, do I use only those points up to which the plastic strain increases and ignore the remaining points? Why does the true stress vs log plastic strain plot have decreasing strain trend which eventually moves towards negative values? Thank you for your time!