Principal compression and tension stress and strain plots

Hi,

I have a question with regards to the various stress-strain plots that I see from FEM softwares.

They have plotted principal compression and tension stress plots  and strain plots for a 2d geomterical analysis from a FE software (MARC). They have used a finite strain theory. So does this mean that the principal compression and tension values for the various elements just correspond to the minimum and maximum values of a Cauchy green tensor or some other finite tensor like first or second piola kirchoff stress tensor??

 I am wondering as to which tensors maximum and minimum values are being plotted here in their results.

 An example of the pots are here http://link.springer.com/article/10.1007%2Fs10439-007-9384-9#page-1

Please refer figure 7 and 8 in this link 

N