Engineering strain (%)

**,Hello Mates,

I have been trying to simulate a triaxial test on the granular (Sand) material.

The aims are:
Try the sample with a certain set of properties and Apply the triaxial confining pressure by means of strain-loading (that is, load sample with a certain strain rate to induce a certain confining pressure).

Then, plot the Stress-strain curve and Volumetric curve based on ENGINEERING STRAIN (%). However, I really do not know how can I modify the Saving variables section to get engineering strain (%) directly from YADE ( sample size is 90*109*90mm).

I really appreciate it and let me know how can I do that.

The current "function saving variables" is as follows:

#from yade import plot

### a function saving variables
def history():
   plot.addData(e11=-triax.strain[0], e22=-triax.strain[1], e33=-triax.strain[2],
        ev=-triax.strain[0]-triax.strain[1]-triax.strain[2],
      s11=-triax.stress(triax.wall_right_id)[0],
      s22=-triax.stress(triax.wall_top_id)[1],
      s33=-triax.stress(triax.wall_front_id)[2],
      i=O.iter)

if 1:
  ## include a periodic engine calling that function in the simulation loop
  O.engines=O.engines[0:5]+[PyRunner(iterPeriod=20,command='history()',label='recorder')]+O.engines[5:7]
  ##O.engines.insert(4,PyRunner(iterPeriod=20,command='history()',label='recorder'))
else:
  ## With the line above, we are recording some variables twice. We could in fact replace the previous
  ## TriaxialRecorder
  ## by our periodic engine. Uncomment the following line:
  O.engines[4]=PyRunner(iterPeriod=20,command='history()',label='recorder')

O.run(100,True)