Tensile test

Hi friends, I am trying a tensile test on a cylindrical sample of spheres. The sample is confined inside a cylindrical boundary using the facetCylinder. I intend to make the last (top)-most particles (at open-end) as a plate which then moves with a certain velocity and pulls the entire sample (body of spheres). The current script is as follows:

from yade import pack,plot
pred=pack.inCylinder((0,0,0),(0,0,0.25),0.10) #create a cylinder geometry
sp=pack.randomDensePack(pred,radius=0.01,spheresInCell=300) #pack cylinder with spheres
O.bodies.append(sp) #add spheres to the body
yade.qt.Controller()
facets = geom.facetCylinder((0,0,0.125),0.1,0.25,wallMask=2|4) #create cylindrical boundary using facets
O.bodies.append(facets) #add facets to the body
global plate # name a new geometry "plate" for the application of load
plate=O.bodies [-1] # the last of the particles act as the plate
plate.state.vel=Vector3(0,0,-0.1) #assign velocity to the plate
O.engines=[ForceResetter(),InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Facet_Aabb(),Bo1_Wall_Aabb()]),InteractionLoop([Ig2_Sphere_Sphere_ScGeom(),Ig2_Facet_Sphere_ScGeom(),Ig2_Wall_Sphere_ScGeom()],[Ip2_FrictMat_FrictMat_FrictPhys()],[Law2_ScGeom_FrictPhys_CundallStrack()]),NewtonIntegrator(),PyRunner(iterPeriod=100,command="addPlotData()")] #set up interactions and run for the data desired
O.dt=0.5*PWaveTimeStep() #time step
def addPlotData(): #define the plot data function
  Fz=O.forces.f(plate.id)[2] #load(force) on particle(plate) along z-direction
  Dz=plate.state.pos[2]-plate.state.refPos[2] #displacement of plate along z-direction
plot.addData(i=O.iter,Fz=Fz,Dz=Dz) #add the data to be plotted
plot.plots={'Dz':('Fz')}
plot.plot() #output plot
O.run()

My questions are:
(1) Is there a better alternative to performing the test with the above mentioned scenario?

(2) While running the script I received the error that "Fz" is not defined. I guess that could be something to do with the line plate=O.bodies [-1]. Have been looking for the right method. Any clues?

Thanks....