Uniaxial test with CohFrictMat

Hi everybody,

I am trying to determine which cohesive contact law I should be using in order to model rock fragmentation. So I have been implementing tests on JcfPM, CohFrictMat and CpmMat.

I have adapted the uniaxial tension-Compression Test from CpmMat examples in order to use it with both CohfrictMat and JcfPM.

Seems to work not too badly with JcfPM (although I can't see fracture), but there is an issue with the CohfrictMat.

When I try to run the test there is an initial value of averageStress that is very high, up to -102754.2 pa.

This values quickly goes back to around zero, and then increases in an 'expectable' way, but that messes up the curve a lot, and I haven't been able to find to what it is due ?

I have tried setting the equilibrium distance following this thread : https://answers.launchpad.net/yade/+question/266828, but that didn't change much ( plus I wouldn't expect initial overlapping forces to be that great ?)

I have also noticed that another script posted by a Yade user, also an adaptation of the CPM uniaxialcompression test but using CohFrictMat, on this thread https://answers.launchpad.net/yade/+question/372295, may also be facing the same problem.

The averageStress that I get running this code ( modifying the packing in order for the warning about porosity to disappear) is -1027113730.45 Pa. Then this value also decreases inexpectantly.

The adaptation with JCFpm shows no sign of such a problem, although I am using the exact same set of parameters....

Any ideas on what is going on ?

Here is the code

# -*- coding: utf-8 -*-
from __future__ import division

from yade import plot,pack,timing
import time, sys, os, copy

#import matplotlib
#matplotlib.rc('text',usetex=True)
#matplotlib.rc('text.latex',preamble=r'\usepackage{concrete}\usepackage{euler}')

def addPlotData():
 yade.plot.addData ({'t':O.time,'i':O.iter,'eps':strainer.strain,'sigma':strainer.avgStress+isoPrestress,
  'sigma.25':forcesOnCoordPlane(coord_25,axis)[axis]/area_25+isoPrestress,
  'sigma.50':forcesOnCoordPlane(coord_50,axis)[axis]/area_50+isoPrestress,
  'sigma.75':forcesOnCoordPlane(coord_75,axis)[axis]/area_75+isoPrestress,
  })

# default parameters or from table
readParamsFromTable(noTableOk=True, # unknownOk=True,
 young=1e9,
 poisson=0.25,

 densityCohFrictMat = 2600.,
 FrictAngSphere = 30.*pi/180.,
 NCohesionCohFrictMat = 4000,
 SCohesionCohFrictMat = 4000,

 intRadius=1.5,
 dtSafety=.8,
 damping=0.4,
 strainRateTension=.0005,
 strainRateCompression=.005,
 setSpeeds=True,
 # 1=tension, 2=compression (ANDed; 3=both)
 doModes=3,

 specimenLength=0.05,
 specimenRadius=0.1,
 sphereRadius=3.5e-3,

 # isotropic confinement (should be negative)
 isoPrestress=0.
)

from yade.params.table import *

if 'description' in O.tags.keys(): O.tags['id']=O.tags['id']+O.tags['description']

# make geom; the dimensions are hard-coded here; could be in param table if desired

O.materials.append(
 CohFrictMat(
 young=young,
 frictionAngle=FrictAngSphere,
 poisson=poisson,
 density=densityCohFrictMat,
 normalCohesion=NCohesionCohFrictMat,
 shearCohesion=SCohesionCohFrictMat,
 fragile = True,
 label = 'concreteId'))

sps=SpherePack()
sp=pack.randomDensePack(
 pack.inCylinder(
 (0,0,-.5*specimenLength),(0,0,.5*specimenLength),specimenRadius),
 spheresInCell=2000,radius=sphereRadius,
 memoizeDb='/tmp/ triaxPackCache.sqlite',returnSpherePack=True)

sp.toSimulation(material='concreteId')

bb=uniaxialTestFeatures(axis = 2)
negIds,posIds,axis,crossSectionArea=bb['negIds'],bb['posIds'],bb['axis'],bb['area']
O.dt=dtSafety*PWaveTimeStep()
print 'Timestep',O.dt

mm,mx=[pt[axis] for pt in aabbExtrema()]
coord_25,coord_50,coord_75=mm+.25*(mx-mm),mm+.5*(mx-mm),mm+.75*(mx-mm)
area_25,area_50,area_75=approxSectionArea(coord_25,axis),approxSectionArea(coord_50,axis),approxSectionArea(coord_75,axis)

O.engines=[
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb(aabbEnlargeFactor=intRadius,label='is2aabb')],verletDist=.05*sphereRadius),
 InteractionLoop(
  [Ig2_Sphere_Sphere_ScGeom6D(interactionDetectionFactor=intRadius,label='ss2sc')],
  [Ip2_CohFrictMat_CohFrictMat_CohFrictPhys(setCohesionNow=True)],
  [Law2_ScGeom6D_CohFrictPhys_CohesionMoment(useIncrementalForm = True, label='cohlaw')]
 ),
 NewtonIntegrator(damping=damping,label='damper'),

 UniaxialStrainer
        (
        strainRate=strainRateTension,axis=axis,asymmetry=0,
        posIds=posIds,negIds=negIds,
        crossSectionArea=crossSectionArea,blockDisplacements=False,
        blockRotations=False,setSpeeds=setSpeeds,label='strainer'),
 PyRunner(virtPeriod=1e-6/strainRateTension,realPeriod=1,command='addPlotData()',label='plotDataCollector',initRun=True),
 PyRunner(realPeriod=4,command='stopIfDamaged()',label='damageChecker'),
]

for b in O.bodies :
  b.dynamic=False

O.step()

for i in O.interactions :
  i.phys.unp = i.geom.penetrationDepth

for b in O.bodies :
  b.dynamic=True

yade.qt.Controller(), yade.qt.View()

#O.miscParams=[Gl1_CpmPhys(dmgLabel=False,colorStrain=False,epsNLabel=False,epsT=False,epsTAxes=False,normal=False,contactLine=True)]

# plot stresses in ¼, ½ and ¾ if desired as well; too crowded in the graph that includes confinement, though
plot.plots={'eps':('sigma',)} #,'sigma.50')},'t':('eps')} #'sigma.25','sigma.50','sigma.75')}

O.saveTmp('initial');

O.timingEnabled=False

global mode
mode='tension' if doModes & 1 else 'compression'

def initTest():
 global mode
 print "init"
 if O.iter>0:
  O.wait();
  O.loadTmp('initial')
  print "Reversing plot data"; plot.reverseData()
 else: plot.plot(subPlots=False)
 strainer.strainRate=abs(strainRateTension) if mode=='tension' else -abs(strainRateCompression)
 try:
  from yade import qt
  renderer=qt.Renderer()
  renderer.dispScale=(1000,1000,1000) if mode=='tension' else (100,100,100)
 except ImportError: pass
 print "init done, will now run."

 O.step(); # to create initial contacts
 # now reset the interaction radius and go ahead
 ss2sc.interactionDetectionFactor=1.
 is2aabb.aabbEnlargeFactor=1.

 #O.run()

def stopIfDamaged():
 global mode
 if O.iter<2 or not plot.data.has_key('sigma'): return # do nothing at the very beginning
 sigma,eps=plot.data['sigma'],plot.data['eps']
 extremum=max(sigma) if (strainer.strainRate>0) else min(sigma)
 minMaxRatio=0.5 if mode=='tension' else 0.5
 if extremum==0: return
 import sys; sys.stdout.flush()
 if abs(sigma[-1]/extremum)<minMaxRatio or abs(strainer.strain)>(5e-3 if isoPrestress==0 else 5e-2):
  if mode=='tension' and doModes & 2: # only if compression is enabled
   mode='compression'
   O.save('/tmp/uniax-tension.yade.gz')
   print "Saved /tmp/uniax-tension.yade.gz (for use with interaction-histogram.py and uniax-post.py)"
   print "Damaged, switching to compression... "; O.pause()
   # important! initTest must be launched in a separate thread;
   # otherwise O.load would wait for the iteration to finish,
   # but it would wait for initTest to return and deadlock would result
   import thread; thread.start_new_thread(initTest,())
   return
  else:
   print "Damaged, stopping."
   ft,fc=max(sigma),min(sigma)
   print 'Strengths fc=%g, ft=%g, |fc/ft|=%g'%(fc,ft,abs(fc/ft))
   title=O.tags['description'] if 'description' in O.tags.keys() else O.tags['params']
   print 'gnuplot',plot.saveGnuplot(O.tags['id'],title=title)
   print 'Bye.'
   O.pause()
   #sys.exit(0) # results in some threading exception

plot.plot(subPlots=False)
#O.run()
initTest()
waitIfBatch()