# Facing problem in stabilizing the DEM simulation sample when using Hertz-mindlin contact model

Asked by Sarath Chandra Reddy on 2021-05-30

Dear Friends,
I am facing problem in stabilizing the DEM model sample and reaching the isotropic compression stage when I am using Hertz-mindlin contact model. The following is my code. Please run the code once and give your suggestions on where I need to correct the code.

####################
from yade import pack

############################################
### DEFINING VARIABLES AND MATERIALS ###
############################################
# The following 5 lines will be used later for batch execution
nRead=readParamsFromTable(
num_spheres=10000,# number of spheres
compFricDegree = 5, # contact friction during the confining phase
key='_triax_base_', # put you simulation's name here
unknownOk=True
)
from yade.params import table

num_spheres=table.num_spheres# number of spheres
key=table.key
targetPorosity = 0.368 #the porosity we want for the packing
compFricDegree = table.compFricDegree # initial contact friction during the confining phase (will be decreased during the REFD compaction process)
finalFricDegree = 30 # contact friction during the deviatoric loading
rate=-0.02 # loading rate (strain rate)
damp=0.2 # damping coefficient
stabilityThreshold=0.01 # we test unbalancedForce against this value in different loops (see below)
young=5e7 # contact stiffness
mn,mx=Vector3(0,0,0),Vector3(.02,.02,.02) # corners of the initial packing

## create materials for spheres and plates
O.materials.append(FrictMat(young=young,poisson=0.2,frictionAngle=radians(compFricDegree),density=2600,label='spheres'))
O.materials.append(FrictMat(young=young,poisson=0.2,frictionAngle=0,density=3000,label='walls'))

## create walls around the packing
walls=aabbWalls([mn,mx],thickness=0,material='walls')
wallIds=O.bodies.append(walls)

## use a SpherePack object to generate a random loose particles packing
sp=pack.SpherePack()

clumps=False #turn this true for the same example with clumps
if clumps:
## approximate mean rad of the futur dense packing for latter use
volume = (mx-mn)*(mx-mn)*(mx-mn)
mean_rad = pow(0.09*volume/num_spheres,0.3333)
## define a unique clump type (we could have many, see clumpCloud documentation)
c1=pack.SpherePack([((-0.2*mean_rad,0,0),0.5*mean_rad),((0.2*mean_rad,0,0),0.5*mean_rad)])
## generate positions and input them in the simulation
sp.makeClumpCloud(mn,mx,[c1],periodic=False)
sp.toSimulation(material='spheres')
O.bodies.updateClumpProperties()#get more accurate clump masses/volumes/inertia
else:
sp.makeCloud(mn,mx,num=num_spheres,rMean=0.0005,periodic=False, seed=1)
#sp.makeCloud(mn,mx,-1,0.3333,num_spheres,False, 0.95,seed=1) #"seed" make the "random" generation always the same
O.bodies.append([sphere(center,rad,material='spheres') for center,rad in sp])
print(len(sp),' particles generated.')
#or alternatively (higher level function doing exactly the same):
#sp.toSimulation(material='spheres')

############################
### DEFINING ENGINES ###
############################

triax=TriaxialStressController(
## TriaxialStressController will be used to control stress and strain. It controls particles size and plates positions.
## this control of boundary conditions was used for instance in http://dx.doi.org/10.1016/j.ijengsci.2008.07.002
maxMultiplier=1.+9e4/young, # spheres growing factor (fast growth)
finalMaxMultiplier=1.+2e3/young, # spheres growing factor (slow growth)
thickness = 0,
stressMask = 7,
internalCompaction=False, # If true the confining pressure is generated by growing particles
)

newton=NewtonIntegrator(damping=damp)

O.engines=[
ForceResetter(),
InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Box_Aabb()]),
InteractionLoop(
[Ig2_Sphere_Sphere_ScGeom(),Ig2_Box_Sphere_ScGeom()],
#[Ip2_FrictMat_FrictMat_FrictPhys()],
#[Law2_ScGeom_FrictPhys_CundallStrack()]
[Ip2_FrictMat_FrictMat_MindlinPhys( betan =0.2 , betas = 0.2, krot = 0.2 , eta = 0.2 )],
[Law2_ScGeom_MindlinPhys_Mindlin()]
),
GlobalStiffnessTimeStepper(active=1,timeStepUpdateInterval=100,timestepSafetyCoefficient=0.25),
triax,
TriaxialStateRecorder(iterPeriod=100,file='WallStresses'+table.key),
newton
]

#Display spheres with 2 colors for seeing rotations better
#Gl1_Sphere.stripes=0
#if nRead==0: yade.qt.Controller(), yade.qt.View()
triax.max_vel=1
#the value of (isotropic) confining stress defines the target stress to be applied in all three directions
triax.goal1=triax.goal2=triax.goal3=-100000

while 1:
O.run(1000, True)
##the global unbalanced force on dynamic bodies, thus excluding boundaries, which are not at equilibrium
unb=unbalancedForce()
print ('unbalanced force:',unb,' mean stress: ',triax.meanStress)
if unb<stabilityThreshold and abs(-100000-triax.meanStress)/100000<0.001:
break
print ("Particle Distribution is Stable")
O.save('confinedState'+key+'.yade.gz')
print ("### Isotropic state saved ###")

###################################################
### REACHING A SPECIFIED POROSITY PRECISELY ###
###################################################

import sys #this is only for the flush() below
while triax.porosity>targetPorosity:
## we decrease friction value and apply it to all the bodies and contacts
compFricDegree = 0.95*compFricDegree
setContactFriction(radians(compFricDegree))
print ("\r Friction: ",compFricDegree," porosity:",triax.porosity)
sys.stdout.flush()
## while we run steps, triax will tend to grow particles as the packing
## keeps shrinking as a consequence of decreasing friction. Consequently
## porosity will decrease
O.run(500,1)

#O.save('compactedState'+key+'.yade.gz')
#print "### Compacted state saved ###"

##############################
### DEVIATORIC LOADING ###
##############################

##We move to deviatoric loading, let us turn internal compaction off to keep particles sizes constant
triax.internalCompaction=False

## Change contact friction (remember that decreasing it would generate instantaneous instabilities)
setContactFriction(radians(finalFricDegree))

##set stress control on x and z, we will impose strain rate on y
triax.stressMask = 5
##now goal2 is the target strain rate
triax.goal2=rate
## we define the lateral stresses during the test, here the same 10kPa as for the initial confinement.
triax.goal1=-100000
triax.goal3=-100000

##we can change damping here. What is the effect in your opinion?
newton.damping=0.1

##Save temporary state in live memory. This state will be reloaded from the interface with the "reload" button.
O.saveTmp()

#####################################################
### Example of how to record and plot data ###
#####################################################

from yade import plot

### a function saving variables
def history():
plot.addData(e11=-triax.strain, e22=-triax.strain, e33=-triax.strain,
ev=-triax.strain-triax.strain-triax.strain,
s11=-triax.stress(triax.wall_right_id),
s22=-triax.stress(triax.wall_top_id),
s33=-triax.stress(triax.wall_front_id),
i=O.iter)
plot.saveDataTxt('calibration-hz.txt')

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:
O.engines=PyRunner(iterPeriod=20,command='history()',label='recorder')

O.run(100000,True)

#plot.plots={'e22':('s11','s22','s33',None,'ev')}

## display on the screen (doesn't work on VMware image it seems)
#plot.plot()

Thanking you in advance,
Sarath

## Question information

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Last reply:
 Revision history for this message Launchpad Janitor (janitor) said on 2021-06-14: #1

This question was expired because it remained in the 'Open' state without activity for the last 15 days.

 Revision history for this message Jan Stránský (honzik) said on 2021-06-14: #2

Hello,

> I am facing problem

please be more specific . Do you get errors? Does it take too much time? The target cannot be reached? ... ?

> when I am using Hertz-mindlin contact model

with other model(s) it is ok or not? (like why you mention explicitly "using Hertz-Mindlin model").

> Please run the code

please describe the problem such that running the code is not the primary source of information .

Cheers
Jan

 Revision history for this message Sarath Chandra Reddy (sarath4321cityu) said on 2021-06-15: #3

Thank you Jan for the reply.
>please be more specific . Do you get errors? Does it take too much time? The target cannot be reached? ... ?

I am not getting any error but the unbalanced force and mean stress are not reaching equilibrium even after long time (say even after few days).

>when I am using Hertz-mindlin contact model - with other model(s) it is ok or not? (like why you mention explicitly "using Hertz-Mindlin model").

I think this especially when I am using Hertz-Mindlin model as I only changed small things in original code of Dr.Bruno (mainly about the contact model).

Please give your advice/suggestions on any measures to be taken to attain stability quickly.

best wishes
Sarath

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