New question #671283 on Yade:
https://answers.launchpad.net/yade/+question/671283
Hello everyone,
I did my triaxial compression tests under different confining pressures.
Sigma3 in the horizontal directions is constant all through the test and the
cubic sample is compressed in the vertical direction with a constant strain
rate. Sigma3 is equal to 100kPa or 400kPa in my tests. Both samples contract
first and dilate later as the compression goes on.
As well all know, larger volume contraction and lower volume dilation
afterwards will occur through the test under higher sigma3. But my results are
just reverse to this. In the case when sigma3 is 100kPa, the volume contraction
is larger and the volume dilation is maller afterwards. I really don't know why
this happens.
My sample is trimmed from a big gravity deposited sample. My friend obtained
his sample by expanding spheres inside a cubic box and found the same weird
deformation. We use the same contact law. We have adjusted the elastic
modulus and the poisson ratio, but it doesn't work.
I did triaxial tests with PFC under different confining pressures and
everything is fine.
Does anyone else meet this kind of problem?
Below is my code:
from yade import ymport
from yade import utils
############################################
### DEFINING VARIABLES AND MATERIALS ###
############################################
compFricDegree = 26.565 # initial contact friction during the confining phase
finalFricDegree = 26.565 # contact friction during the deviatoric loading
rate=-0.01 # loading rate (strain rate)
damp=0.7 # damping coefficient
stabilityThreshold=0.01 # we test unbalancedForce against this value in
different loops (see below)
young=5e8 # contact stiffness
mn,mx=Vector3(0,0,0),Vector3(1,1,1) # corners of the initial packing
## create materials for spheres and plates
O.materials.append(FrictViscoMat(betan=0.2,
young=young,poisson=0.3,frictionAngle=radians(compFricDegree),density=2600,label='spheres'))
O.materials.append(FrictViscoMat(betan=0.2,
young=young,poisson=0.3,frictionAngle=0,density=0,label='walls'))
## create walls around the packing
walls=aabbWalls([mn,mx],thickness=0,material='walls')
wallIds=O.bodies.append(walls)
print "Load clumps"
deposited_bodies =
ymport.textClumps('cubic_sample_clump_miu0.5_dep_damp0.1_35608_hor_layer_plane.txt',
shift=Vector3(0,0,0), material='spheres')
O.bodies.updateClumpProperties()#get more accurate clump masses/volumes/inertia
############################
### 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.+2e4/young, # spheres growing factor (fast growth)
finalMaxMultiplier=1.+2e3/young, # spheres growing factor (slow growth)
thickness = 0,
## switch stress/strain control using a bitmask. What is a bitmask,
huh?!
## Say x=1 if stess is controlled on x, else x=0. Same for for y and z,
which are 1 or 0.
## Then an integer uniquely defining the combination of all these tests
is: mask = x*1 + y*2 + z*4
## to put it differently, the mask is the integer whose binary
representation is xyz, i.e.
## "100" (1) means "x", "110" (3) means "x and y", "111" (7) means "x
and y and z", etc.
stressMask = 7,
internalCompaction=False, # If true the confining pressure is generated
by growing particles
)
contact = Law2_ScGeom_FrictViscoPhys_CundallStrackVisco(traceEnergy=True)
newton=NewtonIntegrator(damping=damp)
O.engines=[
ForceResetter(),
InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Box_Aabb()]),
InteractionLoop(
[Ig2_Sphere_Sphere_ScGeom(),Ig2_Box_Sphere_ScGeom()],
[Ip2_FrictViscoMat_FrictViscoMat_FrictViscoPhys()],
[contact]
),
## We will use the global stiffness of each body to determine an
optimal timestep (see
https://yade-dem.org/w/images/1/1b/Chareyre&Villard2005_licensed.pdf)
GlobalStiffnessTimeStepper(active=1,timeStepUpdateInterval=100,timestepSafetyCoefficient=0.8),
triax,
newton,
# save data for Paraview
VTKRecorder(fileName='post/3d-vtk-',recorders=['all'],iterPeriod =
500000)
]
O.dt=.5*utils.PWaveTimeStep()
# for the samples of this size (diametre is around 1.8cm), the O.dt is 8.8e-5
s, which is about a hundred times of
# the small sample, and this leads to much faster calculation speed.
#######################################
### APPLYING CONFINING PRESSURE ###
#######################################
## we define the lateral stresses during the test, here the same 100kPa as for
the initial confinement.
p_const = 100000
#the value of (isotropic) confining stress defines the target stress to be
applied in all three directions
triax.goal1=triax.goal2=triax.goal3=-p_const
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(-p_const-triax.meanStress)/p_const<0.001:
break
O.save('confinedState'+key+'.yade.gz')
print "### Isotropic state saved ###"
triax.depth0 = triax.depth
triax.height0 = triax.height
triax.width0 = triax.width
print "depth0 = ",triax.depth0
print "height0 = ",triax.height0
print "width0 = ",triax.width0
print "porosity = ", triax.porosity
print "p_eff = ", triax.meanStress
print "total_num = ", len(O.bodies)
##############################
### DEVIATORIC LOADING ###
##############################
## Change contact friction (remember that decreasing it would generate
instantaneous instabilities)
setContactFriction(radians(finalFricDegree))
##set stress control on x and y, we will impose strain rate on z
triax.stressMask = 3
##now goal2 is the target strain rate
triax.goal3=rate
O.run(4000000,True)
O.save('final.yade.gz')
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