Yade

Minimum and Maximum values for parameters

Asked by Sam

Hi guys!

I am trying to calibrate the model with the triaxial test.
I used CohFricMat as a material and Cohesionmoment for the Contact law.
The soil is sand with cohesion.
So I have to work with 5 parameters to match the result of the experimental test with the triaxial test.
These parameters are E, Ks/Kn, Phi, eTaRoll and alphaKr.
Hence, I have a general question about YADE which Are there any limitation for parameters? it means is there any minimum and maximum value for contact (i.e. CohFrictMat) parameters?

For instance, I used these value for my model:

from yade import pack,plot

############################################
### DEFINING VARIABLES AND MATERIALS ###
############################################

nRead=readParamsFromTable(
 num_spheres=5000,# number of spheres
 compFricDegree =30, # contact friction during the confining phase
 key='_triax_Toyoura_', # 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 = 0.49 #the porosity we want for the packing
compFricDegree = 30.6# contact friction during the deviatoric loading
finalFricDegree = 10
rate=-0.1 # loading rate (strain rate)
damp=0.70 # damping coefficient
stabilityThreshold=0.01 # we test unbalancedForce against this value in different loops (see below)
young=1e6# contact stiffness
mn,mx=Vector3(0,0,0),Vector3(0.09,0.18,0.09) # corners of the initial packing

## create materials for spheres and plates
O.materials.append(CohFrictMat(alphaKr=40,young=young,poisson=0.2,frictionAngle=radians(30),normalCohesion=0,shearCohesion=0,momentRotationLaw=False,etaRoll=0.05,density=2600,isCohesive=False,label='spheres'))
O.materials.append(CohFrictMat(young=young,poisson=0.2,frictionAngle=radians(30),density=0,label='walls'))

Cheers
Hessam

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Jan Stránský
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Best Jan Stránský (honzik) said : 		#1

Hello,
in general, Yade does not put any limitations for the parameters values. But is assumes you choose them "reasonably". Reasonable means e.g. positive stiffness. However, in certain cases it may treat "non-reasonable" values in a special way, like for etaRoll<0, "If negative, rolling moment will be elastic" (from the docs).

The specific use always depend on your problem and level of simplification. E.g. you can simulate non-spherical particles with spheres and "non-realistic" higher friction angle.
E.g. in your case, young=1e6 (assuming Pa units) seems too low to me, but it may be the best value at the same time :-)

Some notes about a few parameters:
- elastic parameters (E, Ks/Kn and alphaKr in your case) should be identified first, as their response is independent on the inelastic parameters.
- phi, friction angle, has reasonable values in the range [0,pi/2]
- I would first try the model without rolling stiffness and resistance. If it gives good results, I would choose the more simple model.

You can also try some dimensional analysis to reduce parameters for calibration. E.g. you will find that Poisson's ratio depends on Ks/Kn and etaRoll, but not on E etc.

cheers
Jan

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Robert Caulk (rcaulk) said : 		#2

Relevant thread [1]

[1]https://answers.launchpad.net/yade/+question/654408

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Sam (sambahmani) said : 		#3

Thanks Jan Stránský, that solved my question.