resonant column test

Asked by Miguel David Valencia Galindo on 2019-07-16

Hello everyone

I am a new user on Yade and I am new modelling in DEM.
I want to model a resonant column test, which consist on apply an isotropic stress on a cylindrical soil sample and then apply torsion to different strain levels. I have seen the triaxial test examples and there the soil sample used is a cube in a rigid wall, the isotropic consolidation there is applied through servo-controled system and then the deviatoric stress is applied. I have several questions about that topic.

1. I would like to know if the servocontrol system could be used in a cylindrical sample or if the sample response will be different if I use a cubic sample, I haven't found any reference about the topic, but I have seen in several references and in other examples that explain the consolidation could be achieved increasing the particle sizes. That takes me to the next question.

2. If I perform the isotropic consolidation increasing the particle sizes I have a change in the PSD, in some forums and some references I have seen that size doesn't matter in Yade, at least in models where high frequencies effects are not important. But in a resonant column test, the frequencies are foundamental to define the material behavior. And also if I change the particles and I consider the gravity there could be a change...

3. My last question is about the initial particle sizes I should use. I have seen that O'Sullivan (2011) recomends to use the representative volume element, but if I have a distribution of particles sizes, the number of contacts changes, Does that matter?

Thanks

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Last query:
2019-07-18
Last reply:
2019-07-18
Robert Caulk (rcaulk) said : #1

> if the servocontrol system could be used in a cylindrical sample

If you are referring to triax-script1.py (?), TriaxialStressController, the answer is no since it is designed to let the user control 6 walls of a cube. However, there is a hidden triaxial example showing one creative way to apply isotropic stress to a cylindrical specimen in Yade [1]. In that example, Jan controls hundreds of individual facets around a cylindrical cohesive specimen. Such a method might be problematic if you have wide PSD and a non-cohesive packing, since particles will almost certainly "leak out". A good starting point, nonetheless.

[1]https://gitlab.com/yade-dev/trunk/blob/master/examples/concrete/triax.py

Jan Stránský (honzik) said : #2

If I did the example now, I would use boxes instead of facets..
Jan

Thank you for your help Robert. I am going to try with that example. I have seen that the model runs very slow with the real PSD. Do you have some recommendations about this topic having into account the test I want to perform?.

Thanks

Miguel

Jan Stránský (honzik) said : #4

Hi Miguel,

> the model runs very slow with the real PSD

could you post the values you use?
The critical time step depends on particle size, its density/mass and stiffness
t_cr = R * sqrt(density/young) [1]
the smaller particles, the smaller critical time step, the smaller simulation. Real PSD could contain very small particles.

some options:
- artificially "cut" the PSD, not including the smallest particles
- increase mass of the smallest particles to increase critical time step (google selective mass scaling, advanced mass scaling or such).

cheers
Jan

PS: as it is quite different question than the original one, next time please open a new question [2]

[1] https://yade-dem.org/doc/formulation.html#stability-considerations
[2] https://www.yade-dem.org/wiki/Howtoask

Thanks Jan. next time I will open a new question.

My I have defined my PDS as follows:

psdSizes,psdCumm=[0.000075,0.00015,0.0003,0.000425,0.0006,0.00085,0.00118,0.00236,0.00475],[0.2,0.26,0.42,0.52,0.6,0.69,0.77,0.92,1.]

And the time step I have used is

O.dt=.5*PWaveTimeStep()

I understand this calculates the time steep in function the particles' array

Thanks for your answer

Miguel

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