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[luc scholtes <lscholtes63@xxxxxxxxx>]

Hi all,

(Bruno, you probably have an idea about this)

A remark for those who are interested in doing triaxial loadings on samples
with height/width ratio different from 1. Here is the test I did:

1 - take 2 samples, one with a ratio equal to 1 (a cube typically) and
another with a ratio equal to 2.
2 - give them exactly the same micro properties (it appears to me that it is
more interesting with a high E(Yade Young Modulus)/confiningPressure ratio)
3 - from an initial dense state (say that you have already created a dense
sample with the compaction procedure of the triaxial state), apply them an
hydrostatic loading in order to reach a higher confining pressure, with a
sufficiently small wall velocity to avoid inertial effects (setting the
maxVel parameters small enough)
4 - plot the evolution of the 3 principal stresses as a function of 1 one
the 3 principal strains (the corresponding strain rates are equal according
to the servo-controlled controlller) for both of the 2 samples.

-> see the attached figure for the results

As you can see, in the case of the cube, the 3 principal stresses evolve in
an identical way with the deformation, which is a good news, but,
unfortunately, for the sample with a ratio heigh/width = 2, we can see that
it is not the case at all. The stresses reach the desired confining
pressure, but there is a kind of a delay between the axial stress (S2) and
the radial ones (S1,S3) that occurs during the loading: the axial stress
increase more rapidly than the 2 others... This leads to less deformation in
the axial direction than in the radial ones to reach the desired confining
pressure.

I tried with several velocities and with very low damping values
(wallDamping=0, damping(NewtonIntegrator)=0.05), and I always get the same
result. The sample is supposed to be isotropic and homogeneous in regards to
contact forces and orientations distribution.

One of my experienced colleague though about relaxation effects that should
occur in the sample, creating this delay between the axial and the radial
responses of the system.

If you have any advice....

Cheers

  Luc

Attachment: Hydrostatique_StressDelay.png
Description: PNG image