about CPM material model

Asked by Othman Sh on 2019-10-30

Hello,

Regarding the CPM material model, would you please clarify the following:

1- From Yade documentation under the section: class yade.wrapper.Law2_ScGeom_CpmPhys_Cp :
epsSoft(=-3e-3, approximates confinement -20MPa precisely, -100MPa a little over, -200 and
-400 are OK (secant))
Strain at which softening in compression starts (non-negative to deactivate)

what is the difference between epsSoft and epsCrackOnset (in Yade its described as: limit elastic strain)? is epsCrackOnset is the elastic strain limit in tension only, while epsSoft is the elastic strain limit in compression?

2- relKnSoft is defined in Yade documentation as "Relative rigidity of the softening branch in compression (0=perfect elastic-plastic, <0 softening, >0 hardening"

Is relKnSoft a factor to reduce the elastic modulus of the bond in compression?

3- In the CpmMat, is the parameter "young" corresponds to the elastic modulus in both tension and compression?

4- Suppose I compressed two spheres into eachother, based on the CPM model, the stress-strain relationship is linear elastic at the begining, then what will happend to the curve? is there a strain limit after which the bond will be damaged?

Thank you,
Othman

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Othman Sh
Solved:
2019-10-31
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2019-10-31
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2019-10-30
Jan Stránský (honzik) said : #1

Hello,

see [1] for more details.

1) and 3) yes

2) relKnSoft is a factor to reduce the **tangent** (not elastic, since plasticity is involved) modulus of the bond in compression.
Elastic modulus remains unchanged (visible for unloading).

4)
> the stress-strain relationship is linear elastic at the beginning

yes

> then what will happend to the curve? is there a strain limit after which the bond will be damaged?

no, in compression the strain is not limited. The **elastic** strain is limited, followed by plasticity (perfect, with hardening or with softening). But no damage.

cheers
Jan

[1] https://www.researchgate.net/publication/45194493_Cohesive_Particle_Model_using_the_Discrete_Element_Method_on_the_Yade_Platform, section 3

Othman Sh (othman-sh) said : #2

Jan,

Thanks for your reply, just to follow up on point 4, you said that the elastic strain is limited by epsSoft, then plasticity starts. So to control the stress-strain behavior in compression after epsSoft is exceeded, I can use relKnSoft, right?

Also, in Yade documentation, "For commercial reasons, rate-dependence and compression-plasticity is not present in reduced version of the model, used when CPM_MATERIAL_MODEL is not defined". So is the CPM model in the Yade 2018.02b is the full model or the reduced version with no compression-plasticity?

Thanks,
Othman

Jan Stránský (honzik) said : #3

Hi,

> So to control the stress-strain behavior in compression after epsSoft is exceeded, I can use relKnSoft, right?

yes

> So is the CPM model in the Yade 2018.02b is the full model or the reduced version with no compression-plasticity?

the version is reduced (by strain rate effects and plasticity in shear), but compression plasticity is present, see line 394 of pkg/dem/ConcretePM.cpp

cheers
Jan

Othman Sh (othman-sh) said : #4

Thank you Jan. That was really helpful

Regards,
Othman