Question about boundary condition

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1

Hi I am Jinu who is sophomore of Ulsan Univ, Korea

Here is Lammps tutorial and I was studying this one.

  1. I can not understand why we have to set boundary condition before deformation loop

(I think that boundary condition can be changed during deformation)

https://icme.hpc.msstate.edu/mediawiki/index.php/LAMMPS_Fracture

  1. What is difference between ‘fix deform’ command and 'displace_atoms’command?

  2. When I make tensile loading model my self, the material just ‘moves’ (I could not plastic deformation even the force was not so strong)

What was problem?

Thank you very much. God bless you

2

Hi I am Jinu who is sophomore of Ulsan Univ, Korea

Here is Lammps tutorial and I was studying this one.

1. I can not understand why we have to set boundary condition before
deformation loop

(I think that boundary condition can be changed during deformation)

https://icme.hpc.msstate.edu/mediawiki/index.php/LAMMPS_Fracture

# Put fixed boundary condition on edge atoms by setting forces to zero
fix 2 gbhigh setforce 0 0 0
fix 3 gblow setforce 0 0 0

​you are mixing two things up here. the two lines you quote​ refer to
immobilized atoms, not boundary conditions of the box.

2. What is difference between 'fix deform' command and
'displace_atoms'command?

​fix deform changes the entire box geometry (and the atoms with them in a
way so they retain their relative position with respect to the box).
displace_atoms will only ​move atoms.

3. When I make tensile loading model my self, the material just 'moves' (I
could not plastic deformation even the force was not so strong)

    What was problem?

​who knows? probably your initial geometry was not proper.​

3

Also, a fix, like fix deform operates
continuously during a run. Whereas displace_atoms
or displace_box is a one-time operation, done
before or between runs.

Steve