Hi Hadi,
> In my simulation I want to interact tow crystal (Fe2o3/Fe) in z
direction and after relaxation and bounding
> the tow crystals at interface, I want to do the tension loading by "set
boundary velocity" and " velocity ramp"
> in tow sides in opposite direction.
In my opinion, velocity ramp should not be applied to the middle active
region. The atoms that are not the boundary atoms for applying the
tension/compression load to (or those that are active or mobile) should
just respond freely and automatically to the load applied to the boundary
groups.
Dear Ray
Thanks for your reply.
Absolutely you are correct to don't use velocity ramp. but because of use
high strain rate in tension loading
(to reduce the simulation time) I used velocity ramp to avoid shock wave
during simulation.
my reference to use this method is the following paragraph of "Molecular
dynamics simulation based cohesive surface representation
of mixed mode fracture, X.W. Zhou , J.A. Zimmerman, E.D. Reedy Jr., N.R.
Moody"
"During molecular dynamics simulations of mode I crack
propagation, the system is uniformly stretched (by moving
each atom a distance corresponding to a uniform normal
strain increment) in the y direction each time step. Numerical
approaches are then used to update the atom positions
based upon interatomic potential and Newton’s equations
of motion under the condition that the y coordinates of the
top and bottom horizontal layers of boundary atoms
remain fixed. Unlike displacing boundary atoms alone,
a uniform stretching of the system
avoids the creation of shock wave during simulations"
In Lammps Examples and tutorial this method used too. please let me know if
I
am in wrong way
> In first step I just to simulate the tension loading for one crystal
(Fe2O3).
> I couldn't relax the crystal in P P S boundary, but in P P P the
relaxation result is better and in
> "P P S" boundary with "setfors boundary 0 0 0" the relaxation result are
good. but I think it is not a
> regular method to use the "setforce boundary 0 0 0" during relaxation
mode. please advise me?
What errors were present with p p s boundary? How was "better" defined
with the p p p boundary? Why is it not regular to fix some of the atoms
(especially those kept for applying load to) immobile? If you don't care
about the boundary atoms and care only about how the active and free atoms
respond to the applied load, it does not matter if the boundary atoms are
relaxed or not.
when I use "P P S" to relax the Crystal after a few time step the
structure warped immediately and convert to a
amorphous structure and reduce the volume in z direction. I used the
following command. when I used P P P
the relaxation continued till 60 pico second and then diverged. I prefer to
relax P P S with setforce 0 but
because of finally I should interact one side to another crystal then its
correct to setforce just one side and
other side must relaxed freely. ( I am not sure and need help)
units metal
dimension 3
atom_style atomic
boundary p p s
read_data atom.data
timestep 0.0002
pair_style meam
pair_coeff * * library.meam Fe3 O2 FeO.meam Fe3 O2
replicate 4 4 4
compute enperatom all pe/atom
compute pe all reduce ave c_enperatom
compute msdd all msd
thermo 10
thermo_style custom step vol pe c_pe temp c_msdd[1] c_msdd[2] c_msdd[3]
c_msdd[4] press cpu
min_style cg
minimize 1.0e-10 1.0e-10 1000 10000
velocity all create 300.0 234 dist gaussian
fix 1 all nvt temp 300 300 0.05
run 500000