fix deform

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1

dear lammps users,
i want to compress a pillar by fixing the bottom layer.
i am using fix deform to compress the pillar. while using fix deform can this bottom layer be fixed or not?
if answer is yes…can anybody suggest which command should be used?
i have tried using fix set force
and by making the bottom layer velocity zero
this is not working.
i also tried using fix indent…but in this input file is running only for one step after this it not running.
how to make the bottom layer fixed?

thanks in advance

2

Hi Hymavathi,

Maybe you should try ‘fix momentum’ on the bottom layer to lock it in place. For the deformation it will depend if you want to run it displacement or force controlled. For force the ‘fix setforce’ seems like the right option, for displacement controlled deformation ‘fix move’ or ‘fix drag’. But there’s a myriad of force and displacement related fixes in the the LAMMPS docs - http://lammps.sandia.gov/doc/fix.html so maybe you should just browse them.
I think ‘fix deform’ acts on the simulation box in the first place which I guess you don’t really want to do if your main goal is to strain a group of atoms. Also, this may become a very uncomfortable approach if you want to use more complex boundary conditions later.

regards,

nikita

3

I'll add that fix deform normally will scale the atom positions
to the changed box size. But it has a "dilate" option
so that some of the atoms need not be scaled if
that is what you want.

Steve

4

i want to fix the bottom layer so that i can apply force in only one direction

5

sorry, i didn’t reply to the list previously…

I guess it should work if you put the atoms at the end you want to fix into a group and then apply fix momentum to this group to restrain its movement. Then it depends where you want to apply your force. If it’s the opposite end, put the atoms there in a group and ‘fix setforce’ them. You could also use ‘fix aveforce’ I guess.

The fix documentation gives a list of all fixes in LAMMPS with a short description. Maybe you should really go through it and have a closer look at those that sound promising for your purposes. The documentation is pretty straightforward and describes very well what the commads do and how to implement them.

Just have a look here: http://lammps.sandia.gov/doc/fix.html

nikita

6

dear lammps users,
i want to compress a pillar by fixing the bottom layer.
i am using fix deform to compress the pillar. while using fix deform can
this bottom layer be fixed or not?

fix deform is not really meant to be used
for that scenario. you can use the dilate
partial option that steve mentioned, but
i doubt that you'll get what you want.

first you have to be clear about what kind
of deformation scenario you want to model.
you basically have the two extremes:
1) macroscopic, slow, homogeneous
2) microscopic, fast, inhomogeneous

fix deform is best suited for the first case,
where you model a small subset of a large
object and the deformation is so slow that
it'll be homogeneous across the system
due to equilibration being faster than deformation.
in this case you _want_ a periodic system
and not fix any parts.

fix indent, fix move, fix addforce are best suited
for the second scenario and you'll have to keep
part of your system fixed in space by either
not time integrating it or using fix setforce 0.0 0.0 0.0
and setting velocities to zero. this looks more
similar to what you describe, so you should
get to make those work.

if answer is yes...can anybody suggest which command should be used?
i have tried using fix set force
and by making the bottom layer velocity zero
this is not working.

as documented, fix deform scales _all_ coordinates
unless dilate partial is used, and it does so for a
good reason.

i also tried using fix indent....but in this input file is running only for
one step after this it not running.

have you considered the fact, that you may be using fix indent
incorrectly? from what you describe using either fix indent or
fix move would be the proper way to get your simulation working.

in short, if you want homogeneous deformation, then there is
no need at all to keep a part fixed. if not, then you should use
either fix indent or fix move and use it correctly.

axel.

7

thank you very much sir…

8

thank you very much sir…

9

with fix indent
do we have to create the atoms for indenter or it automatically creates a indenter and applies the force?
what i understood is it will create the indenter and applies the force.
i am calculating the stress/atom, and divided pillar over the depth into 10 regions, and using compute reduce adding the stress of all the atoms in a particular region.
in the top layer even without applying any force/pressure it is showing very large -ve stress. and after indenting it with spherical indenter, gradually the stress in the upper regions increasing.
if i use cylindrical indenter is there any possibility of increasing or decreasing strain in the pillar?
actually i want to get the constant stress over the depth when it is compressed.
for this what i should do?

thanks in advance

10

Fix indent does not create any atoms. It simply
uses an idealized representation of an "indenter",
e.g. a sphere and applies force to atoms near the
surface of the ideal sphere.

Steve