Displacement controlled loading

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1

Dear All,

I’m looking for a way to find out how to apply DISPLACEMENT CONTROLLED LOADING. I want to extract the forces to obtain the force displacement curve. I tried Fix SMD but now I’m welling to apply discrete displacements.

I want a clue. which commands and fixes should I use? How to obtain the total forces on atoms?

Thanks in advance.

2

Are there any available examples too?

3

Have you checked out the indent examples located in $LAMMPS/examples/indent?

R.

4

Dear All,

I'm looking for a way to find out how to apply DISPLACEMENT CONTROLLED
LOADING. I want to extract the forces to obtain the force displacement
curve. I tried Fix SMD but now I'm welling to apply discrete displacements.

I want a clue. which commands and fixes should I use? How to obtain the
total forces on atoms?

there are plenty of clues to be had simply from reading the
documentation. particularly the "howto" sections. and as a bonus you
are likely to learn things that you don't need right now, but will be
helpful in the future.

axel.

5

Are there any available examples too?

LAMMPS comes with bundled examples (have you noticed the folder called
examples?) and to make your life easier, there are also files called
README and comments in the specific input scripts to indicate what
those examples are showing.

axel.

6

Dear lammpsmates

I have a problem and a question about using fix indent.

I’m working with small system of Cu atoms. Now if I use such command, the system remains still.

fix 4 all indent 0.0 sphere 7 7 16 5 units lattice

But I want to move the indenter in a constant velocity in direction z.
So, I defined a vz variable as follows:

variable vz equal “16 - 1elaplongdt”

Which makes my system to lose its atoms! I even tried to define the variable vz like this:

variable vz equal 16
which is actually a constant but it doesn’t matter. My system is unstable as soon as I write v_vz instead of “z” in fix indent command.

It’s so wierd and I don’t understand where the problem is.
Any ideas?

I also have a question about simulation of indentation (which at least I can do it by defining another object as an indenter).
How can I obtain a force-displacement curve?

Regards,
Soheil

7

Dear lammpsmates

I have a problem and a question about using fix indent.

I'm working with small system of Cu atoms. Now if I use such command, the
system remains still.
fix 4 all indent 0.0 sphere 7 7 16 5 units lattice

what use is an indenter with a force constant of 0.0?

But I want to move the indenter in a constant velocity in direction z.
So, I defined a vz variable as follows:
variable vz equal "16 - 1*elaplong*dt"

Which makes my system to lose its atoms! I even tried to define the variable
vz like this:
variable vz equal 16
which is actually a constant but it doesn't matter. My system is unstable as
soon as I write v_vz instead of "z" in fix indent command.

It's so wierd and I don't understand where the problem is.
Any ideas?

you are not telling use the whole story here and your initial test is
a pointless test.
outside of guessing or having mind reading abilities, there is little to help.

I also have a question about simulation of indentation (which at least I can
do it by defining another object as an indenter).
How can I obtain a force-displacement curve?

by printing the force and the displacement, of course. please have a
*good* look at the LAMMPS manual, it is full of helpful information
and among many other things also has the answer to his question.

axel.

8

Actually, there is no need for guessing or extraordinary abilities!
I’m trying to simulate an indentation. So, for sure my system has a plane (or half plane!) which should be penetrated with an indenter.
That’s all. Telling other parts of the story is just a waste of time both for me and others in here.

Also, there is no use for having an indenter with a K = 0. But that’s the whole point! Even with a K=0, the system would be unstable if I use a variable instead of the value of z in fix indent. And z=16 for the center of the indenter with a r=5 put in the free space, far from that surface it should penetrate.
(if it works, I surely will not use an indenter with K=0)

So, I may need to clarify my question again. It’s so simple. If I put a variable in fix indent for it’s z value, my system will be unstable. Does anybody know what the problem is and how it can be solved?

Thanks,
Soheil

9

Actually, there is no need for guessing or extraordinary abilities!
I'm trying to simulate an indentation. So, for sure my system has a plane
(or half plane!) which should be penetrated with an indenter.
That's all. Telling other parts of the story is just a waste of time both
for me and others in here.

without providing any way to easily reproduce what you are seeing, you
*do* waste your time and the time of anybody reading this.
there is an example in the lammps distribution and it *does* work fine
as you can see here: https://www.youtube.com/watch?v=jbHLTAeJPfU

Also, there is no use for having an indenter with a K = 0. But that's the
whole point! Even with a K=0, the system would be unstable if I use a
variable instead of the value of z in fix indent. And z=16 for the center of
the indenter with a r=5 put in the free space, far from that surface it
should penetrate.
(if it works, I surely will not use an indenter with K=0)

So, I may need to clarify my question again. It's so simple. If I put a
variable in fix indent for it's z value, my system will be unstable. Does
anybody know what the problem is and how it can be solved?

how should *anybody* know? it works correctly when you use it
correctly, so there must be something wrong in the parts you don't
want to waste your time talking about.
axel.