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Re: [lammps-users] Force based friction test
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Re: [lammps-users] Force based friction test


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Mon, 20 Nov 2017 19:34:43 -0500



On Mon, Nov 20, 2017 at 5:02 PM, Raghu Ram <raghu.mmt@...24...> wrote:
Dear LAMMPS users,

I am doing MD simulations for frictional studies (indentation and scratch tests) between aluminum surfaces. I have used fix indent (LAMMPS indenter) command and it worked good which is displacement controlled. I am  trying to simulate force based indenter to replicate the experimental setup. The indentation process looks fine and comparable to lammps indenter (displacement controlled repulsive indenter), but the scratch process is not working properly. 

I have used fix aveforce command and fix lineforce  command for indentation process and fix setforce (gives normal load) and fix move to scratch during scratch process. 

What I am seeing is the normal force on the indenter is not stable and it is dropping down to zero. I have also not observed any wear in the indenter (group of atoms). I am not sure what mistake I am doing?. Any suggestions will be appreciated. 

​your mistake is, that ​you are time integrating the atoms in the group top twice. check your output! LAMMPS should have warned you about it.
instead of fix move, you can make the first argument of fix setforce "0.0" instead of "NULL" and then use "velocity top set" to give its atoms a velocity. since the force in x is set to zero, the velocity will not change and thus you have the same effect as using fix move, but you only use one time integrator (fix nve).

axel.

 

Here is the important portion of the code:

Indentation: 

fix               4 top aveforce NULL v_force 0.0

fix               5 top lineforce 0.0 1.0 0.0

fix               6 top nve

I have used npt for substrate atoms


Scratch:

fix                4 top setforce NULL -0.08 NULL

fix                5 top move linear 0.05 0 0 units box

fix                6 top nve.

Thank you.

Regards
Raghuram

--

S. Raghuram Reddy
TUBAF, Freiberg, Germany.               
Cell : +49-17655446864


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--
Dr. Axel Kohlmeyer  akohlmey@...12...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.