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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: Sat, 25 Nov 2017 14:58:13 -0500



On Sat, Nov 25, 2017 at 2:34 PM, Raghu Ram <raghu.mmt@...24...> wrote:
Dear Axel

Thank you very much for the response. I have tried what you said but there is other complication with it, the system explodes. I don't know what I am doing wrong?

The scratch test works fine with the velocity set command (you have suggested) when the set force for the top group is 0.0 in all the directions. When I give constant normal force along y-direction (using set force) the system explodes because the indenter moves along y-direction continuously which is not periodic. The reason I understand is, velocity can't be set to zero when there is force acting in that particular direction. But, I need to keep constant normal force on the indenter (in y-direction) and scratch along x-direction with constant velocity.

​this is all nonsense. you have not paid proper attention ​to what i was telling you and what you see is the result. when a system "explodes" that is usually due to bad choice of force field, external forces or velocities.
 

Fix move command works fine even tough  I give constant normal force (using set force). The problem with this command is the normal force is fluctuating a lot and average is less than the given set force. For one of the cases the average is equal to the set normal force but for other cases it is not. I do not understand why this is happening?

​you are *still* applying time integration to the same atoms twice. this is wrong and thus you​ get what you deserve.
at any rate, this is now becoming an issue of understanding basic simulation methodology and not an issue of using LAMMPS commands according to their documentation and thus it is rather something that you need to discuss with your adviser/supervisor. the purpose of the mailing list is not to explain to you how to do your research. that is the concern of your adviser/supervisor.

axel.

 

Below is my input script and it would be great if you can help me in figuring out my mistake. Thank you.


#########################################
# Al material, MD simulation to indent ##
# single crystal                   ##
#########################################

# -------------- Initialisation -------------------#

units           metal

boundary        p s p 

atom_style      atomic

timestep        0.001

#######################################################

##    reading coordinate file and Pair Style init    ##

#######################################################

region           box  block 0 40 0 60 0 40 units box

create_box      2 box

lattice        fcc 4.05 orient x 1 1 -2 orient y 1 1 1 orient z 1 -1 0 spacing 0.408248290 0.577350269 0.70710678

region          substrate block 0 40 0 30 0 40 units box

region          indenter sphere 20 45 20 10 units box

create_atoms   1 region substrate units box

create_atoms   2 region indenter units box

pair_style      eam/alloy

pair_coeff      * * Al.eam.txt Al Al

neighbor        0.5 bin

neigh_modify    delay  0

########################################################

## forming regions       ##

#########################################################

region           bottom block INF INF INF 5 INF INF units box

region           middle block INF INF 5 30 INF INF units box

region           top sphere 20 45 20 10  units box

group            bottom region bottom

group            middle region middle

group            top region top

group            main union bottom middle

########################################################

## Minimization       ##

#########################################################

compute         9 all centro/atom fcc

dump             mydumpmin all custom 10 dump_min.txt id type x y z c_9 

minimize        1.0e-14 1.0e-14 100000 100000    

#########################################################

##   equilibrating temp in substrate region            ##

#########################################################

compute          middle_temp middle temp/partial 1 1 1

velocity         middle create 300 5687952 temp middle_temp

fix              1 main npt temp 300 300 0.1 x 0.0 0.0 0.1 z 0.0 0.0 0.1

fix              2 bottom setforce 0.0 0.0 0.0

fix_modify       1 temp middle_temp

thermo           1000

thermo_modify    temp middle_temp

dump             mydump2 all custom 1000 dump_equi.txt id type x y z c_9 

run              5000

write_restart    restart.equil

undump           mydump2

#########################################################

##            Indentation process                      ##

#########################################################


reset_timestep   0

variable         ind_run equal 500

variable         max_iter equal 320

label            loop

variable         i loop ${max_iter}

if "${i} > ${max_iter}" then "jump in.test"

variable         force equal -0.000121*${i}

fix               4 top aveforce 0.0 v_force 0.0

fix               5 top lineforce 0.0 1.0 0.0

unfix            1

fix              6 top nve

fix              1 main npt temp 300 300 0.1 x 0.0 0.0 0.1 z 0.0 0.0 0.1

fix_modify       1 temp middle_temp

#########################################################

##            Calculating force vs indentation        ##

#########################################################

### Stress Data Setup

compute         ind_pos top com

compute         ave_F top reduce ave fy

compute         sum_F top reduce sum fy

variable        F_xx1 equal "f_4[1]*1.602"

variable        F_yy1 equal "f_4[2]*1.602"

variable        F_zz1 equal "f_4[3]*1.602"

variable        posy equal "c_ind_pos[2]"

variable        Fy_ave equal "c_ave_F*1.602"

variable        Fy_sum equal "c_sum_F*1.602"

variable        t_step equal "step"

variable        t_temp equal "temp" 

fix             def1 all print 100 "${t_step} ${posy} ${F_xx1} ${F_yy1} ${F_zz1} ${Fy_ave} ${Fy_sum}" append force_indentation.txt 

fix             def3 all print 100 "${t_step} ${t_temp}" append temperature.txt

thermo          1000 

thermo_style     custom step press temp

thermo_modify    temp middle_temp

dump             mydump1 all custom 500 dump_indent.txt id type x y z c_9 

dump_modify      mydump1 element Al Al 

restart          5000 restart_ind.equil

run              ${ind_run}

unfix            4

unfix            5

unfix            6

undump           mydump1

uncompute        ind_pos

uncompute        ave_F

uncompute        sum_F

next i

jump in.test-ind-move loop

label break

variable i delete


unfix            def1

unfix            def3


#########################################################

##                   scratch                           ##

#########################################################


reset_timestep   0

compute         ind_pos top com

compute         ave_F top reduce ave fy

compute         sum_F top reduce sum fy

variable        posx equal "c_ind_pos[1]"

dump             mydump2 all custom 500 dump_scratch id type x y z c_9

fix                4 top setforce 0.0 -0.0387 0.0

fix               5 top move linear 0.05 0 0 units box

#velocity          top set 0.05 0 0 units box

fix              6 top nve

unfix            1

fix              1 main npt temp 300 300 0.1 x 0.0 0.0 0.1 z 0.0 0.0 0.1

fix_modify 1 temp middle_temp

fix             def1 all print 500 "${t_step} ${posx} ${F_xx1} ${F_yy1} ${F_zz1} ${Fy_ave} ${Fy_sum}" file scratch_disp.txt

fix             def3 all print 1000 "${t_step} ${t_temp}" file temp-scratch.txt

dump_modify mydump2 element Al Al

restart          50000 restart_ind.equil

run              150000



On Mon, Nov 20, 2017 at 6:34 PM, Axel Kohlmeyer <akohlmey@...24...> wrote:


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.               


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



--

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




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