# Normal force and von mieses calculation

Dear all,

I am simulating the contact of a rigid cylinder with deformable block. I am not integrating the cilinder, but just moving the block with constant v. I have a reservoir in the under the block and both solids in study are in nve. Since normal force is very important in contact mechanics, I have to calculate it. So, I got the fy for every atom and I am summing only the y coordinate of the indentator. Is it ok like that, even if the rigid cylinder is not being integrated? I am getting a little big values.

Another question is what is the best way to calculate the von mieses stress in such a sistem. Is there any effect in the thermodinamics I should consider when simulating the block with constant velocity, since I am using thermo_modify and compute temp/com for the block temperature?

Thanks a lot!

Débora Marques

These are the scripts:

For the Contact Simulation:

dimension 3
boundary p s p
atom_style atomic
neighbor 0.3 bin
neigh_modify delay 2 check no
units lj

# create geometry

lattice fcc 1.0

compute temperatura all temp
compute t1 hi temp/com
compute t2 lo-fixed temp/com
compute t3 lo temp/com
compute 1 lo com
compute 2 lo-fixed com
compute 3 hi com
compute 4 lower-group centro/atom fcc
compute 5 lo msd com yes
compute 6 hi msd com yes

#displace
displace_atoms idn move 0 -4.3 0 units box
#fix
#unfix 3
fix 2 lo nve
fix 3b lo-fixed nvt temp 0.0623 0.0623 0.074
fix_modify 3b temp t2

velocity lower-group set NULL 0.791862285 NULL
velocity idn set 0.0000000 0.000000 0.0000000

#run main

thermo 500
thermo_style custom step temp ke pe etotal c_t1 c_t2 c_t3 etotal c_1[1] c_1[2] c_1[3] c_2[1] c_2[2] c_2[3] c_3[1] c_3[2] c_3[3] c_5[1] c_5[2] c_5[3] c_6[1] c_6[2] c_6[3]

timestep 0.0000148

dump 1a all atom 1000 dump.mat3-cut00_a
#dump 2 all atom 1000 dump.mat3-cut00b.*
dump 3 all xyz 10000 dump.mat3-cut00_c..xyz
dump 4 all custom 10000 dump.mat3-cut00_d.
id type x y z vx vy vz fx fy fz c_4
write_restart restart.mat3-cut00.100000
run 1000000
write_restart restart.mat3-cut00.200000
run 1000000
write_restart restart.mat3-cut00.300000
run 100000
write_restart restart.mat3-cut00.400000
run 100000

For the anneal:

dimension 3
boundary p s p
atom_style atomic
neighbor 0.3 bin
neigh_modify delay 2 check no
units lj

# create geometry

lattice fcc 1.0
region box block 0 50 0 80 0 9
create_box 3 box

mass 1 1.0
mass 2 1.0
mass 3 1.0

# atom regions

region lo-fixed block INF INF 1.1 3.1 INF INF
region lo-slab block INF INF 3.2 11 INF INF
region b1 block INF INF 14.000 24.0900 INF INF
region cilindro cylinder z 25.000 25.000 10.0000 INF INF
region cyl intersect 2 b1 cilindro

# create 2 surfaces with asperity

create_atoms 1 region lo-slab
create_atoms 2 region lo-fixed
create_atoms 3 region cyl

# LJ potentials

pair_style lj/cut 2.2

pair_coeff * * 1.0 1.0 2.2

# define groups

group lo region lo-slab
14400 atoms in group lo
group lo type 1
group idn region cyl
group hi type 2
group lo-fixed region lo-fixed
group lower-group union lo-fixed lo

set group lo-fixed type 3

# initial velocity and compute

compute temperatura all temp
compute t1 hi temp
compute t2 lo-fixed temp
compute t3 lo temp
compute 1 lo com
compute 2 lo-fixed com
compute 3 hi com
#compute 4 lo centro/atom fcc

velocity all create 0.0623 482748 temp temperatura

#deslocando o upper-group

displace_atoms idn move 0 30.0 0 units box

#Fixes

#fix 3a lo nve

fix 3 lo nvt temp 0.0623 0.0623 0.001

#configurações do run

thermo 100
thermo_style custom step temp etotal c_t1 c_t2 c_t3 c_1[1] c_1[2] c_1[3] c_2[1] c_2[2] c_2[3] c_3[1] c_3[2] c_3[3]

timestep 0.0003#0.0148

run_style verlet

#run

dump 1 all atom 100 dump.010_anneal
dump 1b all xyz 100 dump.010_anneal..xyz
dump 1c all custom 100 dump.010_c.
id type x y z vx vy vz fx fy fz
run 50000

Whatever you are doing to sum forces within the script,
you can easily check by dumping forces on all atoms
and summing them yourself to see if you are consistent.

I don't know the details of VM strees, but you can look
at the USER-ATC package which calculates various
kinds of stresses, besides the standard virial stress
that main LAMMPS computes.

Steve

Dear Steve,

I did plot the force over the block and the indentor. I found reasonable values for fy for the indenter, but very strange values in the deformable block. I am attaching the Fy values (summ over fy in the dump file for atoms type 3, deformable block), they should be zero before contact, that is, before 170000. Is there any force envolved in the set velocity command which is appearing here?

Thanks a lot for your help
Débora Marques

2011/10/8 Steve Plimpton <[email protected]>

Grafico.bmp (1.73 MB)

The velocity command does not affect forces.

Steve