[lammps-users] Uniaxial Tensile test

Dear All,
I am doing a uniaxial tensile test on a copper wire.
As per the literature, I should follow these steps.

1) The system should be assigned an initial temperature of 300K and is allowed to relax by holding the length of the wire constant while maintaining a constant temperature using thermostat.

2) No thermal constraints are applied while applying tension.

When I run the simulation, I am getting negative stress at zeroth time step. But it should be positive as the wire has initial stresses. The temperature should increase during tensile test but its not happening in this case.

Am I using the correct commands. I am giving the input script and results herewith.

%%%%%%%%%%%%% input script %%%%%%%%%%%%%%%%
units metal
boundary s s s
atom_style atomic
lattice fcc 3.615
region box block 0 60.0 0 5.0 0 5.0
create_box 3 box


pair_style eam
pair_coeff * * Cu.eam
neighbor 0.3 bin
neigh_modify delay 1


region lower block INF 1 INF INF INF INF
region upper block 59 INF INF INF INF INF
group lower region lower
group upper region upper
group boundary union lower upper
group mobile subtract all boundary

set group lower type 2
set group upper type 3

# temp controllers
compute new3d mobile temp

# eqilibriate

velocity mobile create 300.0 581277.5 temp new3d
fix 1 all nve
fix 2 boundary setforce 0 0 0
fix 3 all temp/rescale 10 300.0 300.0 10.0 1.0
fix_modify 3 temp new3d

thermo 50
thermo_modify temp new3d

dump 1 all xyz 50 tens
timestep 0.001
run 10000

#tension
velocity upper set 3 0 0
velocity mobile ramp vx 0 3 x 2 59 sum yes


compute stpa mobile stress/atom
compute stgb mobile reduce sum c_stpa[1]
variable sxx equal c_stgb/vol

dump 1 all xyz 100 tens
thermo 100

thermo_style custom step lx xhi v_sxx temp

reset_timestep 0
run 500
%%%%%%%%%%%%%% input script %%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%% results %%%%%%%%%%%%%%%%%

Step Lx Xhi sxx Temp
0 216.9002 216.9001 -25145.843 1298.4325
100 217.36799 217.36789 16337.328 295.28236
200 217.78983 217.78973 10625.251 295.30146
300 218.21833 218.21823 8799.2358 292.66175
400 218.57678 218.57668 7989.8318 295.20004
500 219.02176 219.02166 9478.1797 291.13471

%%%%%%%%%%%%%% results %%%%%%%%%%%%%%%%%

Thanks in advance
Sivashankar
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If you hold the length of the wire constant, it cannot “relax”, unless its length just happens to be the exact equilibrium value.

Best,
Rob