Uniaxial tension

Hello,
When I am subjecting the simulation box to tension in x-direction, it soon starts to expand in Y and Z directions too. Is this because of a high strain rate? Also the stress in X-direction is negative from 0.05 to 0.09 strain.

My code is attached below.

The tension application code:

variable a loop 2
label loop

fix 27 all npt temp 300 300 1 iso 0 0 1 drag 1
thermo 1000
thermo_style custom step temp press ke pe etotal vol density
run 90000
unfix 27

fix 28 all nve
thermo 1000
thermo_style custom step temp press ke pe etotal vol density
run 50000
unfix 28

fix 29 all npt temp 300 300 1 y 0 0 1 z 0 0 1 drag 1
variable srate equal 1e10
variable srate1 equal “v_srate/1.0e12”
fix 30 all deform 1 x erate ${srate1} units box remap x

Output strain and stress info to file

# for units metal, pressure is in [bars] = 100 [kPa] = 1/10000 [GPa]

# p2, p3, p4 are in MPa

variable strain equal “(lx - v_L0)/v_L0”
variable strain_y equal “(ly - v_Ly0)/v_Ly0”
variable strain_z equal “(lz - v_Lz0)/v_Lz0”

If you have free surfaces in y,z and stretch/compress the box in x,
then those other dimensions will change b/c they want to. That’s
what real materials do. If you make y,z periodic then they
won’t change. The fix npt command allows you to apply a barostat
to those y,z dimensions only. They they will change as x increase/decreases.
Or the fix deform command has options to change x and adjust
y,z automatically to volume preserve the system.

Steve