Hi,
I have calculated the stress tensor as per LAMMPS from per-atom stress. What kind of stress is that when I am converting the virial stress to mechanical stress using compute pressure? Is it engineering stress or Cauchy stress? I am using {erate} or engineering strain rate in fix deform command. The code is below:
compute peratom graphene stress/atom mobile_temp virial
compute p graphene reduce sum c_peratom[1] c_peratom[2] c_peratom[3] c_peratom[4] c_peratom[5] c_peratom[6]
# Storing pxx, pyy, pzz and pxy as variables
variable p1 equal c_p[1]
variable p2 equal c_p[2]
variable p3 equal c_p[3]
variable p4 equal c_p[4]
#variable stress equal (c_p[1]+c_p[2]+c_p[3])/(3*2477.6588) # per-atom stress is negative of total pressure p
# Calculation of stresses and strains # per-atom stress is sum of diagonal components in stress tensor
variable multiplication_factor equal 101.325*1e-6 # stress converstion from atm to GPa
variable f equal ${multiplication_factor}
variable sigmax equal (v_f)*(c_p[1]/${gvol})
variable sigmay equal (v_f)*(c_p[2]/${gvol})
fix 9 all deform 1 x erate ${erate1} units box remap x
Hi,
in theory if you use compute pressure, for a bulk material, you are measuring the Cauchy stress (true stress) because the pressure is computed in respect with the instantaneous section area of the simulation box. If your sample is graphene, periodic along one or two directions in the plane, I think that you have to fix a conventional thickness for your sample (graphite interlayers distance, for example) and normalize the stress with a factor of (height of the box simulation/graphene thickness).
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