I am using the last version of LAMMPS for simulations of a sample with an edge crack. I fix the bottom part of the sample and pull the top, by using the "velocity" command. In the middle region I apply the velocity with a "ramp" style, as suggested in the example of LAMMPS for the crack problem. In my case the boundary are fixed (f f f) and the box size is larger than the sample size.
I am wondering how the stresses are calculated. In my script I have used 'compute stress/atom' to calculate the local stress on each atom and then the compute reduce sum' to get the value of the stress on the entire sample. I know that LAMMPS outputs the stresses as "pressure*volume". Which volume shall I use calculate the stresses (in the SI units, i.e. MPa) in both cases?
I was using the volume of the sample (for the 'compute reduce sum' command) and the atom volume=(sample volume)/(n°atoms) for the 'compute stress/atom' command, but I have got really high results.