Question about velocity ramp!

I am trying to model “crack propagation” with lammps.
I have seen in the crack example that a ramp velociy is applied over the box. I removed it and I have tried to just pull the upper side without the any ramps in the model; but what happened is the model peeled off without the ramp and the upper side got seperated.
I would like to know what the velocit ramp does in the motion equations? i.e. in each step it calculates the velocity of the particles and then add the “ramp velocity” to particles velocity? Why I cannot just pull the upper side without ramp?

Thanks for your kind support


Ramin Ghelichi

The reason is given in the literature. The velocity command sets the respective velocity of atoms in the region, to whatever values you set, instantaneously. Because of high strain rates in-evitable in LAMMPS, the region experiences a “SHOCK”., because of which your upper region is peeling off. To compensate for this shock only, the velocity is RAMPED in the mobile region from zero to the value you have set in the upper region, linearly.( Given in the Documentation). So, u have two options:

  1. Use this RAMPING for your simulation.

  2. Allow the shock to settle in the whole domain by thermostating it. (Keep in mind also the facts about Shear Wave speed in the solid while doing so.)

I hope this helps.!!!

Good Luck.