[lammps-users] Question regarding velocity ramp

Hi All,

I am trying to simulate crack propagation by pulling in y direction by applying following commands:

velocity btop set 0.0 0.06 0.0 units box

velocity bbot set 0.0 -0.06 0.0 units box
velocity mobile ramp vy -0.06 0.06 y 4.05 121.5 sum yes units box
fix 1 all nve
fix 2 boundary setforce 0.0 0.0 0.0

Above commands are for tensile loading. I have also applied a temp control on velocities only in x and z direction.

My questions are:

  1. What effect command should have on defect nucleation if I use

fix 2 boundary setforce NULL 0.0 NULL

instead of

fix 2 boundary setforce 0.0 0.0 0.0 (like above)

  1. Which is the better choice to use.

fix 2 boundary setforce 0.0 0.0 0.0

or

fix 2 boundary setforce NULL 0.0 NULL

  1. Does for applying Compressive loading one just needs to reverse the sign of velocity like -

velocity mobile ramp vy 0.06 -0.06 y 4.05 121.5 sum yes units box

Any response is deeply appreciated.

Thanks

Poornima
NCSU

For Qs 1,2 - there is no right answer. Those are
different boundary conditions and hence different models.
Which model is right depends on what you want to model.

For Q3, I think you will just flip the simulation from the top
moving to the right (+y) to the top moving to the left (-y).
So I don't think that's compressive vs exanding.

Steve