About the command of Velocity

Dear all,
I am running an MD simulation with lammps.I need to strectch a model via the command of velocity. As behind,截图1.png
I need to controll the temperature with the command of fix temp/berendsen .As follows,
But I find the model can not be fractured no matter how long time I used . I want to know why it happens ? Many thanks for your reply .

There is not enough info in your Q to respond.

However, note that the velocity commands sets

the v of its atoms one time only, e.g. initial velocities.

Those values will immediately change as you run
dynamics. A thermostat like temp/berendsen will

change the velocities continuously during a run.