the problem of peeling the cnt from graphene

but the velocity of the 'move 0.05'is small than 'move 0.25' but the former
crack,could you tell me why?

I can only guess the reason.
Small objects are more difficult to deform than large objects. (Small
objects have fewer bonds to absorb the stress.) Also the large tube
has a larger contact area with the graphene sheet, making it harder to
pull away.

The numbers "0.05" and "0.25" are not velocities. These numbers are
distances (in Angstroms). The "displace_atoms" command moves these
atoms instantly. If you use a different command like "fix move" or
"fix addforce", it will move the atoms slowly. I think this might
work better.

I hope this helps.