People are using outer and inner cutoff values equal as 0.2 nm to simulated the CNTs fracture where detailed chemical reactions are not a major concern.
This way you can avoid overestimation of the bond breaking force.
See the attached links for more references.
http://www.sciencedirect.com/science/article/pii/S1359835X06002727
http://www.sciencedirect.com/science/article/pii/S0927025612004193
Thanks,
Sanjib