Fullerene nanotubes yield to tension in two basic ways. At high temperature (or in the long time limit) a single bond rotation creates a dislocation-dipole favored thermodynamically under large stress. However, at low temperature (or limited time range) this process remains prohibitively slow until further increase of tension causes direct bond-breaking and brittle crack nucleation. This instability proceeds through the formation of a distinct series of virtual defects that only exist at larger tension and correspond to a set of shallow energy minima. The quantum mechanical computations of the intermediate atomic structures and charge density distributions clearly indicate a certain number of broken bonds. (C) 2003 American Institute of Physics.