  The failure stresses and strains of nanotubes given by theoretical or
numerical predictions are much higher than observed in experiments. We show
that defects can explain part of this discrepancy: for an n-atom defect with
2<=n<=8, the range of failure stresses for a molecular mechanics calculation is
found to be 36GPa to 64GPa. This compares quite well with upper end of the
experimental failure stresses, 11GPa to 63GPa. The computed failure strains are
4% to 8%, whereas the experimental values are 2% to 13%. The underprediction of
failure strains can be explained by the slippage that occurred in the
experiments. The failure processes of nanotubes are clearly brittle in both the
experiments and our calculations.
