  Strong and conductive carbon nanotube films are ideal candidates for
lightning-strike protection. Understanding their failure mechanisms by
considering the anisotropic and single-fiber nature is essential to improve
performance. Our experimental studies show that the single-layer,
nanometer-thick films fail under electrification by crack nucleation and
propagation, reminiscent of brittle and ductile fracture of materials under
mechanical loads. Sharp and diffuse patterns of fracture are identified in
aligned and non-woven films, respectively, signaling the strong effect of
material anisotropy that is absent in common engineering materials. The
fracture is driven by local Joule heating concentrated at the crack fronts
instead of force-induced breakage, which is validated by experimental
characterization and simulation results at both continuum and atomistic levels.
