  The propagation front of a crack generates large strain gradients and it is
therefore a strong source of gradient-induced polarization (flexoelectricity).
Herein, we demonstrate that, in piezoelectric materials, a consequence of
flexoelectricity is that crack propagation will be helped or hindered depending
on whether it is parallel or antiparallel to the piezoelectric polar axis. This
means that the theory of fracture physics can no longer assume mechanical
symmetry in polar materials. The discovery of fracture asymmetry also has
practical repercussions for the electromechanical fatigue of ferroelectrics and
piezoelectric transducers, as well enabling a new degree of freedom for
crack-based nanopatterning.
