  Toughness describes the ability of a material to resist fracture or crack
propagation. It is demonstrated here that fracture toughness of a material can
be asymmetric, i.e., the resistance of a medium to a crack propagating from
right to left can be significantly different from that to a crack propagating
from left to right. Such asymmetry is unknown in natural materials, but we show
that it can be built into artificial materials through the proper control of
microstructure. This paves the way for control of crack paths and direction,
where fracture -- when unavoidable -- can be guided through pre-designed paths
to minimize loss of critical components.
