  Catastrophic failure of materials and structures due to unstable crack growth
could be prevented if facture toughness could be enhanced at will through
structural design, but how can this be possible if fracture toughness is a
material constant related to energy dissipation in the vicinity of a
propagating crack tip. Here we draw inspiration from the deformation behavior
of biomolecules in load bearing biological materials, which have been evolved
with a large extensibility and a high breaking strength beyond their elastic
limit, and introduce an effective biomimetic strategy to enhance fracture
toughness of a structure through an intrinsic to extrinsic (ITE) transition. In
the ITE transition, toughness starts as an intrinsic parameter at the basic
material level, but by designing a protein-like effective stress-strain
behavior the toughness at the system level becomes an extrinsic parameter that
increases with the system size without bound. This phenomenon is demonstrated
through a combination of numerical simulations, analytic modeling and
experiments, and leads to a biomimetic strategy which can be broadly adopted to
enhance fracture toughness in engineering systems.
