  Rapid progress in additive manufacturing methods has created a new class of
ultralight and strong architected metamaterials that resemble periodic truss
structures. The mechanical performance of these metamaterials with a very large
number of unit cells is ultimately limited by their tolerance to damage and
defects, but an understanding of this sensitivity has remained elusive. Using a
stretching-dominated micro-architecture and metamaterial specimens comprising
millions of unit-cells we show that not only is the stress intensity factor, as
used in conventional elastic fracture mechanics, insufficient to characterize
fracture but also that conventional fracture testing protocols are inadequate.
Via a combination of numerical calculations and asymptotic analyses, we extend
the ideas of fracture mechanics and develop a general test and design protocol
for the failure of metamaterials.
