Brittleness

Nanotechnology

Characterization (materials science)

Structural engineering

Metamaterial

Engineering

Truss

Composite material

Materials science

Fracture (geology)

Optoelectronics

Stress intensity factor

Fracture mechanics

Fracture toughness

Nature Materials

Rapid progress in additive manufacturing methods has created a new class of ultralight mechanical metamaterials with extreme functional properties. Their application is ultimately limited by their tolerance to damage and defects, but an understanding of this sensitivity has remained elusive. Using metamaterial specimens consisting of 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 and asymptotic analysis, we extend the ideas of elastic fracture mechanics to truss-based metamaterials and develop a general test and design protocol. This framework can form the basis for fracture characterization in other discrete elastic-brittle solids where the notion of fracture toughness is known to break down. Microscale architecting enables metamaterials to achieve mechanical properties not accessible to bulk materials. Here the authors show that established design protocols for the fracture of materials need to be revised to predict the failure of these materials.

The toughness of mechanical metamaterials