Mechanical metamaterials: does toughness characterize fracture?

TL;DR

This paper investigates the fracture behavior of advanced mechanical metamaterials, revealing that traditional toughness measures are inadequate and proposing new testing and design protocols based on extended fracture mechanics.

Contribution

It introduces a novel approach extending fracture mechanics to metamaterials, providing a new testing protocol for their failure analysis.

Findings

Conventional stress intensity factors are insufficient for metamaterials.

Standard fracture tests do not accurately predict metamaterial failure.

A new general test and design protocol for metamaterials is proposed.

Abstract

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.