Fracture resistance of 3D nano-architected lattice materials

Abstract

Exploiting small scale material effects and structural topology, nano-architected lattices represent a recent novel class of mechanical metamaterials, which exhibit unprecedented combination of mechanical properties. Together with scarce resistance to fracture and catastrophic failure, understanding of the fracture characteristics and properties of 3D nano-architected lattices still represents a limiting factor for the design and realization of future engineering applications. Here, using a combination of in-situ tensile fracture experiments and finite element simulations, we first show the possibility to reach stable crack growth in nano-architected materials harnessing only the intrinsic plastic toughening mechanism. Exploring the effect of lattice topology on the fracture properties, we then demonstrate similar performance between the octet and 3D kagome architecture (along one direction). Based on the experimental and numerical results, a power-scaling law of normalized crack initiation toughness with relative density ρ̅ (i.e., fraction of material per unit volume) K_(IC)/σᵧ √L ∝ ρ̅^(1.11), ρ̅^(1.17−1.27) is exhibited by the octet and 3D kagome topology, respectively, given the yield strength σᵧ and the unit cell size L. Owing to the combination of the parent material’s size effect and plasticity (3D-printed photo-resist polymer), the fracture initiation toughness (considering σᵧ) of our octet nano-architected lattices is 8 times that of previously realized macroscopic octet titanium structures. After crack initiation, the two architectures manifest rising (in average 18%) fracture resistance curves (i.e., R-curves), without catastrophic failure. In addition, we find that the fracture toughness of architected lattices, measured by means of compact tension specimens, seems not to be dependent on the sample’s thickness, forcing to re-think the plain strain toughness definition for this class of materials. Our results uncover the basic fracture characteristics of 3D architected materials exhibiting stable crack growth, providing insights for the design of light-weight, tough materials, with implications for future macro-scaled structural applications.