Toughening by Interpenetrating Lattices

Abstract

As structural lattice metamaterials become more accessible through 3D printing, there is a pressing need to go beyond studies of stiffness, strength, or energy absorption, and understand the practical limits of engineered structures. In particular, the fracture toughness of lattices poses a crucial challenge, as the resistance to catastrophic failure generally drops precipitously as the lattices become more open and the unit cell size shrinks. Recently discovered interpenetrating lattices, made by weaving two or more physically separate lattices through the same volume, offer a potential path to significantly improve fracture toughness by increasing the fracture process zone size and inducing unique toughening mechanisms via contact and friction. Interpenetrating lattices possess a steeply rising resistance-curve behavior, with the final toughness associated with unstable catastrophic fracture an order of magnitude greater than the initiation toughness needed to begin advancing a crack. Remarkably, we show that the interpenetrating lattice’s toughness can be five times greater than it’s corresponding fully dense solid base material, trending opposite to both the expected rule of mixtures and the Gibson-Ashby scaling law for cellular solids. Furthermore, by examining two different interpenetrating lattice topologies, we show that the toughening effect can be tailored by controlling the mechanical mismatch of the constituent sub-lattices.

Keywords: Metamaterial, Fracture Toughness, Interface Engineering, Lattice, Additive Manufacturing