Learning from Nature's Failures

Abstract

To create a more sustainable future for aviation, new, lighter-weight structures and materials will need to be engineered. It will also be critical that damage tolerance and safety are not compromised in the process. Lattice materials represents one avenue of exploration; however, two key challenges arise: limited experimental work has been conducted to date regarding tensile mechanical response and lattice materials are generally considered to be less tough than traditional aerospace materials. Advancements in additive manufacturing in recent years creates the opportunity to rapidly produce high-quality complex geometries, allowing for both challenges to be more easily investigated. To address the issue of toughness and damage tolerance, nature is a source of inspiration, as all of nature’s toughest materials derive this characteristic from creating structural hierarchy using intrinsically weak
building blocks.

Two sets of lattice structures were fabricated using stereolithographic (SLA) 3D printing and tested under quasi-static tensile loading. Two sets of lattices were fabricated: lattices with uniform strut thickness, or relative density, and mixed-relative density lattices which create structural hierarchy. Using a novel method to track lattice deformation during loading, lattice stiffness-displacement response has been correlated with beam elongation and rotation behavior and the deformation of individual cells. The stiffness-displacement response of uniform lattices can be classified by relative density as either an elastomeric, elastoplastic, or hybrid response. In hierarchical lattices, cell deformations occurring in different relative density regions are directly correlated to features of the stiffness-displacement response.

Aspects of the mechanical response of hierarchical lattices, particularly fracture toughness and fracture pattern, are heavily influenced by the exact configuration of structural hierarchy, spurring a discussion of what characteristics are most important in the pursuit of increased lattice damage tolerance. While none of the lattices represent an optimal solution, each displayed characteristics which, if combined to
form a hybrid structure, could substantially improve lattice damage tolerance.