UC Berkeley

Architected lattice materials are some of the stiffest and strongest materials at ultra-light density (< 10 mg/cm3), making them ideal for aerospace structural applications. However, many factors inhibit their use in practical application. The first is scalable manufacturing. Though roll-to-roll process have been developed for 2D lattices, 3D architectured lattice materials rely on manufacturing methods that face scalability challenges when considering aircraft-sized applications. The second challenge is safety. Safety-critical applications demand a high understanding of fracture mechanics and fatigue mechanisms, but the fracture mechanics of lattice materials is a relatively nascent field that has demonstrated several challenges compared to continuum fracture mechanics.

This thesis addresses both of these concerns. The first section presents a mass-manufacturable discrete lattice material. The mesoscale, ultra-light (5.8 mg/cm3) fiber-reinforced polymer composite lattice structure is reversibly assembled from building blocks manufactured with a best-practice high-precision, high-repeatability, and high-throughput process: injection molding. The chopped glass fiber-reinforced polymer (polyetherimide) lattice material produced with this method display absolute stiffness (8.41 MPa) and strength (19 kPa) typically associated with metallic hollow strut microlattices at similar mass density. Additional benefits such as strain recovery, discrete damage repair with recovery of original stiffness and strength, and ease of modeling are demonstrated.

The second section characterizes the fracture toughness of the cuboctahedral lattice using experimental and computational methods. Novel experimental methods for use of the compact tension specimen are described, which enable the testing of higher unit cell resolutions than three-point bend fracture bend specimens. 3D printed acrylic compact tension specimens were manufactured and tested with relative densities of 5-15 % and compared to FEA models. The normalized toughness is shown to increase linearly with relative density, matching scaling performance of the higher connectivity octet truss. Effects of specimen geometry are computationally investigated, and limitations of the applicability of long crack fracture theory to cellular materials are discussed. Additionally, a potential toughening mechanism for lattices is addressed by characterizing crack deflection in a heterogeneous lattice. The He-Hedgepeth criterion for crack deflection in a bi-material is adapted to lattice materials, providing a single parameter to predict crack deflection in a reinforced lattice.