Mechanics of Digital Lattice Materials for Re‐configurable Space Structures

Deep space exploration will require innovative material and structural solutions. Considering the expense of mass during missions and the impossibility of resupply, materials for future space exploration must be lighter, stronger, and more reliable than current solutions. Architected cellular solids literature has shown that lattice cellular solids can be designed with record-breaking strengths and stiffness at ultralight densities, appealing characteristics for numerous aerospace applications. However, before these outstanding material properties can be realized in application, many technical challenges must be addressed, including methods for scalable manufacturing, damage tolerant design, understanding of failure modes, and identification of highest impact applications for initial technology infusion. To support the development of safe, ultralight, next-generation aerospace structures, this research investigated quasi-static and fracture material mechanics of discrete lattice materials, as well as potential applications and impact on NASA missions. Though strong and stiff at ultra-light densities, architecture lattice materials have mostly been limited to lab-scale demonstrations due to challenges surrounding scalable manufacturing from high-performance constituent materials. In response to this challenge, this work focused on the implementation of discrete lattice materials, a class of architecture cellular solid that uses discrete building blocks to construct truss systems that behave as a bulk continuum. By doing so, discrete lattice materials can be manufactured with a build envelope unconstrained by the size of typical additive manufacturing strategies. The building-block strategy leverages traditional manufacturing processes that can utilize a wide variety of materials and provide mass manufacturability. Coupled with high fidelity behavior prediction and discrete repairability, these characteristics are promising for practical wide-scale implementation in space structures. The first portion of this work characterized an ultra-light cuboctahedral lattice made from injection-molded, chopped-fiber reinforced building blocks, fastened together with nuts and bolts. These mass-manufacturable building blocks were produced very quickly (17 seconds per building block) and inexpensively (0.01-0.03 USD per cubic centimeter), yet they maintained relative stiffness and strength performance in the regime of state-of-the-art microlattices [1]. Mechanical testing demonstrated low hysteresis and very repeatable stress-strain response [1]. Additionally, the repairability of discrete lattice materials was demonstrated [1]. A lattice was loaded in tension until a single strut failure. The broken building-block was then replaced and the lattice retested. The repaired lattice had the same stiffness as the original sample, and only a small decrease in yield strength (95% of original strength) [1]. This ability to repair greatly influences material life-cycle and is a primary advantage of discrete lattice materials, especially for space applications. This work showed that high-performance discrete lattice materials can be made on a scale that enables practical consideration for use in full-size structures. After demonstrating feasible manufacturing, performance, and repairability, this work next sought to investigate the fracture behavior of the cuboctahedral lattice. Understanding fracture in architected cellular materials is vital to implementation of in safety-critical load-bearing applications. As will be shown in forthcoming publications, a new lattice fracture specimen was developed and used to test the fracture toughness of the cuboctahedral lattice. These experimental results were compared to computational studies of cracks in different specimen sizes and geometries. Results advance the state of the art in experimental validation of lattice fracture and lay the groundwork for damage tolerant, safe design of primary lattice material structures. The final goal of this work was to assess the effect that discrete lattice materials may have on potential applications, presuming scalable manufacturing, reconfigurability, and repairability. A case study was conducted to evaluate large space structures of various types, comparing discrete lattices designs against traditional performance baselines [2]. Two main benefits were examined: the versatility of one system to meet very different design requirements through hierarchical construction and improvement of structural efficiency based on mass. Prototypical aerobrake and large precision reflector sub-structures were designed, and it was shown that hierarchical construction could tune material behavior for either higher loads or higher precision/ resonant frequency requirements. Hierarchical construction meant that using the same material system, each strut in the aerobrake could have variable cross-sectional area based on anticipated loading, increasing efficiency. For the precision reflector, the averaging effects of many parts in hierarchical construction theoretically increase the precision of the assembled structure beyond the precision of the individual parts. Both discrete lattice designs were lower mass than comparable traditional structures baselined [2].