Novel Lattice Structures Based On Ashby’s Designing Criteria

M.F. Ashby, a worldwide famous scientist in materials science, has detecteddeveloping laws for engineering materials, and subsequently summarized severalcriteria for designing novel materials, including developing new alloy or polymerbased on chemical knowledge, controlling micro structures and designing hybridmaterials. Lattice materials, due to their highly efficient topologies and inherentopen space, are termed as the most potential materials with integrated structure andfunction. Those materials can be used in cutting-edge technology involved fields,such as spacecraft, carrier platform, aircraft, and also civil fields of daily life suchas wind energy and automobile. Developing lightweight materials, can promoteengineering technologies updating, and also help build a saving-style country.In chapter1, we first went back to the advances of metallic lattice materials,and then summarized the progress of composite lattice materials. Subsequently,hierarchical structural concepts were introduced with several existing examples;finally, fabrication technologies for micro-and nano-lattice materials wereinvestigated. By comparing with metallic lattice materials, composite and micro-scale lattice materials were still in their early stage, micro architectures of whichwere limited. Thus, the motivation of the present research was to develop novelmaterials or new architectures in a more original way based on Ashby’s designingideas, expecting that the mechanical properties of those structures could fill gaps inthe corresponding property chart and more selections appeared for engineeri ngapplication.By designing micro architecture, hollow composite pyramidal lattice (CPL)structures were developed and fabricated for the first time based on a thermalexpansion molding approach shown in chapter2. Out-of-plane compressiondemonstrated the increased buckling resistance of hollow CPL structures, and thecompressive strength was2times that of CPL structures with solid trusses; single-lap shear tests indicated that the shear strength of hollow CPL depended on both thestrength of lattice members and nodes, and experimental shear strength was greaterthan that of solid CPLs with the same relative density; under the in-planecompression, hollow CPL structures were proved to be as efficient as solid CPLs,but after minimum weight design, hollow CPLs could be17%and21%lighter thanpolymer foam and metallic foam sandwich structures, respectively.Based on the idea of designing hybrid materials, hybrid truss concepts forcomposite lattice structures were examined in chapter3. Lightweight wood andsilicon rubber were selected, and the corresponding wood-core and rubber-core hybrid truss CPL structures were fabricated. The specific strength of wood-corehybrid truss CPL structures was increased by19%compared with solid CPLstructures; the energy absorption capability per unit volume of rubber-core hybridtruss CPL structures surpassed that of hollow CPL structures, but capability perunit mass decreased. After filling rubber into the CPL structure, the resistance ofCPL structures for local impact increased, but the effective damping ratio decreased,due to the low strain energy contribution rate by rubber inside the lattice members,in comparison with hollow CPL structure. However, after appropriate design,hybrid truss concepts could expand the specific strength-density chart, and alsoprovided an additional route for multi-functionization of lattice materials.Hierarchical structures were developed in chapter4by assembling stretch-or bend-dominated construction in different length scales, which formed stretch-stretch-hybrid and stretch-bend-hybrid hierarchical composite pyramidal latticestructures. The fabrication approaches for each structure were also introduced andafter compression tests, the structural efficiencies were evaluated by com paringwith other competing topologies. The hierarchical CPL structures were found to beas efficient as optimized hollow CPL structure and superior to other constructionsof lower order. Subsequently, the effect of structural topology in different lengthscale was discussed. The macroscopic topology would determine the efficiency ofhierarchical structures, but the mesoscopic topology had little effect. Additionally,stacking manner was proved to affect efficiencies of composite lattice structuresand the in-plane stacked structures outperformed those out-of-plane stacked. Thein-plane stacked stretch-bend-hybrid hierarchical CPL structures developed in thepresent study, would be the most promising structure for application among latticestructures in the author’s opinion, due to their regular dimension, high efficiencyand simple fabrication approach.In chapter5, we have investigated the inertial stabilization effect of polymermicro-lattice materials provided by HRL laboratories in California, USA. Softmicro-lattice materials with different lattice geometries were fabricated using aself-propagating photopolymer waveguide process. The parent polymer wascharacterized by dynamic mechanical analysis (DMA) and the glass transitiontemperature was shifted with equivalent strain rate. Quasi-static and dynamiccompression tests were subsequently carried out to investigate the inertialstabilization of lattice member buckling as a function of strain rate and structuralgeometry (e.g. relative density and lattice aspect ratio). The micro-lattice structuresexhibited super compressibility and increased strength. The observed strengthincrease, particularly for high aspect ratio and high strain rate, was attributed toinertial stabilization. The results presented here could be used to improve structuraldesign in strain rate related application.