Design And Mechanical Performance Of Metallic Hollow Lattice Truss Sandwich Panels

The metallic hollow lattice truss structures with the rod element section width in the order of millimeters have become a type of outstanding periodic porous metal materials with excellent mechanical performance and attracted extensive attention from the scientific and engineering communities,due to their flexible designability,exceptional light-weight and high-strength characteristics,and broad potential in multi-functional application.When the porous material is used as a core and bonded with a pair of thin but rigid metal face-sheets,a metallic hollow lattice truss sandwich panel can be formed.This kind of lattice structure can meet more engineering requirements.However,compared with the more mature solid lattice truss sandwich panels,the research on hollow lattice sandwich panels is still in its infancy.There are three limiting factors: the uniqueness of the topology,the limitations of the preparation method,and the complexity of the fabrication process.Only by breaking the shackles of the existing manufacturing methods can the scientific research and engineering applications of such lattice structures be promoted.Particularly,scientific issues about parent material properties,advanced manufacturing technologies,quasi-static mechanical properties and mechanical enhancement design of hollow lattice truss sandwich panels need further research.To this end,304 stainless steel and millimeter-scale hollow pyramidal lattice truss are selected as the parent material and the core of the sandwich panel in this dissertation,respectively.In-depth research on the metallic hollow lattice truss sandwich panel from the above four aspects is then conducted.In order to predict the mechanical properties of stainless steel hollow lattice truss sandwich panels under different loading modes,uniaxial tests on three types of parent materials including as-brazed cold-rolled sheets,3D printed sheets and asbrazed small-diameter welded tubes are conducted and the mechanical properties are then obtained.Furthermore,a generalized three-stage stress-strain model is proposed to accurately describe the full-range nominal stress-strain relationship of the parent materials.At the same time,an interactive computer program is developed to simultaneously optimize the three-stage strain hardening exponents （n₁,n₂,n₃） to rapidly determine the model expression for specific materials.In addition,in order to further reduce the dependence of the three-stage model on multi-parameters,the uniaxial compressive behavior of the welded tubes is taken as an example.Expressions of five parameters including segment point stress σ_p2,n₃ ultimate stress σ_p3,n₃,ultimate plastic strain （p₃%） are given based on numerous experimental results for stub columns.The research shows that the above five expressions help reduce the number of dependent parameters to three without significantly reducing the prediction accuracy.In order to simplify the manufacturing process of the metallic hollow lattice truss sandwich panels and eliminate the dependence of the process on high-precision manufacturing equipments,a node-interlocking and brazing method is proposed in this dissertation to fabricate the metallic hollow lattice truss sandwich panels with strong joints,low defects and thin face sheets,in which the parent material of the core is selected from millimeter-scale stainless steel welded tubes.The out-of-plane compressive and shear properties of the cores of the sandwich panels with relative densities of 1.99%-5.47% are studied by analytical and experimental approaches.It is found that the analytical predictions of the out-of-plane compressive and shear stiffnesses are in good agreement with the experimental results.The generalized material model proposed in this dissertation can accurately simulate the uniaxial tensile and compressive true stress-strain behavior of the cross-sectional material in a core strut before strain localization occurs.The out-of-plane compressive and shear strengths of this type of lattice cores are predicted more accurately by use of the stress-strain model of the parent material under compression.Compared with other competing periodic porous stainless steel materials,the metallic hollow lattice truss structure exhibits better specific strengths and specific energy absorptions.Furthermore,the bending behavior of the metallic hollow lattice truss sandwich beams is explored experimentally and numerically.Particularly,the influence of four typical overhanging boundaries on the bending response of the simply supported sandwich beam is discussed,and a more reasonable cantilever-type boundary is determined.Here,the material damage data used in the finite element models of the sandwich beams are determined by the finite element method.A simplified theoretical model of flexural rigidity is then established for the lattice sandwich beams with the overhanging boundaries determined,and the prediction accuracy of the theoretical and finite element models is verified by experiments.Next,the comparisons of the ultimate failure load and bending specific energy absorption of the above sandwich beams show good agreement between the finite element predictions and experiments.This dissertation also focuses on the strengthening design of the stainless steel hollow lattice truss sandwich panel,and merges an idea inspired by the passive thermal management design of lattice sandwich panels into the reinforcing design of face-sheets.The SLM additive manufacturing technology is improved to integrally fabricate the hollow lattice sandwich panel structure with potential for thermal management applications.The out-of-plane compressive behavior of the multipurpose hollow lattice sandwich panel and the comparative structure is studied through experiments,finite element simulation and theoretical prediction.Subsequently,parametric analysis of the bending behavior of the multipurpose lattice sandwich beam is carried out numerically,and the effects of the thickness of the skin layer of the stiffened face-sheets on its bending response and ultimate failure mode are discussed.Combined with the finite element simulation results of the comparative sandwich structures,the variation trends of the bending properties of these two types of sandwich beams with the thickness of the skin layer is analyzed.