| As the lightweight demand for high-speed trains continues to escalate,the design and fabrication of lightweight and high-efficiency energy-absorbing structures are technical bottlenecks to further enhance the passive safety performance of trains.At the design level,the traditional empirical design method cannot realize the unified layout of energy-absorbing materials and structures at multiple scales;at the fabrication level,it is difficult for the traditional manufacturing technology to meet the accurate and controllable fabrication of complex configurations of energy-absorbing structures.How to realize the macro and micro multi-scale structural design and accurate as well as controllable fabrication of energy-absorbing structures are of positive significance for the continuous improvement of train crash safety performance.Based on the design and fabrication of complex energy-absorbing structures,this dissertation proposes novel lattice structures,a novel multi-scale topology design method for energy-absorbing structures,and the optimal configuration of variable-density lattice-reinforced multi-layer nested thin-walled structure.Some kinds of aluminum alloy Al Si10Mg lattice structures were fabricated by laser additive manufacturing technology and their impact failure modes,energy dissipation mechanisms,and failure mechanisms were investigated.The main work of this dissertation is briefly described as follows:（1）The Johnson-Cook（J-C）constitutive model and failure model of the additively manufactured Al Si10Mg material were established.Quasi-static tensile experiments and dynamic compression experiments of the material were conducted via an electronic universal testing machine and Spilt Hopkinson Pressure Bar（SHPB）system to investigate the regulation of flow stress at different strain rates(0.0001 s-1-5104 s-1).The determination of strain rate sensitivity of Al Si10Mg at medium to high strain rates were completed and the J-C constitutive model as well as failure model were established.Taylor impact tests were conducted and the validity of the J-C model was verified based on the comparison of the projectile dimensions and damage modes obtained from experiments and simulations.（2）A novel bio-inspired lattice structure with dual bionic features derived from the front wing of beetles and the arrangement pattern of honeycomb cells was proposed and fabricated via additive manufacturing.The mechanical properties of the bio-inspired lattice structure were explored under different processing parameters.Quasi-static and dynamic compression experiments were carried out using an electronic universal testing machine and SHPB system to analyze the mechanical properties and deformation mechanisms.Finite element models were established to analyze the static and dynamic energy absorption responses of the novel bio-inspired lattice structure.The results show that the struts collapse in bending and torsion under quasi-static and dynamic loadings and the same collapse pattern extends to other layers until the whole structure is compacted.The additive manufacturing processing parameters have an important effect on the energy absorption and mechanical properties of the bio-inspired lattice structure.The peak specific energy absorption（SEA）of the structure is9.328 k J/kg and the peak crushing load is 1.865 k N at the processing parameters（laser power P=300 W,scanning speed v=1400 mm/s）.The SEA of the structure will be increased by90.8%by increasing the loading rate from 0.0005/s to 1000/s.（3）Based on the bio-inspired lattice structure configuration,a novel layered-hybrid lattice structure consisting of a bio-inspired lattice structure and a dodecagonal body-centered cubic lattice structure was constructed and the energy absorption characteristics of the structure were investigated using numerical simulation and experimental methods.Quasi-static and dynamic compression experiments were conducted on electronic universal testing machine,drop hammer testing machine,and SHPB system respectively to investigate the mechanical properties and deformation mechanisms.Finite element simulation analysis was used to verify the advantages of the layered-hybrid design strategy over the single cell topology in terms of impact resistance.The effect of component ratio on the mechanical properties and energy absorption of the established layered-hybrid structure was investigated.The results show that the SEA of the layered-hybrid lattice structure is 25.0%and 84.7%higher than that of the same mass of the bio-inspired lattice structure alone and the dodecagonal body-centered cubic lattice structure alone,respectively,breaking through the limitations of the conventional bending-dominated or stretching-dominated lattice structure alone.（4）The optimization design theory of multi-scale lattice structure was extended to three-dimensional space.A collaborative optimization design method of three-dimensional multi-scale lattice structure considering multiple control parameters was proposed,which realized the collaborative optimization of macroscopic material density distribution and microscopic cell topological configuration.A parameterized interpolation model suitable for lattice structure with multiple control parameters was proposed and an explicit relationship between the design variables and the macroscopic equivalent properties of lattice structures was established.Through numerical examples and experimental verification,the matching design regulation between macroscopic material density distribution and microscopic cell topological configuration was discussed,which proved the effectiveness of the proposed collaborative optimization design method aiming at multi-scale lattice structure.（5）A multi-layer nested thin-walled energy-absorbing structure based on variable-density lattice enhancement was proposed,realizing the lattice core design with variable-density lattice microstructures.Using finite element analysis and experimental methods,the crashworthiness advantages and deformation mechanism of the multi-layer nested thin-walled energy-absorbing structure based on variable-density lattice enhancement were investigated.The results show that,under axial loading,the SEA and crushing force efficiency of three-stage hybrid type II lattice reinforcement multi-cell tubes were increased by 38.2%and 6.6%,respectively.At the same time,under lateral bending,the specific energy absorption and crushing force efficiency of the three-stage hybrid type II lattice reinforcement multi-cell tube were increased by 20.0%and 5.2%,respectively.The mutual coupling between the thin-walled tube and the lattice core was the main reason why the SEA of the hybrid variable-density lattice-enhanced energy absorption structure was higher than the sum of its individual components.In this dissertation,the static and dynamic mechanical behaviors and energy absorption characteristics of novel lattice structures,the novel method of multi-scale topology design of energy-absorbing structures,and the energy-absorbing characteristics and deformation mechanisms of multi-layer nested thin-walled structures enhanced by variable-density lattice core were systematically investigated,which can provide a theoretical basis and reference for the design and fabrication of a new generation of train energy-absorbing components. |
