| It is anticipated that the future spacecrafts will face more complex and more changeable environments in the deep-space explorations,and the sophisticated instruments in the spacecrafts put forward even strict demands to the load bearing and impact protecting devices.Three-dimensional lattice materials show outstanding specific strength,specific stiffness and designability,etc.,and are believed to be new and most promising porous materials.They have extensively potential applications in light-weight structures,debris impact protection panels and impacting energy absorption apparatus of spacecrafts,as well as other weight saving structures in common vehicles.Based on the above background,the present dissertation is focused on the design,preparation,optimization and compressive mechanical behaviors of metallic lattice materials design with the objective of giving references to the development of advanced lattice materials.The main results are as follows.Three multi-rotation self-supporting lattice materials and their reinforced variants were designed,and the corresponding lattice structure samples were prepared using AlSi10Mg alloy by selective laser melting technology.It is shown by the uniaxial compression experiments that the six-fold rotation structures have the highest strength that is about 152%and 255%higher than that of three-fold and four-fold structures,respectively.Adding vertical struts in the six-fold rotation structures makes the strength further increase and the maximum value is as high as 174%.However,addition of facecentered struts in the lattice structures,the effect on the specific strength is insignificant.When the relative density of lattice structures reaches a critical value,a pronounced change takes place in the compression deformation mode and thus the energy absorption characteristics are affected.The energy absorption diagram shows that the trajectories of shoulder points with respect to the change of density are straight lines with different slopes,different from those of common foam materials.In view of shortages of self-supporting lattice structures in deformation mode and stress-strain response and so on,optimization studies were carried out on the preparation technologies and structure design methods.In terms of preparation technologies,two enhancing type lattice structures were prepared using AlSi10Mg and 7005 aluminum alloys by 3D print patterning combined with investment casting.The compression mechanical behaviors of these two lattice structures were tested and analyzed by finite element method,and also compared with that of 3D printed AlSi10Mg lattice structure.The results show that,by 3D printing combined with investment casting,there were no obvious fluctuations in the stress-strain curves of selfsupporting lattice materials during the plastic deformation,very different from that of direct 3D printed lattice structure.This change is obviously beneficial to the improvement of energy absorption efficiency.It is also found that the strut material and the strut inclination angle affect the deformation mode,while the end diameter of struts plays a determining role in the strength of lattice materials.Based on the above results,the present study gives several theoretical equations describing the relationships of strength and stiffness with the inclination angle,as well as the criterions based on the energy absorption diagram for strut material selection and optimization.In terms of structure design,self-supporting lattice materials are designed with a configuration gradient layer.It is shown through finite element analysis and uniaxial compression experiments that the absorbed energy per unit volume of gradient lattice materials is as high as 4.94 mJ·mm-3,which is 44.87%higher than that of homogeneous lattice materials.The deformation mode of gradient lattice structure is stepped layerby-layer failure while that of homogeneous lattice materials is diagonal failure.This difference would be the main reason that makes the energy absorption capacity of gradient self-supporting lattice structure increase.If preparing sandwich structures using the above two lattice structures as the cores,both the strength and stiffness increased due to the constraint of face panels to the unit cells adjacent to the panels.A kite-like lattice structure was designed that exhibited either stretchingdominated or bending-dominated mechanical characteristics,depending on the compressive direction.Longitudinal compression gives rise to several diagonal failure zones in the longitudinal section,presenting a typical stretching-dominated deformation mode,when compressed in the lateral direction,the lattice structure exhibits uniformly and overall deformation behavior,being a characteristic of bending-dominated deformation mode.Moreover,a composite structure consisted of kite-like and selfsupporting lattice structures was designed and prepared.When compressed longitudinally,not only the serrated waves in the stress-strain curves of composite were decreased but also the strength and stiffness were obviously increased.The smoothness of stress-strain curves was even remarkable if compressed in the lateral direction,and meanwhile,there appeared two new changing tendencies of stress with increasing the strain,i.e.stepped and slow rising.These results suggest that,if lattice materials composed of kite-like and self-supporting lattice structure are used as the load bearing unit,the load bearing should be in the longitudinal direction,but if used as the energy absorption unit,the direction should be lateral.Based on the above experimental results and material mechanics theories,the theoretical stress-strain curves of self-supporting lattice materials were established,and the idealized energy absorption diagram was plotted,from which the design method of the self-supporting lattice material used for buffering energy absorption was proposed.The feasibility and reliability of the design method were verified by the calculated examples and experiments. |
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