Study On Hierarchical Thin-Walled Structures Design And Energy Absorption Performance

The thin-walled structures are widely used in aerospace,transportation and industrial protection domains due to their light and efficient energy absorption characteristics.On the one hand,the advanced structure puts forward a new requirement for the specific energy absorption of thin-walled structures;on the other hand,the energy absorption behavior of thin-walled structures also has a very complex and sensitive connection with the structural forms,loading conditions and material properties.The traditional thin-walled structures have the disadvantages of low average crushing force and low specific energy absorption efficiency.Consequently,the hierarchical structure is introduced to design a new type of thin-walled energy-absorbing structure in this paper.Through the experiments,theoretical analysis and numerical simulations,the influence of the relationship between the micro hierarchy and macro structure of hierarchical thin-walled tubes on the structural crushing deformation as well as the energy absorption and dissipation mechanism under the crushing load is revealed.Main research contents and achievements are as follows:(1)The hierarchical regular hexagonal thin-walled structure is designed and manufactured by introducing the hierarchical structure into the hexagonal thin-walled tube and replacing the solid thin wall with the triangular sandwich wall.Its crushing resistance is experimentally investigated to reveal the folding mechanisms and energy absorption properties of three kinds of plastic folding existing in the hierarchical structures,namely the sub-cell folding,mixed folding and global folding modes.Based on the finite element analysis,the influence of structural parameters on the structural deformation mode and crushing resistance is discussed.Based on the Simplified Super Folding Element theory,the prediction formula for the mean crushing force of hierarchical regular hexagonal thin-walled structure under three folding modes is derived,and the mean crushing forces of testing tubes are predicted accurately.The results show that the sub-cell folding mode of hierarchical structure can effectively reduce the folding wavelength,add the plastic energy absorbing mechanism,and improve the mean crushing force and specific energy absorption of the thin-walled tube.In the experiments,the mean crushing force and specific energy absorption of the hierarchical regular hexagonal thin-walled tube are twice those of the ordinary regular hexagonal thin-walled tube.The hierarchical thin-walled tube is a lighter and more effective energy absorption structure.(2)In order to improve the specific energy absorption of thin-walled tube and reduce the initial peak force efficiently,this paper proposes a hierarchical hexagonal tapered tube combining the advantages of hierarchical structure and tapered structure.Based on the numerical simulation,the quasi-static crushing behavior of the hierarchical hexagonal tapered tube is revealed.The influence of the tapered angle,wall thickness and cell number on the energy absorption capacity of the hierarchical hexagonal tapered tubeis discussed.According to the folding mode of tapered tube,a theoretical model for predicting the mean crushing force of hierarchical hexagonal tapered tube is presented,and the predicted values are consistent with the simulation results.The results show that the hierarchical structure effectively improves the mean crushing force of the thin-walled tube,and the tapered structure effectively reduces the initial peak force of the tubular structure.The combination of these two structures effectively improves the energy absorption efficiency of the thin-walled structure.(3)Based on the self-similar fractal structure by vertex substitution,a hierarchical hexagonal tube with self-similar fractal structure is designed and fabricated.The crushing characteristics of the hierarchical hexagonal tube are studied experimentally and the plastic folding mechanism of the structure is revealed.Combined with the finite element analysis,the influence of the scale factor on the structural crushing resistance is revealed.By using the Simplified Super Folding Element theory,a theoretical model is established to predict the mean crushing force of the hierarchical hexagonal tube,and the predicted value sare consistent with the simulation results.The results show that the fractal structure effectively reduces the folding wavelength and adds the plastic energy absorbing mechanism.Based on the reasonable self-similar structure design,the mean crushing force and specific energy absorption of the fractal hexagonal tube can be effectively improved to be twice those of the ordinary hexagonal tube.(4)The deformation mode and energy absorption characteristics of the hierarchical regular hexagonal thin-walled tubes under oblique impact loads are numerically investigated.The results show that the crushing mode transforms from axial crushing dominant to bending dominant with the increase in the impact angle.In this process,six crushing modes are observed,including two axial crushing modes(progressive folding and integral folding),one bending dominant crushing mode and three hybrid folding modes.These complex crushing modes are mainly due to the structural sensitivity on the variation of the impact angle,cell number,wall thickness and tube length.The results show that the hierarchical topological structure can efficiently improve the energy absorption of the thin-walled tubes under oblique impact.In total,three new hierarchical thin-walled energy absorbing structures are designed.Three kinds of typical deformation modes in the crushing process of the hierarchical thin-walled structures are revealed.A theoretical model for predicting the mean crushing force and specific energy absorption of hierarchical thin-walled energy absorbing structures is established.The mean crushing force and specific energy absorption of the designed hierarchical structure are twice as high as those of the conventional thin-walled tube,which provides an ideal design scheme of lightweight and efficient energy absorption structure for the engineering protection structure.