A Study On The Design Methods Of A New Type Of Thin-walled Square Tubes For Energy Absorption With Surface Nanocrystallization

With the rapid development of economy and urbanization,the number of motor vehicles is increasing rapidly,which directly leads to the increasing of traffic accidents,and the safety of passengers and critical equipment in vehicles has become a hot topic.Because most of the traffic accidents are caused by collision,in order to improve the safety performance of vehicles and other vehicles,improving the energy absorption effect of energy absorption structures in collision accidents has become one of the key technologies in the field of automobile manufacturing.Metal thin-walled structure is the most widely used basic structure in energy absorbing structures,which can absorb a lot of energy under axial impact.The traditional designs of energy absorption structure are mainly realized by changing the cross-section shapes of the structures,prefabricating defects/dents,designing multi cell structures,filling foam inside the energy absorption structures.etc.The above method not only improves the energy absorption effect of the structure,but also directly changes the shape and overall quality of the structure,and indirectly increases the processing difficulty and manufacturing cost.Therefore,it is necessary to develop a new type of energy absorption structure without changing the external geometries.Surface nanocrystallization is a new technology which can improve the mechanical properties of metal materials.The hardness,yield limit,fatigue performance,wear resistance,corrosion resistance and other mechanical properties of metal materials can be significantly improved after surface nanocrystallization treatment.In this paper,numerical simulation and experimental methods are used to study the energy absorption of thin-walled square tubes.By utilizing the advantages of local surface nanocrystallization,the local mechanical properties are enhanced,the deformation modes of the metal thin-walled structure under axial load are induced and controlled,so as to achieve the goal of improving the energy absorption properties.The research contents and achievements are as follows:In this paper,a new type of surface nanocrystallized energy absorption structure based on thin-walled square tube is proposed.Based on the tensile test of the untreated and nanocrystallized 304 stainless steel,the increase of elastic limit is revealed.According to the characteristics that surface nanocrystallization changes the mechanical properties of materials,models with local surface nanocrystallization layouts are established to control the peak crushing force(PCF)and buckling deformation modes of thin-walled square tubes.Taking the square thin-walled tube as the research object,the influence of local surface nanocrystallization layout on its energy absorption properties is analyzed.It is found that the PCF and buckling deformation modes of the thin-walled square tube are directly related to the local surface nanocrystallization layouts.The PCF is controlled and the deformation mode is induced by designing the local surface nanocrystallization layout.According to this method,the influence of the nanocrystallized stripes' direction on the PCF and deformation mode is studied.Regarding to different square tubes,the local surface nanocrystallization layouts are designed.Through the optimization design of nanocrystallized stripe parameters,the optimized nanocrystallization layout is obtained.Results show that the specific energy absorption(SEA)of the square tube with the local nanocrystallization layout of circumferential staggered stripes is 64.29%higher than that of the untreated square tube.The SEA of the square tube can be further improved by 74.87%by locally optimizing the local nanocrystallization layouts.To further investigate the effect of loading directions and try to reduce the PCF,structures with different shapes(including square tubes with slope end surfaces and tapered square tubes)and corresponding nanocrystallization layouts are designed.Subsequently,the energy absorption properties of local nanocrystallized structures are verified by compression and drop hammer impact experiments.In addition,based on the axial nanocrystallized stripes design,an impact resistant energy absorbing thin-walled square tube is proposed.By studying the influence of the number and distribution position of the axial stripes on the PCF of the structure under axial compression,it is found that the structure with the axial nanocrystallized stripes concentrated on the edge of the square tube has a higher PCF.Compared with the untreated structure,the PCF of the design is increased by 74.45%.Through the research on the energy absorption of the structure with axial nanocrystallized stripes,it is found that the design with nanocrystallized stripes concentrated on the edge can improve the structural energy absorption capacity,with the highest SEA increased by 63.97%,while the design with nanocrystallized stripes concentrated in the plane has little effect on the SEA of the structure.Therefore,for the energy absorption structure which requires high impact resistance,the nanocrystallization layout design of axial stripes should be selected,which can maintain the integrity of the structure under low impact loading and can absorb large amount of energy through its own plastic deformation when the load is increased.Finally,a design method of two-staged combined energy absorption structure is proposed by combining the single thin-walled square tubes.When the external load is greater than the first PCF and lower than the second PCF,the structure deforms locally and only part of the structure absorbs energy,whereas when the external loading exceeds the second PCF,the entire structure will deform and absorb energy successively.Compared to the untreated structure with same material properties and geometric dimensions,the SEA of the local nanocrystallized structure is enhanced by 34.36%.Meanwhile the energy absorption allocation is designed between the tubes on two layers.Results show that through adjusting and optimizing the local nanocrystallization layouts,the two PCFs and their ratio is controlled in certain range,the ratio between two PCFs can be adjusted in the range of 0.20-0.55,and the energy absorption allocation ratio between the two stages varies in 0.31-0.45.Experiment validates the design method and proves that the two-staged combined energy absorption structure is practicable and effective,which provides a guidance in design of the energy absorption structures.