Mechanical Research Of An Auxetic Cellular Structure And Its Application In Commercial Vehicle Crashworthiness Optimization Design

As a new kind of functional material,auxetic structure is found to have great mechanical properties,such as negative Poisson's ratio(NPR),porous media and increased stiffness.The relationships between the periodic configuration of auxetic structure and its mechanical response exhibit applicable potentials in function-oriented design.Due to its unique properties,the technological applications for auxetic cellular structures are actively being pursued in many fields,like the Aeronautics and Astronautics industry,the automotive industry and the body-protection industry.A further study of auxetic structures in its deformation mechanism,mechanical properties and parametric design will promote the progress of auxetic structures in theoretical research and engineering application.With the rapid growth of car parc,commercial vehicles have become a significant part of the automotive industry in China.A more comprehensive system of test rules has been developed,and in which safety rules and standards are attracting more attention.Although experiencing a fast developing period,compared with passenger vehicles,there are still many deficiencies in the crash safety research of commercial vehicles,especially in terms of crashwothiness analysis and improvement.In this dissertation,an auxetic structure composed of periodic re-entrant hexagonal cells has been developed and discussed.The mechanical properties of the honeycomb and lattice auxetic structures have been investigated via various ways like theoretical deduction,numerical simulation and virtual test.Then the re-entrant auxetic structure has been innovatively applied in vehicle energy absorbing box.With the new designed crash-box and some other structural improvements,the crashworthiness capacity of a M1 type commercial vehicle has been improved immensely.The main contents and achievements of the research in this diss can be summarized as below:(1)On basis of the conventional homogenization method,a novel strain-based homogenization approach has been proposed for auxetic cellular structures.By adopting the coordinate reduction approach in Guyan Reduction,in the developed method,the expansion process of field quantities like displacement and strain is performed from a new mechanics view that the field quantity will be separated into a homogenized field item and a variation field item.With the separated field quantity,the basic state equation of discrete system can be solved,and the effective elastic constants of the homogenized cellular structure will be achieved.(2)A mechanical model for the in-plane elastic analysis of the honeycomb auxetic structure has been established on the periodic re-entrant hexagonal cell.By using the developed method,the in-plane effective elastic constants of the honeycomb auxetic structure have been achieved.With the further study,the relationships between the elastic constants and the geometric parameters of the cell have been discussed.(3)The finite element model of the honeycomb auxetic cellular structure has been established by using the Euler-Bernoulli beam elements,which has then been validated for its accuracy and efficiency by compar:ison with a solid element based model.The mechanical behavior of the honeycomb auxetic cellular structure under axial compressive loading has been further analyzed by performing the in-plane quasi-static compression simulation.A 4×4 type honeycomb auxetic cellular structure has been fabricated by using additive manufacturing technique,and then been used to perform real compression test.The comparison between the simulation results and the test data has verified the accuracy of the finite element simulation.In addition,the failure mechanisms of the honeycomb auxetic cellular structure have been analyzed,and the function expression of the critical stress for each failure mode has been determined.(4)The finite element model of the lattice auxetic cellular structure has been established by using the Euler-Bernoulli beam elements with a verified element size.By performing the axial impact simulation,the deformation modes,the dynamic response and the energy absorbing property of the lattice auxetic have been detailed discussed.What's more,the influences of relative density and impact velocity on the impact characteristics have been analyzed.The comparison of the impact characteristics between the structure with positive Poisson's ratio and the auxetic structure leads to a conclusion that the auxetic structure responds more stably and efficiently under axial impact loading.(5)A new vehicle energy absorbing box has been designed by combining the lattice auxetic structure and the conventional thin-walled crash-box.The axial crash simulations have been performed on both the new box and the conventional box at a low impact velocity and a medium velocity.The comparison of the crashworthiness capacity between the two boxes shows the better energy absorbing capacity of the new developed NPR crash box.The effect of material choice on the capacity of NPR crash box has been analyzed,and Nylon has been identified as the material of auxetic structure for its comprehensive performance.A parametric model has been built for rapid modeling of the NPR crash box.To perform the multi-cases crashworthiness optimization,the meta-models of the objectives and constraints have been constructed by combining the orthogonal experimental design and the least squares support vector regression(LS-SVR).An improved time-based particle swarm optimization algorithm for multi-objectives has been developed and applied to solve the proposed multi-objective optimization problem.The complete Pareto frontier has been obtained,and the optimal solution determined from the frontier proves to improve the crashworthiness capacity significantly.The optimization design of the NPR crash box indicates that the the proposed methodology is reliable and can be a useful guidance for vehicle crashworthiness design.(6)The crashworthiness analysis and improvement of a M1 type commercial vehicle has been implemented under the new statute for vehicle safety assessment GB11551-2014.The differences between the former statute GB 11551-2003 and the new statute GB11551-2014 have been investigated.A real 100%frontal impact test of the M1 type commercial vehicle has been performed under the GB11551-2014 statute,and the crash safety of the vehicle has been fully assessed.To effectively perform the vehicle crashworthiness design,a full scale finite element model of the vehicle has been established and validated via the test results.The 100%frontal crash simulation of the vehicle shows that the vehicle lacks sufficient energy absorbing parts in vehicle frame and frontal components.Thus,an improvement strategy on vehicle crashworthiness containing two bouts of improvements has been proposed and implemented.The first bout consists of adding the new developed NPR crash-box and layout optimization for frontal components.The second bout is mainly to perform the multi-objective size optimization for the main energy absorbing parts in front frame structures.Finally,the improved vehicle proves to exhibit a significant improvement in its vehicle deformation,energy absorption and occupant protection under the 100%frontal crash.