Study On Mechanical Properties For Double-arrowed Structure And Automobile Energy Absorber With Negative Poisson's Ratio

With the development of new structures/materials,structures/materials with negative Poisson's ratio have emerged.The structure/material laterally contract when axially compressed,and laterally expand when axially stretched,thereby having special mechanical properties.It has good performance in terms of energy absorption,porous permeability,shear resistance and fracture resistance,and is widely used in automobiles,ships,aerospace,medical and other fields.In this dissertation,the double-arrowed structure with a negative Poisson's ratio(NPR)is taken as the research object,and its static and dynamic mechanical properties,deformation mechanism and energy absorption characteristics are systematically and deeply studied.Additionally,it is applied into a crash box of automobiles.The optimal crash box can be obtained with the multi-objective optimization algorithm and surrogate surface technique.The optimized crash box can absorb more energy and better protect the occupants during car accidents.The main contributions of this dissertation are as follows:(1)Based on the homogenization method of coordinate reduction,a theoretical model for predicting the equivalent elastic mechanical properties of two-dimensional(2D)/threedimensional(3D)double-arrowed NPR structure is established.The accuracy of the theoretical model is verified by numerical and experimental methods.Based on the theoretical model,the influence of angles and thickness of the long/short beams,and the half-width of the cell on the relative density,equivalent Young's modulus and Poisson's ratio of the NPR structure are studied.(2)Based on the cantilever beam model with elliptical integral solution under large deflection,a theoretical model for predicting the equivalent mechanical properties of the2D/3D double-arrowed NPR structure under large deflection is established.The theoretical model is verified by numerical and experimental methods,which shows that the model can fully consider the influence of the geometric nonlinearity of the NPR structure on its mechanical properties.The mechanism of the double-arrowed NPR structure under large deflection are explored with the theoretical model.(3)The deformation modes of the 2D double-arrowed NPR structure under impact with different velocities in X-axis and Z-axis are studied.It is observed that there are three deformation modes(bending mode,uniform mode and progressive mode)in the Z-axis direction,and there are only two deformation modes(uniform mode and progressive mode)in the X-axis direction.Based on the numerical analysis method,semi-empirical formulas for critical velocities distinguishing between different deformation modes are derived.The formulas show that the critical velocity is mainly determined by the relative density of the NPR structure.(4)Based on the deformation process of a typical unit,theoretical models for predicting the impact and support plateau stress of double-arrowed NPR structures in the X-axis and Z-axis directions are established.Different impact velocities will cause different deformation unit processes;therefore,theoretical models are divided into two kinds: theoretical models for high-velocity and low-velocity impact.Numerical results prove that the theoretical models can accurately predict the impact and support plateau stress of double-arrowed NPR structures.In the theoretical models,the dynamic stresses of the double-arrowed NPR structures are closely related to the structural parameters and impact velocities.(5)A parametric modeling method based on MATLAB and LS-DYNA for 3D doublearrowed NPR structure is proposed.The accuracy of the finite element(FE)models established by the parametric modeling method are verified by quasi-static compression test of metal specimens.The parametric analysis of the crashworthiness of the double-arrowed NPR structures from two scales was carried out.In macrostructure scale: The effect of slenderness ratio of the macrostructures on crashworthiness and the size effect of the cell were studied.In cell scale: the effect of the angles between the long/short beams,the thickness of long/short beams and the half width of the cell on crashworthiness were studied.The results show that larger slenderness ratio leads to the bending of the negative Poisson's ratio structure and reduces the crashworthiness of the structure;the angles and thickness of the long and short beams,and the half width of the cells all have a significant effect on the crashworthiness of the NPR structure.(6)A crash box based on a double-arrowed NPR structure is proposed,which can absorb sufficiently great energy in a car collision to improve the passive safety of the car.In order to obtain the optimal double-arrowed NRR crash box,a multi-objective chaotic ant lion algorithm based on feedback mechanism is proposed.The ant lion ability and population improvement rate are introduced into the ant lion algorithm as feedback information to improve convergence;improved Tent chaos algorithm is introduced to improve the global search ability and avoid falling into local optimum.Combining the RBF models for specific energy absorption and peak crushing force of double-arrowed NPR crash box,the optimal one is designed.It can absorb greatest energy and guarantee the peak crushing force not exceeding 80 k N.SEA(Specific energy absorption)increased from 2.97 k J/kg to 9.25 k J/kg by 211%.Integrating NPR crash box into the vehicle can effectively reduce the acceleration of the B-pillar,thereby reducing the occupant's damage;absorb more energy during the collision,thereby reducing the energy absorption of other structural components and ensuring the occupant's living space.