Crushing Behaviors Of Modularized Honeycombs And Crashworthiness Of Honeycomb-filled Vehicle Crash Box

Gradient design is a well-known method that is able to improve the mechanical properties and the adaptativity of honeycombs in engineering fields.The one-dimensional graded design can change the deformation mode,reduce the initial impact peak and improve the energy absorption ability.However,the lack of gradient configurations limits the potentials in improving the mechanical properties of honeycombs.Taking hexagonal honeycomb as the research object,this paper investigates the influences of different gradient designs and multiple dimensional gradient on the mechanical properties of honeycombs numerically,theoretically and experimentally.Considering the graded honeycombs as the filling core,the crashworthiness of honeycomb-filled crash box under low crash velocity is investigated,and multi-objective optimization is then conducted to optimize the crashworthiness performances.The work is listed as follows:(1)The differences of strengths of regular hexagonal honeycombs between x and y directions are investigated.Theoretical and numerical results show that the y-directional strength is larger than that of the x-direction.This relation is varied with the change of base material and crushing velocity.Modified theotetical model are derived to predict the plateau stress of the regular hexagonal honeycomb in the y direction.Then,the equi-biaxial compression of regular hexagonal honeycomb is conducted.The deformation mode,stress and energy absorption ability of honeycombs under equi-biaxial compression are investigated.Effects of relative density and crushing velocity on the mechanical properties are discussed.Empirical formulas for predicting the turss stresses in the x and y directions are provided.Compared with uniaxial compression,the equi-biaxial compression enhances the stress and energy absorption abilities of honeycombs,but the densification strain decreases dramatically which influences the travel distance of the energy absorption process.(2)Effects of cell-wall angle on the mechanical properties are investigated.Based on the numerical analysis and quasi-static experiments,the relationship between cell-wall angle and quasi-static plateau stress is obtained.Modified theoretical model for predicting the quasi-static plateau stress is presented.Then,cell-wall angle-induced graded honeycomb is proposed.Comparative study between cell-wall angle-induced graded honeycomb and traditional cell-wall thickness-induced graded honeycomb is conducted.Experimental study is conducted to validate the deformation mode of cell-wall angle-induced graded honeycomb based on the 3D printing prototypes.Under quasi-static compression,the graded honeycomb shows weak-to-strong layer-by-layer deformation mode while it turns to top-to-bottom deformation mode under high crushing velocity.The stress of cell-wall angle-induced graded honeycomb underperforms that of the corresponding cell-wall thickness-induced graded honeycomb.However,the difference of stresses between these two designs reduces as the crushing velocity increases.Theoretical models for predicting the quasit-static and dynamic plateau stresses in each stage are derived.Finally,the energy absorption and impact resistant characteristics are also analyzed.Results show that the cell-wall angle-induced graded honeycomb has weaker ability in energy absorption.The traditional series design limits the energy absorption ability when crushing distance is relatively short.(3)To make sure the structure is always working under high strength,a parallel graded design method is proposed to enhance the mechanical properties of honeycombs.The main idea is to arrange the sub-honeycombs in the direction that is perpendicular to the loading direction,forming a parallel arrangement.The theoretical models for predicting the plateau stress are derived based on the weighted method.Quasi-static experiments are also conducted to validate the theoretical model,and good agreements are achieved.A 5-module graded honeycomb with parallel design is then analyzed.Effets of design parameters on enhancing the mechanical properties are discussed.The enhancement of honeycomb is related to the graded coefficient,the number of modules and the crushing velocity.Quasi-static results show that this 5-module graded honeycomb is able to enhance its quasi-static stress of the honeycomb by 70% compared with the corresponding uniform honeycomb.Dynamic analysis shows that parallel design is able to enhance the dynamic strength of honeycomb and reduce the sensitivity of honeycomb to the dynamic loadings.(4)This section extends the one dimensional gradient to two dimensional gradient,called modularized design,to generate a honeycomb that its modules are arranged along two mutually orthogonal directions.Theoretical model for predicting the quasi-static and dynamic plateau stress are derived.A modularized honeycomb with 5×5 same-sized modules are created.The enhancing effects of modularized design to plateau stress and energy absorption capacities in the x and y directions are investigated.Results show that modularized design is able to enhance the strength of honeycomb in its primary direction by 117% for the quasi-static strength.Under 100m/s crushing velocity,modularized design is able to enhance the strength by 40%.The reduction of enhancement under high crushing velocity results from the design principle and the inertial property of the modularized honeycomb.Then,the y-directional compression of the modularized honeycomb is conducted.Results show that modularized design can also enhance the strength in the y direction.Modularized design enhances the energy absorption ability of the honeycomb,providing an new path to enhance the energy absorber.(5)Taking the modularized honeycomb as the filling core,a honeycomb-filled thin-wall crash box is proposed for improving the crashworthiness.First,a comparative study is conducted to investigate the crashworthiness of thin-walled crash box,uniform honeycomb-filled crash box and modularized honeycomb-filled crash box under low-velocity front impact.Results show that crashworthiness of modularized honeycomb-filled crash box outperforms those of other crash boxes.Then,the surrogate model and multi-objective particle swarm optimization are used to conduct multi-objective optimization,aiming to minimize the maximum crushing displacement and the initial peak force.The Pareto noninferior solution is obtained.Representatives of optimal design are selected and compared with the original design.Results show that the optimal designs perform better than the original design.Compared with the original design,the optimal designs are able to satisify various design standards from various vehicles.This study provides the reference for designing modularized honeycomb-filled crash box under different design requirements.