Investigations Of Mechanical Behaviors Of Graded Cellular Metals Subjected To Impact Loading

Compared with continuous media,cellular materials enhance some advantages,such as low relative density,high specific strength,high specific surface area,light-weight,sound and thermal insulation,and good permeability etc.Multifunctional design could utilize the features in one aspect or more of the material to endow some special property.Cellular metal is a novel material incorporating physical peculiarities and structural characteristics.It can be designed as multifunctional density graded structures attributing to its superiorities and open architectures,serving in hazards and some other extreme conditions successfully.Therefore,graded cellular materials draw much attention.Recent findings uncovered that graded cellular material has better energy absorbing capacity than that of uniform material,but there was also some divergence in these conclusions.This indicates that this ability of graded cellular material is not always better than that of uniform one,thus,it is not reasonable to deem the graded material superior but depends on many factors,for instance,compressive displacement,density distribution and loading,which is just one of the target and focus of multi-functional graded design.Recently,it is not much systematical about the impact resistance of graded cellular metal and not much clear about the energy distribution and dissipation mechanism of multi-layer graded cellular materials under intensive loading.Meanwhile,stress wave propagation in graded material should be paid more attention.Hence,contents below are included in this work.Firstly,the work concerns the deformation,stress and energy absorption of multi-layer circle-arc and Voronoi random honeycombs subjected to constant velocity impact.For the grade materials,except for inertia effect,quasi-static yield stress of each layer becomes a vital factor affecting deformations,especially in the case of low and moderate velocity impact.However,inertia effect overwhelms the influence of yield stress and dominates the deformation when the loading velocity is sufficient high.Stress at impact side depends on the impact velocity and density gradient while that at stationary side is sensitive to yield stress or deformation mode.Secondly,for better understanding the influence of strain hardening on the properties of foams,3D Voronoi model was employed to study strain hardening effect systematically,and it was found a convergence phenomenon of this effect.Finally,previous analytical models to predict the plastic response of graded foam mainly stem from R-P-P-L model which neglects the strain hardening effect during the crushing process but just involves the plateau stress and lock strain.The hypothesis in this analytical model does not match with actual foams,thus in this work,this strain hardening effect is included in the stress wave propagation model of continuously graded foams based on the rate-insensitive rigid-plastic hardening model(R-PH model).Meanwhile,continuously graded Voronoi foam models are obtained from controlling the density distribution by governing local cell parameters.Comparisons of both theoretical results and FE outcomes verify that the analytical model could capture the response of continuously graded foams under constant velocity impact.This model also provides an effective method to probe the deformation mechanism and responding characteristics of graded foams.