Mechanical Behaviors Of Metallic Foams Under Multi-axial Loads

Aluminum Foam,as a cellular structural material,is a good energy-absorption material with low density,relatively small stress strength,large crushing strain and strong ability in energy absorption.It is widely used as energy absorbers and protective components in various areas,such as aerospace,automotive and military defense industry.Metal foams have attracted a wide range of research interests as high energy-absorbing materials.However,the existing researches are mainly focused on the mechanical behavior of metal foam under uniaxial loading.In practical applications,metal foams are often subjected to complex loading,and thus studying the macro-mechanical behavior of metal foam under the multi-axis loading is of great significance.In this paper,we took the "developing experimental methods-studying mechanical behavior-establishing yield surfaces" as the main line,and developed two kinds of multi-axis loading devices,including the constrained passive loading device and the combined compression-shear loading device.Macro-mechanical behavior and deformation characteristics were studied.Firstly,effects of constraint on the mechanical behavior of aluminum foam were studied,including the influence on the yield strength,densification strain and energy absorption ability.Then,the yield surface of aluminum foam under combined compression-shear loading was determined.Firstly,the size effects(axial height and cross-sectional area)on the mechanical properties of the aluminum foam were studied.As a result,the size of the specimen in the paper was fixed as ?32 × 10 mm,which can eliminate the size effects of the material.Then,the effect of constraint on the mechanical behavior of aluminum foam was studied by adding a rigid sleeve on the tri-axial testing machine.The results show that,the plastic collapse strength of the closed-cell aluminum foam is independent of constraint,only related to the relative density.Aluminum foam has a more obvious strain hardening under radial confinement.The application of the restraint slightly reduces the densification strain,but the total energy absorbed is also greatly increased.Then,by adding the beveled bars and column sleeve,the static compression-shear experiments on the closed-cell aluminum foam were carried out on the tri-axial testing machine.The overall mechanical properties of the aluminum foam under compression-shear loading were studied.The yield surface of the material is determined through force analysis.At the same time,the finite element software LS-DYNA was used to simulate compression-shear loading experiment.The reliability of the simulation results were first verified by comparing to the experimental results.Results show that,yield initiates from the stress-concentrated corners of the specimens under combined compression-shear loading and the stress distribution at the specimen/beveled bars interfaces is no longer uniform.The LS-DYNA was also employed to further analyze the effect of density and cell size on the yield surface.Finally,the mechanical responses of aluminum foam under dynamic compression-shear loading were studied by improving the SHPB experimental technology.The failure process of the material under compression-shear loading was analyzed,and then the strain rate effects of the material were analyzed.The yield surface of the material is determined through finite element analysis.Results indicate that,the yield surface of the material increases with the loading speed,and the effect of the loading rate on the yield surface of the aluminum foam is confirmed.