Investigations On The Dynamic Properties Of Metal Foam

Metal foams have been widely used in engineering fields such as aeronautics,aerospace,and vehicle due to their superior performances,such as specific stiffness,specific strength,impact resistance and good energy absorption capacity.Researchers have paid a lot of attention to the mechanical properties of metal foam under quasi-static and dynamic uniaxial or multiaxial loadings.Different from the continuum material,metal foams exhibt localized deformatian and stress enhancement under impact loadings due to the porosity.Meanwhile,metal foams compact under hydrostatic loadings.The yield behavior of metal foams differs fundamentally from that of fully dense metals.This thesis is aimed at investigating the mechanical behaviour of metal foam under uniaxial/multixal loadings.For uniaxial compression of metal foam,this thesis establishes the idealization material model which is based on the dynamic locking strain,the theoretical dynamic energy absorption model,and inplements the quasi-static/dynamic uniaxial compression tests.For multiaxial compression of metal foam,the modified yield criterion based on the plastic Poisson's ratio is proposed.Quasistatic/dynamic compression-shear combined tests are carried out,and the yield mechanism is studied based on an octahedron mode.The research of this thesis is shown as follows:(1)In this thesis,the quasi-static/dynamic uniaxial compression are carried out by universal machine and Direct Hopkinson Pressur Bar(DHPB),respectively.High speed camera is used to record the impact process.Stress enhancement and localized deformation,as two mainly dynamic properties of metal foam,are observed in the experiments.With increasing the impact velocity,the localized deformation becomes more distinct.It indicates that the locking strain is not a constant.(2)The mesoscopic modelling technology of metal foam is investigated.Firstly,the mesoscopic finite element model is constructed based on the Voronoi model.Secondly,based on the X-ray computed tomography(CT),the image-based mesoscopic finite element model is constructed.The Voronoi finite element model is mostly used to study the large deformation of metal foam due to its high efficiency.The CT image-based finite element model is used to study the yield behavior of metal foam due to its high accuracy.(3)This thesis firslty propose the dynamic locking strain,and eslablish the rigid,perfect plastic,dynamic locking strain(RPPDL)model.It shows that with increasing the impact velocity,the dynamic locking strain enlarges.The dynamic locking strain can reveal the compaction degree of the localized deformation area.Comparing with the RPPL model,RPPDL model can achieve more accurate prediction of the plateau stress of metal foam.(4)We investigated the effects of plastic shock wave on dynamic energy absorption properties of metal foam,and firstly proposed the energy absorption theoretical model.The plastic shock wave of metal foam occurs under impact,and the propagation and the reflection of the plastic shock wave result in the variation of kinetic and internal energy of metal foam.According to the 1D shock wave theory and law of conservation of energy,the theoretical model,which can be used to predict the energy absorption behavior precisely,is established.Meanwhile,we also investigated the effects of inertia and base material strain rate on the plastic shock wave.(5)The local plastic Poisson's ratio is determined,and the modified yield criterion is proposed based on the local Poisson's ratio.Several virtual experiments,i.e.triaxial compression tests and biaxial compression-shear combined tests are carried out to investigate the yield behavior of metal foam,and the numerical yield surface is obtained.The theoretical yield surface is in better agreement with the numerical yield surface when compared with another yield criterion.The effect of plastic Poisson's ratio on the yield criterion is studied.It shows that the theoretical yield surface expands when increasing the plastic Poisson's ratio.(6)The quasi-static/dynamic compression-shear combined tests are carred out,and the yield mechanisms are investigated by the octahedron model.The phenomenological yield criterions in shear-normal stress space are established.It is noted that with increasing the loading angle,the normal stress decreases,however,the shear stress increases under dynamic or quasi-static compression-shear loadings.The normal stress and shear stress both show obvious stress enhancement,and the yield surface shows isotropic hardening under dynamic loadings.Due to the shear loading,a collapse band occurs along the shear direction in the foam specimen.The initial deformation is concentrated on this collapse band.In this thesis,the quasi-static and dynamic properties of metal foam under uniaxial/multiaxial loadings are investigated by theoretical,numerical and experimental approaches.The research solutions and results can provide an important guidance for the designing and application of metal foam.