| When the projectile penetrates the target,the built-in measurement and control device often bears tens of thousands or even hundreds of thousands of g value acceleration overload.As the "black box" on the projectile body,the crashworthiness of the measurement and control device in this harsh environment directly determines the success or failure of the test.In view of this weapon safety technology question,A large number of engineering practices and studies have shown that the sacrificial metal foam-filled structure can provide better safety protection effect with high-g value.However,the flow stress of high density foamed metal increases rapidly in the platform segment,which is not conducive to the stable and orderly feedback load of the buffer structure.High polymer foams often provide stable platform stress,and high polymer foams materials have strong viscoelasticity.They have better safety protection performance when resisting multiple shocks.In addition,the composite foam filled with brittle hollow particles has the advantages of low cost,environmental friendliness and high buffer energy dissipation,It is of great value in the field of military and civilian buffer protection.However,it is difficult to obtain higher filling ratio and lower foam density when hollow microspheres are used to fill solid polymers due to the influence of volume fraction of microspheres.Combining the above two aspects,In this paper,a new type of composite foam was prepared by using hollow microspheres as the main material,polymer foam as the binder and porous metal as the reinforcement phase.Through static and dynamic compression experiments,the mechanical properties of the new composite foam were studied and the dynamic response and buffering mechanism of its filled shell in the safety protection of the light-weight components with high-g were investigated.The development of new high performance composite foam preparation methods and the exploration of new ways of comprehensive utilization of fly ash cenosphere will provide ideas for the new composite foam in high-g security protection technology of military light components.Fly ash cenosphere polyurethane syntactic foam with different density and size of cenosphere was prepared by pressure infiltration technique(Fly ash cenospheres polyurethane syntactic foams,CPSFs).The reinforced composite foam was prepared by introducing aluminum honeycomb as the reinforcing phase(R-CPSFs).Through using two kinds size of cenospheres,the plain foams(LGs and LTs)and reinforcement foams(RLGs and RLTs)were prepared respectively.Through the observation of SEM,it can be seen that cenospheres and binders in the prepared composite foam are distributed uniformly and the bonding interface is close.The aluminum honeycomb reinforced phase does not affect the microstructure of the material,just divides CPSFs into a number of six-prism aluminum honeycomb holes.A large number of micropores are introduced by cenospheres and binder(Rigid polyurethane foam,PUR),which can provide larger compression space.The porosity and matrix density of the four CPSFs have been analyzed theoretically,and the proposed empirical formula can predict the porosity and matrix density of the foam accurately,which is consistent with the experimental results.Based on universal material testing machine and split Hopkinson pressure bar experiment,the quasi-static and dynamic mechanical properties of the four types of composite foam have been tested,moreover,the effect of material density and particle size on mechanical properties of CPSFs and the strengthening mechanism of composite foam also have been studied.Firstly,Four types of CPSFs with different densities(0.4~0.7g/cm3)must be subjected to quasi-static compression experiments,the results show that The strength,platform stress and relative density of the four CPSFs all satisfy the power function,and the mechanical properties of RLTs are most significant with the increase rate of relative density,and the mechanical properties of LTs are the lowest with the relative density enhancement.When the relative density of CPSFs is less than 0.29,the particle size has little effect on the mechanical properties;when the relative density is greater than 0.29,the mechanical properties of RLGs and RLTs can be significantly improved by using smaller cenospheres.However,the mechanical properties of LGs are slightly better than LTs in the test density range.Secondly,the mechanical behavior of CPSFs under static compression is analyzed based on speckle measurement and digital image correlation techniques.The results show that whether it is a plain composite foam or a reinforcement composite foam,the initial failure modes of CPSFs under quasi-static compression are controlled by the randomly generated and distributed deformation concentration bands,which is related to the non-uniformity and randomness of the mesostructure of the foam.The quasi-static compression failure mode of plain composite foam is dominated by the shear deformation band.In the reinforcement composite foam,when the CPSFs are completely restricted by the surrounding aluminum foil,the axial compression deformation is mainly deformation mode.Conversely,when the CPSFs is weakly restricted by the surrounding aluminum foil or uniaxially compressed,the material mainly produces shear damage.By comparing and analyzing the mechanical properties relationship between plain CPSFs and enforcement CPSFs,we can see that RLGs enhancement mechanism can be directly attributed to the addition of honeycomb aluminum,while for RLTs the additional enhancement of their mechanical properties are affected by both the failure mode transition and the density.Thirdly,in order to obtain the dynamic mechanical behavior of composite foam under high-g impact,the mechanical properties of four kinds of CPSFs under high strain rate were studied by SHPB experiment.The results show that the dynamic strength improvement of plain composite foam LGs and LTs is about 31%~65% and 39~46%,both above 30%.The dynamic strength of the enforcement foam RLGs and RLTs increases by about 25% to 35% and 28 to 35%,respectively,both below 35%.The effect of material strength improvement of plain composite foam under impact loading is much higher than that of reinforced composite foam.The platform stress of LGs does not have strain rate sensitivity,and the platform stress and energy absorption are basically the same under static and dynamic conditions.The dynamic absorption energy at densification strain of the three composite foams of RLGs,LTs and RLTs is about 19%~23%,25%~30% and 24%~30% respectively.Finally,combined with high-speed photography and DIC analyze the deformation mode of CPSFs with different material compositions at high strain rate,the results show that the failure modes of plain foam LGs and LTs are sensitive to strain rate.The initial failure mode of the material under quasi-static changes from shear failure to axial crack failure at high strain rate.Due to the lateral deformation limitation of aluminum honeycombs in RLGs and RLTs,the static and dynamic failure modes of internal CPSFs in which are mainly axial compression.The transformation of the CPSFs deformation mode leading to the property of the reinforcement composite foam is different from that of plain composite foam at high strain rate.In summary,aluminum honeycomb as a reinforcing phase can not only improve the mechanical properties of composite foam,but also improve the dynamic mechanical behavior of CPSFs,which is beneficial to the application of composite foam in impact protection.Based on the results of these experiments,using ANSYS/LS-DYNA finite element software,a high-g buffer numerical simulation model of CPSFs filled shell for lightweight components was established and the application of CPSFs filled shell in high-g safety protection of lightweight components was discussed.Loaded with a typical excitation acceleration of 60000 g amplitude and 180?s pulse width,the influence of the strain rate effect of CPSFs on the buffering effect has been investigated.The results show that when considering the material strain rate effect,the energy absorption by CPSFs and its proportion in total energy have been significantly improved in the same structural parameters and high-g impact and the dynamic mechanical properties of syntactic foam can improve the energy absorption ratio of each component in the composite structure.Then,for CPSFs and aluminum foam with comparable platform stress and energy consumption characteristics both filled tubes of high-g safety protection effect have been compared and analyzed,the results show that under the action of a limited high g-value impact load,foam aluminum has a step-up rise in buffer acceleration due to instability of flow stress,which is not conducive to the composite structure to provide a stable feedback load for components.Due to the stable dynamic platform stress,CPSFs can effectively reduce the unevenness of the buffer acceleration and provide better safety protection and energy absorption.In order to further expand the application technology of cenospheres polyurethane syntactic foam,two kinds of structured foams with grade density(GD)and layer density(LD)were prepared by regulating the distribution of cenospheres of different sizes.Compression experiments show that the mechanical properties and deformation behaviors of thses composite foam have typical segmentation and controllability,but thsese layered structures lead to typical anisotropy properties.The mechanical properties of structural composite foams depend not only on the macroscopic density of the material,but also on the internal structure.That is,the layer thickness of the LGs and LTs,the degree of layer structure,the interface combination and the number of layers can seriously affect the macroscopic mechanical behavior of the material.In comparison,the mechanical behavior of variable pore diameter aluminum foams under longitudinal and transverse compression is also studied,the experimental results show that the transverse compression of functional density graded aluminum foams(FGs)has higher compressive strength,but the reduction of both platform stress and the densification strain lead to lower energy absorption under transverse compression than longitudinal compression.The longitudinally compressed FGs failure mode is the progressive compression of the deformation concentrated zone,while the transversely compressed deformation zone is randomly distributed.Lateral compression causes the specimen to have a large lateral expansion effect,which leads to more tear and transverse tensile deformation of the sample cell.In the design of composite structures involving gradient foams,it is necessary to consider the problem of the protective performance reduction of foamed materials under the transverse impact of the load,which can provide basis for relevant engineering optimization design. |
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