| Multilayer foam core sandwich structures have attracted extensive attentions due to their superior mechanical properties,designability and multifunctional applications.Since sandwich structures are vulnerable to impact from foreign objects in service,recent studies have focused on the impact resistance performance of multilayer foam core sandwich structures.So far,there have been a large amount of analytical,experimental and numerical investigations on the mechanical behavior of monolayer foam core sandwich structures under low-velocity impact;however,relevant researches on the multilayer foam core sandwich structures were rarely reported.Aiming at the closed-cell aluminum foam and carbon fiber reinforced composite sandwich structures with Al foam core,the present dissertation systematically investigates the low-velocity impact response,failure modes and energy absorption mechanisms of monolayer and multilayer Al foam core composite sandwich structures through analytical,numerical and experimental methods.Besides,the damage tolerance and failure mechanisms after impact were characterized and analyzed.On this basis,the effect of local stiffness variation on the impact resistance performance and damage tolerance of the multilayer Al foam core sandwich structure was further illuminated.First,the micro-mechanical model of closed-cell aluminum foam was established based on the experimentally observed geometrical morphology and deformation modes of the critical cells within the deformatio n bands,and the equivalent elastic modulus and yield strength of closed-cell aluminum foam under uniaxial compression load was derived by using force equilibrium and EulerBernoulli beam theory,obtaining the equivalent mechanical properties parameters of closed-cell aluminum foam.The real three-dimensional(3D)meso-structure of closed-cell aluminum foam was reconstructed by using X-ray computed tomography,on this basis a 3D finite element model was established,and the mechanical response and deformation mechanisms of closed-cell aluminum foam under uniaxial compression load was investigated.It is found that the proposed micro-mechanical model overcomes the questionable assumption about the equiaxed critical cell by the classical analytical models.Compared with experimental result,it is found that the errors of the predicted equivalent modulus and strength reduce from 41.79% and61.10% by the classical analytical models to 4.23% and 10.98%,respectively.Furthermore,the simulation results verify the simplification on geometric shape of the critical cell in the micro-mechanical model.Second,the Al foam core was simplified as homogeneous and isotropic material by using the equivalent mechanical properties parameters of closed-cell aluminum foam,an analytical model for multilayer Al foam core composite sandwich structures was developed by using the principle of minimum potential energy,the principle of conservation of energy and spring-mass model.The analytical model took into account the penetration processes of the structure under low-velocity impact,and the face sheet/core coupling effect as well as the progressive damage process of the composite sheets during this process.The model was validated by the following experimental and numerical investigations.By introducing the face sheet/core coupling energy,the model successfully predicted the reinforcement of the face sheet by the core.The present model provides a theoretical foundation for the following studies on the low-velocity impact response,energy absorption mechanisms and local stiffness optimization of Al foam core composite sandwich structures.Then,based on the analytical model for multilayer Al foam core composite sandwich structure under low-velocity impact,experimental and numerical investigations on its mechanical behavior was carried out,focusing on the mechanical response and failure modes of the structure,and revealing its energy absorption mechanisms.It is found that the strength of the core dominates the main energy absorption mechanisms of double-layer Al foam core composite sandwich structure:the plastic strain energy of the core takes more significant contribution to the energy absorption of the structure when using high strength cores,and the deformation energy of the face sheets dominates the energy absorption of the structure when using low strength cores;while for the composite face sheets,membrane stretching is their main energy absorption mechanisms.Compared with a monolayer sandwich structure of the same mass,the crush force efficiency of multilayer Al foam core sandwich structure was increased by 15.5%.Therefore,multilayer Al foam core sandwich structure has a better impact resistance performance.A further study revealed that the energy absorption and maximum contact force are competitive relations,so requirements should be carefully considered when optimizing and designing the local stiffness of multilayer Al foam core sandwich structure.Finally,considering that impact may induce internal damage of sandwich structures,and decrease the load-carrying capability of the structure,the damage tolerance of Al foam core composite sandwich structures was evaluated and characterized by quantitatively studying the residual compressive strength after low-velocity impact.It is found that the residual compressive strength of the specimen with initial damage decreases obviously compared to the intact specimen.The reason is that the unimpacted and impacted Al foam core composite sandwich structures exhibit different failure modes.Under the same or similar mass(weight),the change of the structural configuration will change the failure mechanism of the entire sandwich structure,so the residual compressive strengths of both unimpacted and impacted double-layer Al foam core composite sandwich structures increases 11.88%and 13.98%,respectively.With the same(or similar)mass(weight),the damage tolerance of double-layer Al foam core composite sandwich structure could be improved by increasing the local stiffness close to the top face sheet. |
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