Mechanical Behavior Of Multicellular Element Sandwich Structures Under The Fragmentation And Water Hammer

Multi-cellular element sandwich structure is a new type of lightweight and high-strength material structure,which is widely used in aerospace field,high-speed rail transportation and automobile field because of its high strength,high stiffness,low density and excellent energy absorption performance.As an energy-absorbing structure with progressive hardening characteristics made of ductile metal,the study of the performance of the structure often starts from quasi-static experiments to strong dynamic load loading analysis.In this thesis,the uniaxial flat compression performance of multi-cellular element sandwich structure under quasi-static conditions is tested,and the in-plane uniaxial compression mechanical properties of negative Poisson’s ratio multi-cellular element sandwich structure with different number of cell layers are analyzed;then,based on its mechanical performance,the experimental study of multi-cellular element sandwich structure under strong dynamic load coupled with breaking sheet and water hammer action is carried out to clarify its impact dynamic mechanical behavior and damage characteristics;on this basis,the study of On this basis,the simulation study of the multi-cellular element sandwich structure under the compound strong dynamic load was carried out,and the advantages of its application in the new lightweight protective structure were analyzed.The specific research contents and results are as follows:(1)The in-plane foundation mechanical properties of the multicellular element sandwich structure under quasi-static conditions were analyzed.The in-plane uniaxial compression mechanical properties of the negative Poisson’s ratio multi-cell element sandwich structure are analyzed from the perspectives of peak stress,platform stress,densification strain,and energy absorption rate.The results show that the deformation mode of the multi-cell sandwich structure becomes more complicated as the number of cell layers in the core layer increases,but the energy absorption efficiency,densification strain,platform stress,and initial collapse stress of the sandwich structure are not affected by the number of cell layers and remain stable all the time.(2)Based on the results of in-plane uniaxial flat compression test of multi-cellular element sandwich structure with different number of layers,it is known that when the number of multi-cellular element layers is less than 3,its deformation model mainly shows a single internal shrinkage,so the 3-layer multi-cellular element sandwich structure is selected to construct the strong dynamic coupled loading specimen.Using the light air cannon loading system and water hammer loading device,combined with the three-dimensional digital scatter measurement technology(3D-DIC),the strong dynamic coupling mechanical behavior and damage mechanism of the multi-cell interlayer structure considering the effects of fragmentation head type(ball-tipped and flat-tipped bullets)and initial velocity velocity were studied.The results of dynamic response analysis show that when subjected to the coupled fragmentation and water hammer loads,the overall dynamic response of the multi-cell sandwich structure is axisymmetric parallel to the cell plane with the intrusion point as the center,and cracks along the hole depth will occur on the near-water side of the structure,while petal cracking will occur on the air side of the panel.Under the initial velocity conditions of the fragmentation studied in this thesis,the multi-cell element core layer did not exhibit significant cohesive deformation toward the intrusion point.Meanwhile,the flat-tipped projectile shows obvious ballistic deflection during the intrusion process,but the ball-tipped projectile has less velocity decay than the flat-tipped projectile.(3)Based on the results of the fragmentation of the multi-cellular element sandwich structure and the loading test under the action of water hammer,the typical failure of the strong dynamic coupling of the multi-cellular element sandwich structure is analyzed by using the numerical simulation method.Four failure modes of the multi-cell element sandwich structure under the coupling effect of fragmentation and water hammer were obtained,namely,plastic deformation of the near water side plate accompanied by partial compression in the core layer thickness direction,intrusion of the near water side plate accompanied by compression of the core layer,plastic deformation of the air side plate,and intrusion of the air side plate,and the damage map of the structural failure mode based on the structural strain energy was clarified.Meanwhile,the effect of water hammer effect on the damage of multicell element sandwich structure is comparatively analyzed.In the case of no liquid,the structural damage area decreases with the increase of the breaking speed,but the water hammer pressure inside the liquid-containing compartment increases with the increase of this breaking initial speed,which produces transient impact on the multi-cell element sandwich structure and leads to the increase of the structural damage area.In addition,when the fragmentation velocity is less than the ballistic limit of the liquid compartment,the damage area of the liquid-containing structure is smaller compared to that of the non-liquid-containing structure.