Research On Impact Resistance Of Asymmetric Honeycomb Sandwich Structure

In modern military applications,radar stealth has become an important demand for next generation aircrafts.The honeycomb sandwich structure is an effective strategy to meet the light-weight requirement of aircraft.In order to endow the honeycomb sandwich structure with both high mechanical strength and electromagnetic wave absorption(EMA)function,it is necessary to carry out the integrated design of mechanics and radar stealth properties.Thus,the top facesheet of honeycomb sandwich structure requires both electromagnetic wave transmission and mechanical bearing capability,the bottom facesheet needs to be reflective for electromagnetic wave and mechanically loadable.While featuring the honeycomb core with strong EMA ability through coating or filling with additional EMA materials.These additional electromagnetic wave absorption design results in asymmetry in both geometry and material system,which notably affects the mechanical properties of the honeycomb sandwich structure.This paper focuses on the impact resistance design of asymmetric honeycomb sandwich structure.Under dynamic loading,the failure strength of composite material varies with the strain rate.Firstly,the failure strength and strain rate of the composite were fitted by Cowper Symonds model.Then the finite element model of asymmetric honeycomb sandwich structure was established by LS-DYNA.The failure mechanism of asymmetric honeycomb sandwich structure under different impact energy is studied.The simulation results show that in the energy range of 1.5-3.5J,the main factors are the epoxy matrix tensile failure of top facesheet,honeycomb buckling as well as the debonding of them.The deformation of honeycomb core only occurs at the impact area.The introduction of functional filling material can inhibit panel failure.Delamination damage is reduced by 4.1 %.What's more,considering the low density of EMA functional materials,the light-weight and high-strength feature of the honeycomb sandwich structure is well maintained.The effects of honeycomb core height,geometric and material asymmetry on the impact resistance of asymmetric honeycomb sandwich structure were studied.It is found that the core height within the range of 5-15 mm has little influence on theresponse to contact force and the asymmetric honeycomb sandwich structure with a core of 10 mm shows small damage area and good impact resistance.The top facesheet plays the dominate role in influencing the response of contact force.Without changing the integral density,increasing top to bottom facesheet thickness ratio leads to smaller damage area of the matrix.When the same thickness is given to top and bottom facesheet,an optimized smallest total damage is obtained.Moreover,under the condition of constant top facesheet thickness,changing the thickness of bottom facesheet has little influence on the response of contact force.Impact response was mainly determined by the material system of top facesheet.With fixed thickness,different top facesheet material result in dramatically different impact resistance performance.The glass fiber/epoxy top facesheet shows better impact resistance.The low-speed drop hammer test and compression-after-impact test were carried out.The accuracy of the finite element simulation model was verified by the drop weight test.With the same nominal density,the compression strength of asymmetric honeycomb sandwich structure is obviously lower than that of symmetric honeycomb sandwich structure,which is about two-thirds of the strength of the symmetric ones.After a 3.5J impact,the residual strength of asymmetric honeycomb sandwich structure decreased by 17.6%,while that of symmetric honeycomb sandwich structure decreased by 39.5%,indicating that the impact resistance performance of asymmetric honeycomb sandwich structure is better than that of symmetric honeycomb sandwich structures.The response of asymmetric honeycomb sandwich structure under high-speed penetration was studied.The results show that the ballistic limit of asymmetric honeycomb sandwich structure is 10.5% lower than that of symmetric honeycomb sandwich structure.10% of the energy of the asymmetric honeycomb sandwich structure is absorbed by the honeycomb core,50% of the energy is absorbed by carbon fiber/epoxy,and 40% of the energy is absorbed by glass fiber/epoxy.Under the same density,the penetration resistance of asymmetric honeycomb sandwich structure is slightly lower than that of symmetric honeycomb sandwich structure.The bird strike problem of asymmetric honeycomb sandwich structure is studied.At a speed of 115m/s the asymmetric honeycomb sandwich structure will be broken through.The damage of the top facesheet is small,the ability to maintain the original shape is higher than that of the bottom facesheet.And the top facesheet has good impact resistance.The bottomfacesheet has more energy absorption,and the protection ability of the internal structure is greater than that of the top facesheet.