Design,Fabrication And Mechanical Properties Of Carbon-Fiber-Reinforced Foldcore Sandwich Panel And Shell

Structural lightweight has always been one of the technical problems that need to be urgently solved in the fields of aerospace and high-end equipment.Carbon-fiberreinforced composites,which possesses the high specific strength and stiffness as well as the uniquely high fatigue strength characteristics,has become more acceptable for the rocket launcher,radome,solid rocket motor shells etc.in recent years.Compared with the conventional carbon-fiber-reinforced anisogrid structures,sandwich structures greatly improve the overall bending stiffness of the structure under the premise of the mass.Foldcore sandwich structure as a new type of sandwich structure has aroused considerable research interests due to open and continuous channels for humidity circulation and excellent mechanical properties.In this dissertation,the design,manufacturing and mechanical properties of carbon-fiber-reinforced foldcore sandwich panels and shells were carried out with the main load-bearing structure of a new heavy-duty rocket as the research background.The typical specimens of carbon-fiber-reinforced Miura-ori core sandwich panels(MSPs)for in-plane compressive and vibration tests were successfully designed and fabricated by using a hot-pressing folding and twice curing technique.Analytical models considering the five typical failure modes were established to predict the critical failure loads and corresponding failure mechanism maps were constructed.The governing equation of vibration motion of MSPs was built by Hamilton’s principle.Analytical expression for the equivalent transverse shear modulus of carbon-fiber-reinforced Miuraori core was derived based on an energy approach.The theoretical models for the critical failure loads and natural frequencies of MSPs were verified by in-plane compressive tests,modal tests and numerical simulation,respectively.Furthermore,the influences of structural parameters of MSPs on the failure modes,critical failure loads and natural frequencies were discussed.The result shows that carbon-fiber-reinforced MSPs are less prone to local buckling failure and possess higher natural frequencies than corrugated core sandwich panels.According to the vertex coordinate calculation method,the three-dimensional geometry of the curved foldcore was calculated and designed,and the functional relations among the geometrical parameters were obtained.The manufacturing technique of hotpressing folding and twice curing was extended to carbon-fiber-reinforced curved foldcore sandwich shells(CUFSSs).A set of sectional die for the fabrication of CUFSSs was designed through machining process,and the typical specimens for bending tests were successfully prepared.Analytical models for the central point deflection and critical failure loads corresponding to the different failure modes of CUFSSs under three-pointbending load were established.The influence of face sheet thickness on the failure modes of CUFSSs was investigated by three-point-bending test and numerical simulation.The accuracy of the theoretical and numerical models was verified.Futhermore,the influences of the relative density and geometrical configuration of the curved foldcore on the bending characteristics of CUFSSs were investigated by numerical simulation.The result shows that the rich geometrical parameters of the curved foldcore meet the requirements of the high degree of designability of CUFSSs.Compared with curved corrugated core sandwich shells,carbon-fiber-reinforced CUFSSs possess higher bending stiffness and strength under three-point-bending load and higher ability to resist local buckling failure of the face sheets.In view of the difficulty in preparing carbon-fiber-reinforced cylindrical foldcore sandwich shells(CFSSs),the hot-pressing folding and splicing assembly technique was put forward.Based on the mould for CUFSSs,a set of assembling and curing mould was designed and machined.The typical specimens of carbon-fiber-reinforced CFSSs for axial compressive and vibration tests were successfully designed and fabricated.Analytical models considering the four typical failure modes were established to predict the critical failure loads and corresponding failure mechanism maps were constructed.Based on the first-order shear deformation theory,the governing equations of vibration motion of CFSSs were developed.Analytical expressions for the equivalent transverse shear moduli of carbon-fiber-reinforced cylindrical fodcore were derived based on an energy approach.The theoretical prediction models for the critical failure loads and natural frequencies of CFSSs were verified by axial compressive tests,modal tests and numerical simulation,respectively.Furthermore,the influences of the layup sequences,the geometrical configuration of cylindrical foldcore and the overall dimensions of shells(including the length-diameter ratio and thick-diameter ratio)on the axial compressive and vibration performance of carbon-fiber-reinforced CFSSs were explored.The result shows that although carbon-fiber-reinforced CFSSs lose some axial load-bearing capacity compared with the axial cylindrical corrugated core sandwich shells,the topology configuration design of the cylindrical foldcore greatly improves the circumferential stiffness and strength of CFSSs and the ability to resist local buckling of the face sheets.The purpose of reducing the anisotropy of axial cylindrical corrugated core sandwich shells is realized and the comprehensive mechanical properties of the structure are improved.