Deflection/Vibration Mechanical Behavior Of Sandwich Structure With Aerogel Core On An Elastic Plate Base

Thermal protection structure,as the outermost structure of the aircraft,protects the internal structure of the hypersonic aerospace vehicle.During service process,there is a strong coupling between aerothermal environment and thermal protection material/structure.This article focuses on a new type of stitched thermal protection structure with mullite fiber reinforced composite material as the panel and mullite fiber reinforced Si O₂ aerogel as the core.To our knowledge,the mechanical properties of this new structure has not been understood comprehensively,and there is still a lot of design margin for improvement and optimization in designing this kind of structure.In this paper,experiments,simulations and theoretical research on the mechanical behavior of the sandwich structure with aerogel core on an elastic plate base under deflection/vibration environment were carried out,including the following aspects:Firstly,the deflection/vibration experiments of the sandwich structure on an elastic plate base were carried out.The macroscopic mechanical behavior,the damage process and failure mode of the sandwich structure were analyzed.The results show that the failure process consists of three stages under the working conditions studied.Initially,a slight crack first appeared at the dumbbell-shaped position in the core layer.Then the crack gradually spreads to the end to form a through crack.Finally,the stitching thread breaks under the combined action of tensile force and shear force.Besides,the location of the crack is random.And when the crack growth at one end of the structure is dominant,then the crack at the other end will stop cracking.The mechanical properties of the structure also show a strong discrete due to the immature processing technology.Secondly,the equivalent material constitutive of the core layer with stitching thread was derived based on the homogenization theory,and the numerical analysis model of the sandwich structure on an elastic plate base was established.The static bending simulation was carried out,and the validity of the finite element model was verified in static simulation compared with the experimental data.The validity of the model on vibration characteristics,including mode shape and mode frequency,was verified by comparing the constrained modal results of simulation and experiments.The boundary conditions of the model were optimized to carry out the deflection/vibration simulation.And the results show that the model could be used for kinetic calculation within the allowable error range.Based on the above research,material constitutive models and a finite element numerical simulation method were constructed and used in deflection/vibration conditions.Finally,a laminated beam theory considering the transverse shear deformation of the core layer was proposed for the statically indeterminate beam structure with variable cross-section.The accuracy of the theory was verified compared with the simulation results based on the high-precision method obtained in chapter III.Taking the integrity,heat insulation and light weight of the thermal protection structure as the optimization goal,the density and thickness optimization analysis of the core layer was carried out under the action of centrosymmetric loading and uniform force,respectively.And the length of the sandwich structure was also analyzed under the action of centrosymmetric loading.The optimization curves of structural parameters were given through the structural optimization analysis.Based on the above work,the theoretical basis and guiding direction for the design of thermal protection structure were provided.