Research On Buckling Stability Of Large Size Honeycomb Sandwich Composites Under Compression And Shear Loads

Composite sandwich structures have been widely used in the aerospace field due to their light weight,high ratio strength,high ratio stiffness,excellent absorption and designability.As the primary structure of general aircraft fuselage,the composite honeycomb sandwich structure is prone to buckling and instability under in-plane loads,which will lead to failure before reaching the ultimate load.At present,the experimental studies are used ASTM standard based on small size structures which gnored the size effects.Most of previous studies have shown that the experimental and simulation models based on small size which is not fully consider the overall stability of the structure cannot fully reflect its macroscopic mechanical behavior.Therefore,it is of great significance to study the buckling stability of large-size composite sandwich structures which can reflect the actual mechanical properties of the structures.This paper will study the experiment,theoretical analysis and macro-meso FEM model to predict the buckling stability of the large size aircraft fuselage composite sandwich structure.This study has the great significance to guide the engineering design and apply in the fuselage structure of the aircraft.The mechanical properties of the composite honeycomb sandwich structures under axial compression and in-plane shear load are studied in detail using experimental methods.The specimens of the sandwich structure were prepared by autoclave process and secondary bonding technology.The special fixtures were designed and prepared accordinge to the in-plane compression and shear test standards.In order to ensure the reliability of the experimental results,the deflectometers,strain gauges and DIC system were used and compared with each other.The strain field of the sandwich structure were verified by the experimental results.The buckling loads and failure modes of the sandwich structure under axial compression and in-plane shear tests are studied.Based on the improved unit cell model,the relationship between the macroscopic mechanical stiffness of composite materials and the properties of meso-materials is established.The damage analysis model is established by defining the material damage criterion and stiffness degradation criterionusing the subroutine of the ABAQUS finite element analysis software.The Finite element model in this paper is verified by the tensile experiment of the open-hole composite plate.The stress-strain relationship,buckling loads and failure modes predicted by the finite element are in good agreement with the experimental results.According to the failure modes of the sandwich structure,the stress analysis methods of the sandwich structure are given and the theoretical buckling load is calculated based on the improved unit cell model.The damage and stability analysis model of the composite honeycomb sandwich structure was established based on the improved unit cell model theory.The displacement,strain,instability loads and failure modes of the composite honeycomb sandwich structure were studied.The FE results of the buckling load under axial compression are obtained and compared with the experimental and theoretical results.The buckling load and failure models of the specimen predicted by the finite element are well consistent with the experimental failure phenomenon.According to the finite element analysis,the failure load of the composite honeycomb sandwich structure is slightly larger than the buckling instability load,which reveals the failure mode of the composite honeycomb sandwich structure under the axial compression load.The in-plane shear buckling FEA model is established based on the P-element and H-element analysis methods.The FEA results have been verified by comparing the shear load-displacement curves and the load-strain curves with experimental results.The finite element analysis in predicting mechanical information such as strain and deformation is more accurately.Finally,the effect of lay-up of the composite panel on the buckling load of sandwich structure is studied.