Bearing Characteristics And Progressive Failure Behavior Of Composite Honeycomb Sandwich Structures

With the increasing demand for lightweight materials in aviation structural design,honeycomb sandwich structures are widely used in aerospace and other fields because of their high strength/modulus,excellent antifatigue and resistance to impact.Gnerally,the analysis of sandwich structures models the honeycomb configuration as an orthotropic structural material to address the issue of discrete non-uniformity.This treatment presents the significance of the equivalent elastic parameters of Nomex honeycomb.The core layer thickness and the geometry of honeycomb configuration considerably affect the damage path and failure mode of the Nomex honeycomb sandwich(N-HS)structure.And the N-SH structure presents the complicated failure mechanism,such as breakage or crushing and wrinkling as well as their coupling,which difficultly estimates the structural damage and failure law.Besides,these damages not only reduce the local stiffness and strength but also affect the structural bearing capacity and stability.Thus,it is necessary to conduct the systematic and indepth investigation into the load-bearing capacity and failure characteristics of N-HS structures with different sizes under specific loads.The equivalent theory was applied to model the honeycomb configuration so as to improve the efficiency of the finite element(FE)method used in the analysis of the damage propagation and failure mechanism of N-HS structures.Based on the Kelsey model and considering the influence of the wall thickness of the Nomex honeycomb,the accurate formula was given to calculate the equivalent out-of-plane shear modulus of the honeycomb.Applying the Euler beam theory,the formulas were derived to estimate the equivalent in-plane elastic parameters of the Nomex honeycomb structure with double-wall thickness.Also,the rigid folding model was adopted to analyze the out-of-plane compression process of the honeycomb core.Based on the analytical result,the analytical expression was proposed to relate the ultimate strength of the N-HS structure and the relative density of the honeycomb.Further,the validity of the model was verified by comparing the analytical results with the experimental results,which set up the foundation for the accurate FE analysis of the N-HS structures.The macroscopic equivalent model was established by introducing the mesoscopic damage model into the honeycomb structure.Through the experimental studies of the mechanical properties of the Nomex honeycomb configuration,it obtained the equivalent constitutive model of the Nomex honeycomb configuration in the out-of-plane compression direction and out-of-plane shear direction.Furthermore,it established the three-dimensional CDM progressive damage model of the N-HS structure.The model included the fiber damage,the matrix damage in the panel and the out-of-plane compression and shear damage of the honeycomb core.Then it assessed the efficiency and accuracy of the FE mesoscopic model and FE equivalent model of honeycomb structure in comparison with the out-of-plane compression test.These provided a basis for the subsequent experimental and simulative analysis of the N-HS structures under complex loading conditions.The three-point bending test was then carried out to investigate the load bearing and damage characteristics of the N-HS structure under the combined action of bending and shear.A three-dimensional CDM damage model was established and verified by the accuracy to the experimental result.The bending beam theory was used to analyze the shear forces of the core layer and the panel in the N-HS structure,leading to a simplified calculation formula.Together with the test result,the formula evaluated the influence of the thickness of the panel and the direction of the honeycomb core on the structural failure load.It revealed that increasing the thickness of the panel and changing the direction of the honeycomb could improve the ultimate load of the overall structure,and the local collapse and shear failure were noted as the main causes of the overall instability.Applying the theory of structural stressing state,the structural stressing state model of the N-HS structure under an in-plane axial compression load was constructed.The Mann–Kendall state mutation criterion was used to detect the buckling starting load of the structure,and an analysis method was proposed to determine the buckling starting load value of the N-HS structure.Axial compression load tests were undertaken for the large-scale N-HS structures to investigate their buckling bearing and post-buckling damage characteristics.The progressive damage model of the N-HS structure under an axial compression load was established,and the damage propagation process of the structure was simulated numerically.Furthermore,the buckling loads and post-buckling bearing capacity of the four types of N-HS structures were evaluated through experiments,revealing that the thickness of the ply and the height of the core layer of the panel led to the different failure modes of the structure,and the influence on the post-buckling bearing capacity of the N-HS structure was also clarified.Finally,in order to explore the buckling bearing characteristics of the largescale N-HS structure with in-plane shear,the buckling and post-buckling bearing capacity of the structure were analyzed by the experiment and the finite element simulation.An in-plane shear test was carried out on the N-HS structure,and the test data were subjected to force modeling analysis.The Mann–Kendall criterion was used to determine the in-plane shear buckling starting load of the N-HS structure.The NHS structure was constructed based on the finite element equivalent model of the shell element;then,the critical buckling load was calculated to verify the buckling load determined by the Mann–Kendall criterion.Furthermore,the experiment verified the in-plane shear bearing characteristics revealed by the stressing state analysis,i.e.,increasing the thickness of the panel is more effective than increasing the height of the core layer.