Numerical Analysis On Damage And Repair Of Honeycomb Sandwich Composite Panels

The aim of structure lightweight is the eternal subject for the developing of aeronautics and astronautics.With great mechanics properties of high specific strength,specific stiffness,and capability to resist buckling,honeycomb sandwich composite panels have been widely used in external and interior bearing structure of many aircrafts.While during the process of manufacture,assembly and using,honeycomb sandwich composite panels are susceptible to impact damage,and have to be repaired.Various damage may occur in composite laminates,adhesive,and honeycomb core of sandwich panels after repair.Damage prediction and strength evaluation have become an urgent problem to be solved in the research of composites.In this paper,the research on progressive damage analysis and strength prediction are carried out by combining the experiment and simulation analysis method with the honeycomb sandwich composite panel and its typical adhesively scarf repair structure.Three-dimensional(3D)finite element model(FEM)based on composites CDM model and adhesive CZM model is established for progressive damage analysis(PDA)and simulation of laminates with adhesively scarf repair.User material subroutines(UMAT)based on Hashin criterion is used to implement fiber and matrix damage simulation.The numerical results are in good agreement with experiment.The FEM is proved useful for the sandwich panel research.Flat compress and shear experiment is conducted to obtain the equivalent material properties of Nomex? honeycomb core.The test results demonstrate that the damage and deformation process of the core is "elastic-collapse-wrinkle-dense/tear".The core retain part of carrying capacity after collapsing,and the residual strength is 40~45% of ultimate strength.Based on the experimental results,the honeycomb equivalent model in Sandwich panel theory is improved by introducing of core damage and residual strength.FEM based on Besant criterion is established for honeycomb core damage simulation.USDFLD and UMAT are coded for damage evolution rules of sudden/progressive stiffness degeneration respectively.Effectiveness and efficiency of FEM have been validated by comparison between simulation and experiment of honeycomb sandwich panel under three-point bending.Finally,3D FEM of progressive damage analysis is established for the typically adhesively scarf repair structure of honeycomb sandwich composite panels.Numerical analysis is conducted for damage simulation and strength prediction.Numerical results indicate that the strength of sandwich panel is s ignificantly recovered after repair.Honeycomb core and composites in unrepaired region damage due to local buckling.The result of FEM simulation has high consistency with experiment.Parameter analysis is carried out to provide repair designing advice.The analysis results present that,the increasing i n thickness and bonding width of additional repair patch weaken the structure strength owing to additional bending moment and premature buckling.The smaller the scarf angle and bonding width of repair patch,the higher the shear stress inside the adhesive layer.It is recommended that repair design should avoid the mechanical performance degradation induced by oversized local stiffness change.