Failure Mechanism And Optimization Design Of Aircraft Composite Stiffened Structures

Stiffened composite structures can take full advantage of the laminate characteristic of good designing to reduce structural weight efficiently.So this form of lightweight composite structure is widely used in the aeronautic and astronautic field.The rational application of such structure involves many aspects,such as stiffened types and cross-section design,manufacture process,type-selected analysis,structural optimization,load-bearing capacity verification etc.While,in every aspect there are some key mechanical problems related.To solve these basic problems will do greatly improve the efficiency of such structures and substantially reduce the structural weight.In this paper the top-hat stiffened panels are used to study prepreg resin pressure in silicone rudder thermal expansion molding process,and some critical process mechanical problems have been discussed.Also,experimental and numerical analysis for different types of stiffened panels under axial compression is presented for a comprehensive understanding of the failure behavior and mechanism.Afterwards,top-hat and I-shaped composite stiffened panels are selected to carry out optimization analysis.In addition,the structural efficiency analysis for I-shaped composite stiffened panel under uniaxial force with multiple constraints is conducted and some useful conclusions are obtained.Finally,based on the study of unidirectional stiffened panels' mechanical performance,the theoretical study and overall buckling behavior of the mechanical properties analysis for grid stiffened structure are carried out,and corresponding practical software for engineering application is developed and verified.The detailed contents are listed as following.1)The carbon fiber composite laminates are prepared by using silicone rubber thermal expansion process.And the rules of the thermal expansion pressure variation and resin pressure variation in the forming process are tested and analyzed by using self-designed mold with resin pressure test system online.Then the effect of process clearance and temperature distribution is examined.And the laminate dense states under different process conditions are observed and analyzed using microscopic.The results show that resin pressure test system online can realize the test of resin pressure within prepreg in thermal expansion process.The process clearance and temperature distribution have a significant impact on resin pressure and silicone rudder thermal expansion pressure.Mover,the silicone rubber thermal expansion process,which is used to prepare the carbon fiber composite top-hat stiffened panels,can control the shape of the product and the precision of the size.Also,this process can reduce manufacturing defects in engineering practice.Different forms of soft silicone mold is simulated to analyze their change of the shape and size due to the temperature increase during the curing process.The analysis results are valuable for the soft silicone mold design to maintain the dimension best.It is also significant to improve production efficiency and reduce the times of experimentation.2)Using the Method of combining experiment and numerical calculation,The buckling behavior and failure mode of unidirectional I-stiffened,hat-stiffened and J-stiffened composite panel under compression were studied.The buckling deflection of the overall panel surface was detected by means of nondestructive non-contact testing with Optical measurement.The evolutions of buckling and failure model were captured.Based on optical experimental data,the numerical analysis and image reconstruction were done.A numerical simulation method based on finite element analysis was proposed,and the buckling and post buckling behavior of the hat-stiffened composite panel were predicted.Numerical analysis results of the buckling shape of the panels were compared with the experiment,as well as the difference of surface strain and displacement.Experimental study on damage behavior and bearing capacity of stiffened plate under in-plane shear loading was also carried out with the combination of optical and electric measurement as well as numerical simulation.And the failure mechanism of the structure under static loading was obtained.Meanwhile,the experience in the numerical simulation and the test for the buckling behavior of the stiffened plate subjected to compressive loads was accumulated.3)Using the finite element method(FEM),The influence of geometric parameters of I-stiffened and hat-stiffened composite panel on the failure mode and the load bearing capacity of them was studied systemically.Based on the proposed mechanical hypothesis and the analytical model,a new method is developed for analyzing the unidirectional stiffener composite panels in axial compression under the multi-constraint case.Analyses and optimization for the typical unidirectional stiffener composite panels are done using present method.And the structure efficiency which changes with the design variables is obtained as well as the relatively optimum structure parameters.And the results can be directly applied to the design of the composite stiffened panels.Present method synthetically accounts for the global buckling,local buckling,static strength constrains and interaction of them.Upon the hypothesis the discrete variables of the composite structure are changed to independent continuous variables,thus it can observably simplify the courses of the analyses and optimization.Finally,results from the test and FEA were compared with those from present method,and they exhibited in good agreements.The results of the study provide a direct reference for the design of the composite stiffened panel.The proposed method has good practical engineering application value and reference value.4)By secondary development of CATIA and PATRAN/NATRAN,a General CAE software for analysis and optimization was developed,with the function of CAD modeling,CAE modeling and analysis as well as verification and optimization?All the modeling operations and graphics display can be completed on the CATIA platform.And according to the CAD model created by the user,the finite element model of the structure can be created by using PATRAN autocalled through the background interface program.Then the finite element analysis software NASTRAN was used to analyze the displacement,stress and strain of the model.Finally,the Software will complete the verification work by calling and using the checking model automatically,and the results of the calculation will be shown graphically in the CATIA graphic display area.After the static strength analysis,a simplified model for optimizing will be created for user by the help of the developed software.And according to the established optimization strategy and optimization algorithm,the software will do the necessary optimization analysis.The software can complete structure modeling,structure analysis,verification and optimization in the design of the wing,fuselage and tail of the plane.