Study On Buckling Behavior Of Stiffened Composite Panels

The stiffened composite panel as a typical thin-walled structure is widely used in aircrafts,and its main failure mode is buckling.The study shows that the composite panel can bear greater load after initial buckling.Due to the complex failure mechanism and for lack of effective analytical methods,the post-buckling design of stiffened composite panel tends to be conservative.In order to improve the bearing efficiency and reduce the structural weight,an accurate analysis method considering the post-buckling is urgent for the load carrying capacity prediction of stiffened composite panels,and the study for post-buckling behavior of stiffened structures under various load cases based on tests is carried out,so as to fully exploit the post-buckling bearing potential of composite structures.In addition,the composite structures are prone to defects in the process of manufactures and in service which causes the decline for stability resistance and ultimate strength of structures.Therefore,the influences of all kinds of defects,especially the debonded defect on the post-buckling behavior are needed to be investigated in order to lay down criterions for the defect acceptance and repair tolerance of stiffened composite structures.The post-buckling behavior of intact stiffened composite structures and damage structures with the initial interface debonded damage under compression,shear and bending loads is studied based on the comparison between the analysis and test.In addition,the lightweight engineering design of composite panels is also investigated.The main work and research results are summarized as follows:(1)The post-buckling analysis techniques including damage onset and evolution of intra-lamina and inter-lamina de-lamination for the nonlinear buckling analysis of composite stiffened structures is studied systematically.The degradation of material properties based on failure criteria and fracture state variables of composite is adopted in progressive damage analysis of composite plates.Cohesive elements are used to simulate the inter-lamina damage behavior,and the mixed mode propagation criteria and the grid size of cohesive element are discussed in detail.An improved Newton-Raphson algorithm is introduced to improve the convergence of nonlinear solutions.(2)The progressive damage model with high accuracy and efficient post-buckling solution methods for stiffened composite panels is studied.The prediction reliability of three strength criteria for the domestic composite material CCF300/BA9916 is explored,and the concise Hashin criterion is recommended.An improved Hashin criterion considering the influence of lamina in situ effect and the transversal stresses on the shear strength of matrix are suggested.A simple determination scheme for the stiffness degradation based on the damage degree of the matrix and fibers is also presented.Then,the feasibility of the model is verified by experimental data.Further,a high accuracy FEM model including element type and mesh size is studied based on the tests of typical stiffened panels.In addition,the comparison of efficiency between implicit algorithm and explicit algorithm is carried out.A concise analysis method is suggested and validated by a typical I shape stiffened panel failure test.(3)The post-buckling load carrying capacity of intact panels and damage panels under compression,shear and bending load are studied in detail.The load-strain curves of hot spots and failure modes for a series of stiffened composite panels under various load cases are discussed,which shows that FEM results coincide well with test.By the way,the buckling and post-buckling behavior of typical panels with initial interface debonding between stiffener and skin under compression,shear and bending loads are investigated,and the influence of debonded damage area on buckling,interface separation,failure load is obtained and the delaminate propagation law during post-buckling process are also mentioned.(4)The lightweight engineering design of composite panels including stiffener sections,stiffener terminations and local reinforcement of cladding layer are discussed and validated by tests.The post-buckling behavior and failure mechanism of panels with three different stiffener sections such as I shape,J shape and T shape under compression are studied based on tests.The results show that the failure load of I shape panel is larger than others,and its load carrying capability reaches154536 N/kg.Further,the parametric influence of the dimension for I shape section on load capability is carried out,it is revealed that the best design configuration obtained is B/A=1.4,and its load carrying ability reaches 200362 N/kg.The designs of stringer terminations for hat shape stiffened panel are investigated by considering the load carrying capacity based on FEM and test,and results demonstrate that the onset and evolution of interface debonded damage between the stringers and skin is the direct cause of the failure of the panel.Moreover,the optimum tapered angle of the stringer terminations for the hat shape stiffened panel is 120°,which the bearing capacity could be improved by 9.2% in comparison with the un-tapered stringers.The influence of the cladding layer on the failure mode and carrying capability has been explored by numerical simulations and tests,and it is found that the post-buckling loading of the reinforced panel increases by 43.4%,which shows that the cladding layer could be highly resistant to the failure of skin/flange interface,and improve the buckling load and failure load.The post-buckling behavior and damage mechanism for intact stiffened composite panels with initial interface debonded damage are fully discussed under compression,shear and bending load in this dissertation.The methods for tapered designs of stiffener terminations and reinforcement designs of the cladding layer are presented,which can be used to improved the load carrying capacity of composite stiffened panels.The results given in the dissertation are helpful for the lightweight design and maintenance of stiffened composite structures and also are of great significance for structure designs in aerospace.