Study On Measurement Method Of Buckling And Post-Buckling Behavior Of Stiffened Panels Via Phase-Shift Coding Fringe Projection Profilometry

Composite stiffened panels are usually used in important parts of modern aircraft such as vertical tails,stabilizers and wings.During the maneuvering process of the aircraft,it is prone to buckling under the combined force of compression,shear and thermal load from the airflow.In addition,different buckling modes will appear at different stages of the load.What’s more,low-speed impact often produces tiny permanent indentations in the panel due to the impact of the external objects during the service process.At the same time,damage of the panel below the impact site reduces the strength and rigidity of the structure.Especially after long-term flight service,these typical components will inevitably appear fatigue and damage,and the composite stiffened panel with local damage will appear more complicated buckling and post-buckling phenomena.The large local deformation caused by these buckling will cause a variety of complex intra-layer and interlaminar failure forms including fiber fracture,matrix cracking,delamination,debonding,and slab separation,and eventually cause early failure.Therefore,it is of great significance to study the mechanical behavior of composite stiffened panels under low-velocity impact and the residual compressive strength after impact.The development and evolution of the buckling mode is the usual behavior of axial compression deformation of stiffened panels.By optimizing the design of the composite structure and establishing a corresponding numerical model,the buckling characteristics of the damaged composite structure can be accurately predicted.It is necessary to master abundant experimental data to strictly verify the correctness of the numerical prediction model.Advanced non-contact,full-field measurement experimental methods can effectively monitor the deformation process of composite stiffened panels.In this paper,a variety of phase-shift coded fringe projection profilometry methods are developed to avoid the effects of nonlinear errors and noise.The full-field deflection information is obtained during the compression buckling process of the composite stiffened panels,which provides an experimental basis for the effective analysis of the compression failure behavior of the stiffened panel with low-velocity impact damage.The main research contents and results are as follows.（1）A phase-shift coding fringe projection profilometry with anti-non-linear error is proposed.Based on the working principle of the high-pass filter,the measurement task under the influence of nonlinear errors is realized via increasing the number of phase shift steps to improve the robustness against harmonics.It is found via simulation results that the proposed method improves the correct rate of unwrapping from 18% to 82% when γ=2.5 and from18% to 100% when J=3 respectively compared to the original method under the condition of using 7-step phase-shift method.In addition,all of the unwrapping errors can be eliminated under certain conditions in practical experiments with the comparative error below 3%.（2）An anti-noise and non-linear compensation phase-shift coding fringe projection profilometry is proposed.Based on the working principle of the high-pass filter and the error analysis model,the measurement task under coupling effect of nonlinear error and noise is realized as well as the problem of insufficient robustness of the original phase-shift coding method in practical applications is solved by increasing the number of phase shift steps to improve the robustness against harmonics and adding another phase shift coded image with complementary noise-sensitive area.It is found via simulation results that the proposed method improves the correct rate of unwrapping up to over 95% when σ=2%cA₁^c & γ=2.5 and up to over 94% when σ=2%cA₁^c & J=2 respectively under the condition of using 7-step phase-shift method in which cases the original method could hardly work.In addition,all of the unwrapping errors can be eliminated after image post-processing in practical experiments.（3）A finite element calculation method for the post-buckling behavior of stiffened panels,which is highly consistent with the experimental results of the buckling mode evolution process captured by the improved phase-shift coding fringe projection profilometry is proposed.Aiming at the problem that the initial deformation mode of composite stiffened panels in the numerical calculation of post-buckling deformation is closely related to the initial defect position and morphology introduced,but is inconsistent with the actual test results,the deformation mode measured by the fringe projection profilometry is introduced into the update of the finite element model which obtains the calculation result being highly consistent with the experimental result.The experimental results show that the difference between the limit load predicted by the updated finite element model and the experimental value is merely 6%.In addition,the load history and buckling waveform are basically consistent with the experimental ones.The simulation data obtained by this high-precision finite element model can reflect the entire process of post-buckling deformation,which has scientific significance and engineering value for the research on damage failure mechanism that can hardly be revealed entirely via theoretical analysis or experimental test.（4）Based on the phase-shift coding fringe projection profilometry,the buckling mode and bearing capacity prediction of composite stiffened panels under compressive loads and the residual compressive strength after impact are carried out.By analyzing the compression experiment results of the J-stiffened composite panels,conclusions that have important reference value for improving the load-bearing efficiency of the composite stiffened panel structure are obtained,such as the distribution density of the ribs,the geometric size of the ribs,and the cross-section of the ribs.The bending moment of inertia and the load-bearing capacity of the skin are all important factors that affect the load-bearing potential of the structure.Through the experimental test and numerical simulation study on the compression behavior of composite stiffened panels after impact,the influence regulation of impact damage on the remaining compressive strength of stiffened panels is analyzed.For instance,the stiffened panel with impact damage at the center of the convex side,the mechanism of the significant decrease of the residual compressive strength is directly related to the buckling mode of the skin.