Theoretical And Experimental Study On Damage And Failure Behavior Of Composite WEB Plates With Cutout

With the rapid development of low-cost manufacturing technology and aircraft structure technology,extensive application of fiber reinforced polymer(FRP)composites is a prominent characteristic of airframe design for the new generation large commercial aircraft.However,the complexity of the composite and the diversity of its structure,as well as the sensitivity of service environment,make the load-bearing mechanisms of composite structure very complicated.For example,during the manufacture and service process,aircraft structures inevitably have various types of damage,such as defects in the production process,cutouts for functional requirements,and impact damage caused by external objects in repairing operation,etc.The existence of damage will reduce the structural stiffness and lower the loading capacity,which is a key factor to the structure security.Currently,the analysis and design of structure with cutout is a great challenge in aero structure design,and the failure mechanisms of composite structure containing damage are always the emphasis and difficulty in analysis.It is very important and urgent to investigate its bearing capacity and predict its performance,which has important academic value and significance in engineering practice.Based on the demand in practical engineering,the typical composite web plates with cutouts in aero structures were taken as the research object to carry out theoretical,numerical and experimental study of the typical composite web plates with cutouts.It was aimed at further exploring the relationship between the stability and strength,predicting the bearing mechanisms,and developing strength analysis method of composite web plates with cutouts.In order to provide the theoretical basis and design reference for the cutout design and engineering application of composite aircraft structures,the main work is organized as follows:The elastic buckling behavior of I-shaped web plates with cutout under pure shear load is studied.Firstly,the buckling calculation formula of simple supported orthotropic laminated plates is derived by Galerkin method,and the main mechanical parameters affecting structural buckling are given.Then the elastic shear buckling properties of a single rectangular plate and a rectangular plate with a central circular cutout under the shear load are analyzed by means of linear eigenvalue buckling method.The variation of the aspect ratio,the aperture ratio and the shear buckling coefficient are explored respectively.The mutual influence law between the aspect ratio and the aperture ratio is revealed,and a simplified formula for calculating the buckling of long orthotropic plates with cutout is proposed.Finally,the constraint degree of the flange to the web is quantized by introducing the dimensionless parameter ?,which is related to the ratio of the torsional stiffness of the flange and the bending stiffness of the web,and the range of the embedding coefficient of flange to web is 1.01~2.87.A simplified method for calculating the buckling coefficient considering the elastic restraint of beam flange to web is proposed.Experimental investigation is performed on the static bearing behavior of web plates with single cutout under shear load.According to the experimental results,the failure modes and causes of structural failure are analyzed.It is found that the material damage starts from the stress concentration area of the cutout edge,and various damage modes are also observed.Local buckling of the sub-layer is the main reason of structure failure for composites with delamination.Based on the experimental verification model,the influence of cutout shape on the stress concentration and the critical buckling load of the web plates are analyzed.It is confirmed that the shape of the opening has little effect on the buckling load,but it has a great correlation with the post-buckling bearing capacity.Finally,the strength enhancement method for typical cutout reinforcement of composite structures is evaluated,and the preferred reinforcement form and degree of reinforcement are given.Nonlinear simulation and experimental verification are carried out on the bearing behavior of beam web with single cutout under shear load.Based on the Tsai-Wu failure criterion,a simulation model of the bearing behavior of the web plates with an oblong cutout considering geometric nonlinearity is established.The numerical simulation results can accurately detect the first-play-failure load and predict the damage area.A material failure model based on stiffness reduction scheme is proposed to simulate the process of damage initiation,damage accumulation and propagation in the web plates with cutouts.It is found that the shear failure mode of the web with cutouts is a variety of damage modes,such as fiber delamination and interlaminar stripping,caused by sub-layer buckling along 45° direction.Using three-dimensional CDM model to simulate in-layer damage and the interface element to simulate the delamination damage,a general method for the progressive damage analysis for the composite web plates with coutouts is established,which realizes gradual failure process of web plates with cutout from damage initiation,sub-layer buckling expansion and instability failure.The bearing behavior of composite I-beams with double diamond cutouts under shear load are analyzed in detail by means of experiment and simulation.The results show that the stress and strain concentration on the double-cutout edge is more serious than that on the web with single cutout,but the strain distribution in the non-opening area is more uniform.The positions where failure may occur become more,making structural failure more complex.The two cutouts themselves bear and transfer loads,and the strain distribution and stress concentration in the double-cutout region are different and influence each other.Then Based upon the experimental verification model,the effects of boundary conditions,cutout and column reinforcement on the stability and bearing characteristics of the web are discussed to reveal the bearing characteristics and deepen the understanding of the failure modes for the web structure with double cutouts.A test loading device capable to meet the low-velocity impact requirements of the webs with cutout is designed.Experimental study on impact resistance and residual shear strength of P2352W-19 composite web plates were carried out.Firstly,through the impact energy groping test,it is determined that the impact energy required for visually invisible damage of the web is 55 J.The low-speed impact characteristics of the two types of profiled webs under three different impact energies were obtained by means of drop-weight test.The basic characteristics and variation laws of impact response contact force,energy,velocity and deformation were analyzed.The pit depth measurement and C-scan non-destructive testing of the beam web after impact showed that the delamination failure of the composite laminate under low energy impact load was mainly concentrated on the back of the laminate,which generally showed an upward trend with the increase of energy.And the pit depth also increases.Through the residual shear strength test of the beam web after impact,the damage degree and shear failure mechanism of the impact energy and the impact position on the open beam web after impact are explained.