Research On Buckling Of 7B04 Aluminum Alloy Integral Panel Using Multi-point Dies

As a structural loading component,the integral panel is made of a single piece of billet.There are no mechanical connections between the skin and stiffener.Being high in strength,light in weight,long in life,the integral panel is widely used in the modern aircraft and the high-speed vehicles.7XXX series aluminum alloy has the advantages of small density,high strength and strong corrosion resistance,so it is the first choice for aircraft manufacturing.At present,for the integral panels with high stiffeners,the incremental press bending with low efficiency is usually used in industry.Multi-point forming,as a new flexible technology,can be with high efficiency and low cost.So combined the multi-point forming and the press bending,aluminum alloy integral panel can be manufactured perfectly.During the process of multi-point press bending,the stiffeners under tangent pressure may be unstable.The forming accuracy and structural strength of aluminum alloy integral panel are affected by buckling.Therefore,it is important to calculate the critical buckling load and to predict the critical buckling time accurately.And it is necessary to put forward the reasonable measures to suppress the buckling of stiffeners.The main contents and conclusions of this paper are as followings:1.Theoretical research on stiffeners during press bendingBased on the mechanic analytic method,the strain and stress of I-typed stiffener are discussed during the elastic and plastic deformation period.Moreover,the boundary is calculated.According to the mechanical theory of buckling,the expressions of critical pressure on elastic instability and elastic-plastic instability are deducted.Based on the energy characteristics of a buckling stiffener,the calculation methods of critical load on elastic instability and elastic-plastic instability are provided.2.Experiment of multi-point forming and research on buckling of I-typed stiffenerCarry out the experiment on an I-typed stiffener using multi-point dies and the buckling stiffener is obtained.Take simulations for this process,and the accuracy of the finite element model can be verified.According to the results using finite element method,the wrinkles andthe buckling time can be predicted accurately.Effects of bending radius,aspect ratio and punch arrangements on the buckling of stiffeners are studied.Results showed that the bigger bending radius were and the smaller aspect ratio were,the more stable stiffener became.When the stiffener was located in the gap of multi-point dies,the inhibition of buckling was the most obvious.3.Effect of process parameters on buckling of single curved integral panelTake a single curved integral panel for example,the influences of filler types,stiffener distributions and punch sizes on buckling of integral panel are researched.Results showed that the integral panel with polyurethane filler of 90 A Shore hardness was in good shape.The more intensive distributions of stiffeners were,the smaller wrinkles were.When the punch size was smaller,the integral panel became more stable.Considering the load capacity and production cost,the multi-point punch with size of 50×50mm2 was selected to manufacture a single curved integral panel with 300×300mm2.4.Research on buckling of double curved integral panel and control methodsTake the spherical hyperbolic integral panel for example,the buckling of integral panel with multi-point multi-step forming is analyzed.Results showed that the more forming steps were,the more stable stiffener was,but too many forming steps will lead to work hardening.The maximum wrinkling height of integral panel formed in the path of arithmetical curvature was less than that in arithmetical radius.Take the drum shaped hyperbolic integral panel for example,the buckling of integral panel with multi-point press forming was researched.Results showed that with the adjusted upper and lower punches,the wrinkling height was reduced obviously.Finally,to prevent the buckling,measures are proposed in the view of the structure of integral panel,the processing parameters of multi-point forming and the paths of deformation.