Research On Process And Numerical Simulation Of Flexible Clamp Stretch Forming

Stretch forming is an important forming method of aircraft skin, which has simple dieand small springback value. The skin parts are the main parts for structuring aerodynamicconfiguration, it has many kinds, small number, complex structure and large dimension.With the wide use of large-scale three-dimensional sheet metal in the fields of aeronauticsand astronautics, high-speed train, ships, modern architectures as well as other industries, iturgent needs a new kind of stretching process to ameliorate stretching quality, improvematerial utilization and reduce production costs. Therefore, it is important to carry outstudy of new type of stretch forming equipment and technology.Flexible clamp stretch forming is a new kind of stretch forming technology, the basicprinciple is the rigid overall clamp is replaced by flexible discrete clamp, the technique hasgood flexibility, high material utilization, large forming range and good forming quality, inaddition, it also makes the sheet metal attach the die easily. In this paper, the finite elementmodel of flexible clamp stretch forming was set up, and the numerical simulation offlexible clamp stretch forming process were performed. The modified parameters offlexible clamp stretch forming machine were studied, the springback and wrinkle offlexible clamp front and back stretch forming were investigated. Then, the application ofmulti-point die used in flexible clamp front and back stretch forming were discussed, andthe related experiment were verified.The main contents and conclusions of this paper are as follows:1. Principle and characters of flexible clamp stretch formingThe structural features of the traditional horizontal stretch forming machine andvertical stretch forming machine were analyzed, and their applied scope were pointed out.The deform conformity method and principle of flexible clamp stretch forming wereelaborated, and the characteristics of flexible clamp stretch forming machine was described.Then, flexible clamp stretch forming experiment of irregular shaped part was carried out, the forming precision of experimental part was measured by using three-dimensionaloptical scanner, and the application of flexible clamp stretch forming technology wasintroduced.2. Establishment of finite element model of flexible clamp stretch formingMechanics analysis of stretch forming was done, and the theoretical calculation of thestress and strain are introduced. The finite element equation of numerical simulation forsheet plastic forming and related issues were elaborated. Then finite element model offlexible clamp stretch forming was set up. Flexible clamp stretch forming experiments andnumerical simulations for toroidal and saddle-shaped parts were done under the sameconditions, the results show that the experimental results are consistent with the simulatedresults, which verified the correctness of the finite element model.3. Numerical simulation of flexible clamp stretch forming processThe loading methods of force loading and displacement loading were adopted forflexible clamp stretch forming based on the moving characteristics of clamp, and thecomparative analysis for two kinds of loading methods was done by numerical simulation.The contrast between rigid clamp stretch forming and flexible clamp stretch forming forspherical and saddle-shaped parts were carried out, the results show that the flexible clampstretch forming mode can decrease the stress in the edge of clamp jaws greatly and avoidthe stress concentration phenomenon, enable the strain distribution of the forming zonemore uniform, reduce the thinning ratio and shorten the minimum transitional length. Theeffects of the magnitude of clamping force, the shape of clamping block, the number ofclamp, the loading curve, the lubrication conditions and the transitional length on theforming quality of parts were investigated through the flexible clamp stretch forming forspherical part, saddle-shaped part and M-shaped part. The results show that a smallerclamping force can make the formed part has a more uniform stress in the case of theformed part completely attach to the stretching die and not to clamp shedding, which alsoreduce the risk of crack. With the decreasing of clamping area, the stress and equivalentstrain of formed part decrease slightly when the shape of clamping block is different. Abetter fitting quality of complex surfaces such as the M-shaped part can be got when thenumber of clamp is larger. The higher the power of loading curve, the smaller the frictioncoefficient, or the longer the transitional length is, the more uniform stretching strain andthickness, or the better the forming quality will be.4. Structural modification and parameter simulation of flexible clamp stretchforming machineThe structural modification of flexible clamp stretch forming machine was conducted, and the structural diagram was given. Taking the spherical part as the objective part, theeffects of the location of vertical cylinder, the magnitude of vertical force due to thevertical cylinder and the fillet radii of clamping block on the forming quality wereinvestigated. The results show that the smaller the distance between the vertical cylinderand the edge of clamp jaws or the larger the vertical force due to the vertical cylinder or thelarger the fillet radius of the clamping block is, the smaller the maximum stress, strain andthickness thinning ratio will be, and more uniform stress, strain and thickness of formingzone will be. The numerical simulations of flexible clamp back stretch forming andflexible clamp front and back stretch forming were compared, and the results show that theminimum forming force, the stretching strain and thickness thinning ratio of spherical partformed by flexible clamp front and back stretch forming is less than that formed byflexible clamp back stretch forming.5. Numerical simulation of springback and wrinkle for flexible clamp front andback stretch formingThe mechanics analysis of springback was done, and the combination of implicit andexplicit method was determined to compute springback. The influences of material,lubrication conditions, pre-stretched ratio and thickness on the springback value ofspherical part were studied. The results show that the larger the elastic modulus and yieldstrength, the better the lubrication conditions, the larger the pre-stretched ratio or thethicker the thickness is, the smaller the springback value will be. In the above fourinfluence conditions, the springback value of flexible clamp front and back stretch formingis minimum, that of flexible clamp back stretch forming is middle, and that of rigid clampstretch forming is maximum. The judge method of wrinkling defect based on the formingcharacteristics of sheet metal in the unconstrained region was determined. Numericalsimulation of wrinkling process for saddle-shaped part formed by flexible clamp front andback stretch forming was carried out, and the reason of wrinkle in different areas ofsaddle-shaped part was analyzed. The effects of material, lubrication conditions,pre-stretched ratio and thickness on the wrinkle of saddle-shaped part were studied. Theresults show that the larger the elastic modulus and yield strength, the worse the lubricationconditions, the larger the pre-stretched ratio or the thicker the thickness is, the smaller thewrinkling value will be. In the above four influence conditions, the wrinkling value offlexible clamp front and back stretch forming is minimum, that of flexible clamp backstretch forming is middle, and that of rigid clamp stretch forming is maximum. Thenon-wrinkle critical graph of saddle-shaped parts with different thickness was obtainedunder the three kinds of stretching methods. 6. Application and numerical simulation of multi-point die in flexible clamp frontand back stretch formingThe flexible clamp front and back stretch forming machine was developed andexperiments were conducted by using multi-point die. The results show that the workpiecesfit the die completely in the case of transitional length is very short, the clamps changewith the shape of stretching die, and the sheet metal in the gap regions of adjacent clampsachieves free flowing. Indentation is the unique forming defect of multi-point forming, andtwo kind distributed forms of indentation are elaborated. The numerical simulations ofindentation for spherical and saddle-shaped parts formed by flexible clamp back and downstretch forming show that indentation can be described by thickness strain. The effects oftransitional length, thickness, curvature radius and material on the indentation of sphericaland saddle-shaped parts were studied. The results show that the longer the transitionallength, the thicker the thickness, the larger the curvature radius or the larger the yieldstrength is, the more gentle the indentation will be. Then, the elastic cushion was used forsuppressing indentation. The relationship between the multi-point die and target workpiece,the calculating scheme of punch coordinate, the multi-point CAD/CAM software andspringback compensation method were investigated. Numerical simulation of springbackcompensation and related forming experiment were done. The results show that theforming error reduced significantly through two-times compensation, and the formingexperiment verified the feasibility of the springback compensation method.