Numerical Research On The Local Deformation, Forming Defect And Their Control Method In Multi-point Stretching Process

The skin parts are main parts for structuring aerodynamic configuration. It has many kinds, a small number, the complex structure and large dimension, and account for 70% of the aircraft parts. As the rapid development of aviation industry, the demand of skin parts are increasing and the quality of parts is becoming higher and higher. The manufacturing techniques of skin parts are still the traditional solid stretch forming. Each skin part needs a sophisticated stretching die or a set sophisticated stretching dies, and there are many kinds of skin parts. So the number of sophisticated stretching dies is huge. It has longer production period. The stretching dies have great weight, large size and large area of storage. So the traditional solid stretch forming has become inadequate to meet the requirements of modern manufacturing, and a kind of advanced technology needs to be proposed for changing the technology lag status of skin parts.The multi-point stretch forming (MPSF) is a new technology, which is coupled with multi-point forming and stretch forming. The technology can decrease the springback of formed part and realize flexible forming by constructing different surface of die used multi-point stretching die (MPSD). MPSF saves the cost and time on die design, manufacture and debugging, because each part does not need a special stretching die. The low-cost, high efficiency, flexible and digital forming can be achieved by using MPSF. For now, the MPSF has been successfully applied in producing the aircraft and the rapid trains of skin parts. In the paper, a series of numerical simulations on MPSF processes were carried out by using ABAQUS software, and the defect of local deformation, the discrete-die continuity, the discrete type and the effct of the punch elementa parameters, and the forming accuracy and the method of surface compensation in multi -point stretch forming.The main contents and conclusions are as follows:1. Theoretical analysis of stretch forming processOn basis the three materical models, the stretch foming prosess was analyzed.The calculating equations of springback and the residual stress were deduced in bengding deformity and combining bending and stetching deformity. The analysis showed that the springback of stretch forming was less than those of bengding.This is because the bending moment in stretch foming was samller than in the pure bending.The number calculation indicated that the spingback value of stretch forming accounted for 10% of bengding.By compararing of the result of numerical simulation in MPSF with the result of theory calculation in stretch forming, it demonstrated that these equations can be used to calculate and analyse the raults of MPSF when the transversal curvature of formed parts was small.2. Building the finite element modelThe material model of sheet metal and elastic cushion were analyzed. The discretization of model and the constraints were described in the process of building the finite element model. Then the finite element model of MPSF was built by used ABAQUS software. The dimension of elemnts, the number of layers in thickness and the coefficients of friction between the contact surfaces in MPSF process were determined by numerical simulation. Based on the idea of finite element discrimination, the method of ensured loading trajectory on the arbitrarily shaped aircraft outer skin parts is proposed and the programs of calculating loading trajectory were developed.The method of ensuring loading trajectory can guarantee the accuracy of trajectory. An aircraft outer skin part applied in practical production was simulated the multi-point stretch forming process in accordance with the loading trajectory ensured by principle of tangency. These results indicate that the method is feasible.3. Analyzing the local stress and local deformation in multi-point stretch forming processIt was found that the thickness changing tendency of the parts formed by MPSF was similar to by traditional stretch forming. But the fluctuations of thickness changing of MPSF were obvious, and theses demonstrated that the formed parts generated the local stress and local deformation. For surveying local stress and local strain, the blank sheets were dispersed into solid elements and a series of numerical simulations on MPSF processes for stretching cylindrical and spherical parts were carried out. The thickness stress and strain distributions of different layers in sheet metal along thickness direction were analyzed. The stress concentration is more serious in the inner layers than in the outer layers. The local deformation is lager in the outer layers than in the inner layers. The local deformation makes the metal sheet thin in the outer layers and that thicken in the inner layer. The reason is that the blank sheet is bended and pushed by punch elements in the areas where the blank sheet contact with punch element. Theses make sheet under tensile strain in outer layers and pressure strain in inner layers along tangential direction. The bending made the blank sheet thin in outer layer and thickens in inner layer. The pushing made the sheet under tensile strain in outer and inner layers. It can be found that local deformation mannish with thinning in the thickness of sheet metal and decreasing in the deformation degree, and the local deformation of different materials is similar, by analyzing the three effect factor on the local deformation.4. The method of die surface continuity and the defect suppressingBy analying the local deformation of cylindrical and spherical parts, it was found that the thicker the elastic cushion is, the more obvious the fluctuations of thickness stress will be. These indicated that the thicker the elastic cushion is, the better the effect on local deformation will be. By analyzing the accuracy of formed parts, it can show that the shape error was more when the elastic cushion was thicker. The relative thickness of elastic cushion-△εr curve and the relative thickness of elastic cushion-Err curve of spherical parts were obtained. Completely considered the effect of elastic cushion on suppressed local deformation and the accuracy shape, the selection principle about the thickness of elastic cushion was proposed in diiffernt size of punch elements. In forming cylindrical parts, the dimension of thickness elastic cushion and the size of punch element is approximative; in forming spherical parts, the thickness of elastic cushion and the size of punch elements is about 1.25 times.These results can be used to form the cylinder and sphere type parts.A new filled liquidcushion for suppressing the local deformation in MPSF was analyzed. The thinner filled liquid of bladder can suppress the local deformation and improve the shape accuracy of formed part. On the basis of MPSF, the technology of reconfigurable unilateral continuity stretch forming(RUCSF) was proposed. From the simulation results of the cylindrical parts and spherical parts, the validity of lightening the local deformation and improving the shape accuracy by using RUCSF technique is demonstrated.5. The discrete type and the effct of the punch elementa parameters on forming resultsThe size is a crucial factor of punch elements. By analying the effects of the size of punch element on the surface quality and the accuracy, the relation curves between the size of punch element and the formed accuracy were obtained. Acoording to these curves, the size of punch element can be selected in the applications. The effect on ditch shape defect in MPSF can be lightened and the surface quality and the shape accuracy can be improved by changing the arrangement of punch element. By analyzing the effect of hemispherical end radius on the local deformation and the shape accuracy and in consideration of the range of forming part using multi-point stretching die, theλvalue of punch element should be 30°.The hemispherical end radius can be determined on the basis of theλvalue of punch element. A new type swing punch element in MPSF was analyzed. It had a greater radius and can be used in forming the parts, which had more degree of bending. It was verified by simulation that swing punch element can improve the forming quality.6. The forming accuracy and the method of surface compensation in multi point stretching dieThe shape error in the MPSF process was discussed. The shape error was determined, which was leaded to by the uneven deformation of elastic cushion and the springback. The influence factors of the thickness of elastic cushion, the geometry shape, the strain, the thickness of sheet metal and the material on the springback and the forming error were studied. On the basis of analyzing the shape error of formed part in MPSF, the method of surface compensation in multi -point stretching die, which based on the shape error, was proposed. To the modeling technology of die surface in the surface compensation of MPSD, The modeling technology of triangular mesh was proposed.Basic on the idea of triangle elements and a new ICP algorithm, the formed surface and the target surface were registered, and the shape error of formed part was calculated by triangle elements with interpolation. Then the die surface can be compensated. The numerical simulation of the aircraft and the rapid trains of skin parts in MPSF process were carried out. The die surfaces were modified by analyzing the results of shape. It was found that the average shape error decrease above 80% after two amendments to MPSD.The forming test could be made according to the above results. These verified the forming results of MPSF and the validity of method of surface compensation in multi -point stretching die.