Research On Process And Numerical Simulation Of Multi-roll Flexible Stretch Forming

Stretch forming process are extensively used for forming aircraft outer skin parts. The stretched parts will have better shape control and smaller springback after forming because of the metal strain-hardening. The skin parts are main parts for structuring aerodynamic configuration, so the forming quality of skin parts are directly influence the aerodynamic performance of aircrafts. As the rapid development of aviation industry, the demand of skin parts are increasing, the quality is becoming higher and higher and the development cycles are getting shorter and shorter. In addition, as the new generation of aircraft commonly use blended wing body design, the aircraft manufacturers require a lot of complex shaped skin parts, such as S-shaped, saddle-shaped parts. These all brings new challenges to the stretch forming technology and it is important to enhance the flexibility of stretch forming equipments to adapt the development of aviation industry.Multi-roll stretch forming (MRSF) is a new flexible stretch forming technology based on a new principle. The basic principle of multi-roll stetch forming is clamping both ends of sheet metal with flexible floating clamp device firstly, and then using a number of discrete rollers press the free area of sheet metal to form the part. MRSF can improve the forming range of stretch forming and make the sheet metal attaching to the die easily. In addition, it also have other advantages, such as low cost, simple control and good quality. MRSF is provided with higher engineering applied value and academic research value.The main contents and conclusions are as follows:1. Researh on MRSF equipment and forming processThe structural characteristics of the traditional stretch forming machine is analyzed, and the shortcomings of traditional stretch forming machine and the advantages of MRSF are given. The design scheme of MRSF testing machine is discussed, including the structure of roller device, two types of clamp device (rigid floating clamp and flexible floating clamp)and two types of clamp plate (tooth-shape clamp plate and brake clamp plate; the related experimental validation is finished. The typical processes are summarized, which are directly press forming process, bend-press forming process and lift up-press forming process. The control methods of roller device and clamp device are studied.2. Research on the finite element analysis (FEA) modeling method of MRSFBased on the FEA software ANSYS/LS-DYNA, the FEA model and explicit-implicit springback analysis model are established. Using the numerical simulation method, the convergence of the FEA model is analyzed and the element size of sheet metal are determined. For the practical swing situation of the flexible floating clamp, a FEA model program to modify the clamp centroid is established; the simulation validation is finished, and the results indicate that the practical situation of clamp swing can be reflected in this program. the related experimental validation of finite element modeling accuracy is finished, the results indicate that the deformation, springback and dangerous point prediction results of simulation coincide with the experimental results.3. Numerical simulation on MRSF processAccording to the structural characteristics and control method of MRSF testing machine, the calculation formulas of the loading amount of clamp and roller in typical forming process are derived; the related numerical simulation and experimental validation are finished, the results indicate that the formula is feasible. Take the typical surface parts as research object, the processes of traditional stretch forming and MRSF are simulated, the numerical simulation results indicate that the parts formed by MRSF process, comparing with the traditional stretch forming process, have more uniform distributions of stress, strain and thickness. The three typical forming processes of MRSF are simulated, the using scope and effect on formed parts of different forming processes are analyzed. The lubrication effect between the sheet metal and die is studied, the results indicate that good lubrication can make the strain and thickness distributions of formed parts more uniform.4. The effect of clamp device on the forming resultsNumerical simulation and experimental validation of the clamping process of floating clamp device are done, the results proved that the clamp device can be applied to the MRSF process. The effect of rigid floating clamp (RFC) and flexible floating clamp (FFC) on forming results is comparative analyzed, the results shows that the stress, strain and thickness distributions of the parts formed by FFC are more uniform and more prone to wrinkle and rupture by using RFC. Take the tooth-shaped clamp plate and brake clamp plate as research object, the effect of different clamp plate on forming results is comparative analyzed, the advantages and disadvantages are investigated and the related experimental is done, the results indicate that the two types of clamp plate are all feasible. Finally, the calculation formulas of critical clamping resistance of brake clamp are derived.5. The effect of roller device on the forming resultsThe effect of sliding roller and rolling roller on forming results are studied, the results indicate that it is necessary and practical to design a roller unit with a roller head which can roll around itself. The discretization effect of the roller device on forming results is analyzed, and the optimal discretization of roller device in this case is determined. The effect of the distance between roller and the plate edge is analyzed, and the distance ranges of typical parts are calculated. The effect of different positions of roller device on forming results is studied, the results indicate that it is more suitable to form a large deformation 3D-surface when the roller device is on the top of die; For the situation that roller device is on the top of the free area of sheet metal, the effect of the distance between roller device and die on forming results is investigated, the results shows that the smaller the distance is, the more easily the shape can be formed, and the better the quality of formed part can be; In addition, the effect of the distance between roller device and clamp device on the forming results is studied, the results shows that the distance has little impact on the quality of forming results.6. Research on wrinkle defects in MRSF processFor stretch forming process is mainly used to form thin metal plate, so the formed parts is always have wrinkle defects. Take the saddle-shaped surface part and spherical surface part as research object again, the wrinkling is simulated. The simulation results indicate: for the, wrinkling is on the middle of saddle-shaped surface part and on the both side of the spherical surface part, and the reasons of wrinkling are explained by principal strain state of shell element and the law of stress-strain. The wrinkling distribution area, wrinkling process and the cause of wrinkling of saddle-shaped surface and spherical surface parts are analyzed. Take saddle-shaped surface as the research object, the wrinkling is simulated. The simulation results indicate that, compared with the MRSF process, the traditional stretch forming process is more prone to wrinkle. The influence of thickness, lubrication and pre-stretching displacement to the wrinkling is analyzed. The results indicate that the thinner the sheet metal is and the better the lubrication is, the less prone to wrinkle; The pre-stretching can suppress wrinkling and the bigger the pre-stretching displacement is, the less prone to wrinkle. The wrinkle suppression method called fractional steps pressing mode is advanced, the related numerical simulation validation is finished. The simulation results indicate that this method can suppress the generation of wrinkles.