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The finite element model of flexible clamp multi-point stretch forming (FCMPSF) was set up, and extensive contradistinctive analysis of forming limit and forming accuracy between FCMPSF and RCMPSF were done. The results show that under the same forming conditions, spherical part formed by FCMPSF is not easy to fracturing, dimpling and springback, and is easy to fit the die. The reason for this was analysed. The non-fracturing limited graph, the non-dimpling limited diagram and the average springback value of spherical parts formed by FCMPSF and RCMPSF were obtained. Finally, the forming experiment and error analysis of spherical part formed by FCMPSF were done, the results indicate that 3D parts with large transversal curvature can be shaped by FCMPSF and the forming quality was guaranteed.

Flexible DCSF technology was put forward, and its forming character was described. The flexible DCSF machine was developed and related stretching experiment were carried out. The experimental photos show the DCSF technology is feasible and the DCSF machine is practicable. The FE model of flexible DCSF was set up, and extensive numerical simulations for spherical parts, saddle parts and S-type parts were carried out by Abaqus. The numerical results show that the longer the transitional length is, the more homogeneous the stretching strain and the thickness become. The smaller the friction coefficient is, the more homogeneous the stretching strain and the thickness become. The larger the clamp number is, the better the fit degree becomes. This work may provide useful guidance on the flexible DCSF process.

Flexible-gripper stretch forming is a new flexible manufacturing process of sheet metal. Four numerical simulation models of discrete grippers were established using finite element method, the effect of the numbers of grippers on stress distribution, thickness distribution and gripping surface shape of the spherical parts were studied. The results show that the clamped edge of the spherical parts formed by using integral grippers is straight，the tensile stress distribution is uneven and the thickness is obviously reduced. However, the stress distribution on the parts formed by using discrete grippers stretch forming is relatively uniform, with the increase in the numbers of grippers, the thickness increases, and the more gripping surface curvature of the parts is close to the curvature of die, the better forming quality of the parts is.

A new type of discrete clamp stretch forming machine based on flexible holding technology was developed to improve the performance of traditional rigid clamp stretch forming machine used for double-curvature metal sheet forming, and its forming characters were illuminated simply. The FE model of flexible discrete clamp stretch forming was set up, and extensive numerical simulations of spherical parts for both the flexible holding mode and the rigid holding mode were carried out by dynamic explicit finite element analysis. The numerical results show that the distribution of stress, strain and thickness were well-proportioned in the flexible holding mode. The larger the spherical radius is, the smaller the minimum transitional length of sheet metal is. The transitional length of 60mm is suitable for flexible discrete clamp stretch forming in the case of considering the forming quality and material utilization. Then, the stretch forming experiment proves that the flexible holding technology is feasible, and the error contrast illustrates the new type of discrete clamp stretch forming machine is practicable.

In order to improve the performance of back drawing type of flexible multi-gripper stretch forming machine used for double-curvature metal sheet forming, back and down drawing type of flexible multi-gripper stretch forming machine was designed by finite element method (FEM), which has simple structure and cheap cost. The FE model of flexible multi-gripper stretch forming was established, and extensive numerical simulations of spherical parts for two kinds of flexible stretch forming machines were carried out. The variations of stress, strain, thickness and springback value of spherical parts for two kinds of drawing modes were analyzed. The numerical results show that the quality of spherical parts formed by the back and down drawing type of stretch forming machine is better than that by the back drawing type of stretch forming machine. This work provides a machine for developing the technology of stretch forming.