A Discrete Springback Compensation Algorithm For Stampings With Curved Surfaces Using Optical Measuring Technology

Springback is an inevitable phenomenon in sheet metal forming, which will lead to the dimensional inaccuracy of products. With the development of automotive industry and aerospace industry, demand for more accurate stamping product is increasing and the problem of springback becomes more and more prominent because of the wider use of high strength steel. Springback can be partly decreased by applying appropriate processes parameters, but it can not be completely eliminated. Springback compensation method is a more promising way because it can completely eliminate the shape error caused by springback in theory. However, the main difficulty of the springback compensation method lies on the design of compensated tool shape, which is particularly difficult for 3D products. During the past decades, various FEM-based approaches have been developed to compensate the springback by iterative Finite Element simulation and calculation. But the application of these approaches is restricted by the accuracy of FEM. At this time the accuracy of FEM-based springback predicting is still not satisfying. Several trial-and-error iterations are still required to ensure the dimension and shape accuracy of products. In this paper, a discrete springback compensation method based on non-contact optical measuring is proposed with an aim to obtain the correctly compensated tool shape for complex stamping products. Main work of this thesis is as following: In order to solve the springback problem of 2D complex varied-curvature stamping products, a discrete curvature correction springback compensation approach is proposed. Complex varied-curvature part is divided into a number of regular equal-curvature units by discretization. For each unit, a curvature correction methodology is used to get the curvature data of compensation tool surface by using the measuring information from the stamped specimen in the trial experiment. A tool surface for next stamping is reconfigured by a methodology based on differential geometry. This approach has been successfully applied to a wavy-shaped part and a blade part. The result of experiment showed that right products can be obtained by only one trial experiment.The springback of 3D products is treated as the springback problem of a large number of discrete double-curved patches and the coupling of their springbacksFirstly, a springback prediction model based on measurement for 3D double-curved part is proposed. Double-curved part deformation mechanics was investigated theoretically. Strain data measured by non-contact optical measuring device at the end of forming process were used to calculated bending moment and tension of the deformed part via the shift position of neutral layer. And then, springback of double-curved part was predicted. Effect of tension, parameters of material and curvature to springback of double-curved part was also discussed according to the theoretical model.Secondly, a series experiment of double-curved part was carried out. Two kinds of optical measuring devices were used to get strain data at the end of forming process and shape data of the stamped part respectively. A curvature evaluation method is proposed to indicate springback of 3D part. With this method, springback and strain distribution of the double-curved part was analyzed. The theoretical springback prediction of part in different processes was verified by comparionwith the experiment results.Finally,a discrete springback compensation is proposed for 3D part based on the theoretical springback prediction model and curvature evaluation method. In this method, a 3D part was discretized into a number of square patches. Every patch was approximated as a double-curved shell. Each double-curved patch was compensated by a methodology based on the mechanical relation of the initial and next stamping forming process. And then the compensation factor was also corrected by curvature release ratio coefficient of the stamped part, which indicated the interactive springback effect between patches. Eventually, the tool surface for next stamping was reconstructed by these calculated curvature data of the surface. By far, this compensation method has been applied to solve springback problem of a double-curved part. The experiment result validated the feasibility of this method.