SPRINGBACK IN THE STRETCH-BENDING OF SHEET-METAL WITH NONUNIFORM DEFORMATION

Springback is a common phenomenon in sheet metal forming, caused by the elastic redistribution of the internal stresses during unloading. Although fundamental theoretical analyses have been developed to quantify the springback behaviour, their application to sheet metal forming processes is limited since discrepancies are often observed when the theoretical predictions are compared with the measured data. This has led to the development of various empirical equations, each suited to the particular forming process examined. The lack of agreement between the theories and practical observations arises mainly from the variation of the material properties, and the complexity and peculiarity of the individual forming process, which lead to an uncertainty of the forming parameters (such as the tension induced during forming and die conformity). In addition, the bending of the sheet during forming might not be at all uniform, which might result in the sheet being formed into less-deformed regions connected by highly-deformed regions (kinks). This non-uniform deformation could be induced by the way the load is applied to the sheet (i.e., load configuration), or by the intrinsic characteristic of a material which has a discontinuous yield behaviour (such as steel). In the latter case, the sheet produces kinks, also known as 'flutes', on bending. A theoretical study of the springback behaviour in these circumstances must take into consideration the nature of the non-uniform deformation during bending in order to provide a realistic prediction. This paper develops such a mathematical model and quantifies the variation of the springback caused by non-uniform deformation. From the theoretical model, the maximum reduction in springback is derived. The predictions from this model are compared with experimental data and good agreement is obtained. This model may be applied to estimate the springback in general stretch-bending situations where non-uniform deformation is involved, but is particularly suited for springback prediction for fluting material. © 1990.