Abstract: In order to study the deforming essence of the tube-bending systemically, a great number of experiments of tube-bending without mandrel are carried out. Integrating the deformation phenomenon that the material of outer wall bears tangential tension, the probable deformation of concentrated and dispersive tensile destabilizing is analyzed respectively. The formula of the minimum relative bending-radius, which is related to mechanical property of the tube and constrained by tensile destabilizing, is concluded. The formula can be used in technique calculation and is helpful to the design of the bending forming of the tube and the technical analysis. Through corre-sponding experimental analysis, it is pointed out that since the remanent strain generated in tube processing causes some change of the mechanical property of the material in the state of tube shape. Therefore, as the secondary plastic deformation, the practical hardening modulus and the mechanical property pa-rameter of the tube-bending have important influence on the destabilizing deformation of material in the bending process and the calculation of the minimum relative bending-radius.

Key words: Hardening modulus, Tangential strain, Tube-bending, Wall-thickness deforming