One of the most significant manufacturability-criterions of a tubular component in bending and/or hydroforming operation is the formability limit of the selected tube material. Currently the most common tool used to determine the material formability is the North American “standard” forming limit diagram (FLD) developed for low carbon sheet steels.
It was shown, that applying the experimental FLDs developed for high strength steel materials or standard FLC on rotary draw bending process would, however, lead to under-utilization of the tube material. Therefore, on the basis of the elementary theory of bending and introducing the bendforming-related “Displacement of Neutral Axis” as a bend parameter, new relationships were developed for accurate prediction of principal bending strains. Subsequently, the residual strains from tubing process were taken in consideration to determine the exact bendability limit for tubular components in rotary draw bending. The position of so-called Bending Limit Curve “BLC” becomes, beside the wall thickness and the n-value of the tube material, also a function of outside diameter of considered tube.
Series of steel tubes of different materials and geometries were bent with decreasing bend radiuses until their failure. The results of strain measurements indicate that the principal bending strains were a function of bend-ratio only (therefore material neutral) and show a very good correlation predicted strains. Furthermore, the paper explains the developed bending limit curve and demonstrates the influence of tube's outside diameter on its bendability limit. The high accuracy of the calculated bending strain paths and the applicability of the developed bending limit curve were shown using the experimental data and FEA-simulations.
