Simulation of Kinetic Friction in L-Bending of Sheet Metals

This paper aims at experimental and numerical investigation of springback behavior of sheet metals during Lbending process with emphasis on Stribeck-type friction modeling. The coefficient of friction in Stribeck curve depends on sliding velocity and contact pressure. The springback behavior of mild steel and aluminum alloy 6022-T4 sheets was studied experimentally and using numerical simulations with ABAQUS software with two types of friction model: Coulomb friction and Stribeck friction. The influence of forming speed on springback behavior was studied experimentally and numerically. The results showed that Stribecktype friction model has better results in predicting springback in sheet metal forming. The FE prediction error for mild steel and 6022-T4 AA is 23.8%, 25.5% respectively, using Coulomb friction model and 11%, 13% respectively, using Stribeck friction model. These results show that Stribeck model is suitable for simulation of sheet metal forming especially at higher forming speed. Keywords—Friction, L-bending, Springback, Stribeck curves. HE most prominent feature of sheet metal forming process is an elastic recovery phenomenon during unloading which leads to springback [1]. The following statement best describes springback: “A purely elastically bent sheet will return to its original configuration upon removal of the bending moment. After partially plastic bending; permanent deformation and residual stress remain after unloading [2]. Evidently, bending causes the metal on the outside of the neutral axis to be under a state of tension, whereas the inside is subjected to compression. During the bending process, internal stresses are developed in the sheet and upon unloading; the internal stresses do not vanish. After bending, the extrados is subjected to residual tensile stress and the intrados is subjected to residual compressive stress. These residual stresses produce a net internal bending moment which causes springback [3]. The sheet continues to springback until the internal bending moment drops to zero. Despite the broad application of sheet metal forming, the design of the tools and the selection of the sheet materials are still usually based on trial and error efforts, a very expensive and time consuming procedure. Reliable finite element models M. Ramezani is a Research Fellow and Lecturer at Centre for Advanced Manufacturing Technology (CAMTECH) of Auckland University of Technology, New Zealand (phone: +64 223 228 807; e-mail: [email protected]). T. Neitzert is a Professor of Mechanical Engineering and Director of Engineering Research Institute (ERI) at Auckland University of Technology, New Zealand. T. Pasang is an Associate Professor and Head of Department of Mechanical Engineeringat Auckland University of Technology, New for describing the process would be of great value in reducing much of the tool tryout work [4]. For these finite element simulations, accurate models of the material behavior under deformation as well as the friction between tool and metal are needed. For metal forming simulations, a good friction model is very important, as shown by [3]–[5]. For most forming simulations the value of coefficient of friction is chosen as a constant, neglecting the fact that the parameters on which this value depends might change during the process. Coulomb friction model is a simple model frequently used in simulation. In this model, the ratio between friction force and normal force, defined as the coefficient of friction, considered to be constant, as used by [6], [7]. But, friction depends on a large number of parameters, e.g. the micro-geometry, the macrogeometry, the lubricant and the operational parameters: velocity, temperature and normal load. If one of the parameters changes; the coefficient of friction will also change [8]. In the present paper, the L-bending of mild steel and aluminum alloy 6022-T4 sheets is simulated with emphasis on Stribeck-type friction modelling. From the author’s knowledge, there has been no previously published analysis of Stribeck-type frictional behavior in L-bending. The springback behavior of sheets was studied using numerical simulations by ABAQUS software with two types of friction model: Coulomb friction and Stribeck friction. The results have been compared with experimental results. The influence of forming speed on springback behavior of sheets is also studied experimentally and numerically. II. STRIBECK FRICTION MODEL When a metal forming process is observed, it is clear that the conditions in all the different contacts are very different. For most forming simulations the value of coefficient of friction is chosen as a constant, neglecting the fact that the parameters on which this value depends might change during the process. Often, several metal forming simulations with different values for the coefficient of friction have to be performed before the simulation provides acceptable results. It is clear that these simulations have no predicting power at all [4]. From this, it is obvious that a model which describes coefficient of friction as a function of local contact conditions is needed. The limiting strain of a material is not directly changed by friction, but friction changes the stress and strain distribution. The redistribution of stress and strain, can affect defects on metal forming, such as springback. Stribeck is credited for carrying out the first systematic experiments unfolding a clear view of the characteristic curve Simulation of Kinetic Friction in L-Bending of Sheet Metals Maziar Ramezani, Thomas Neitzert, Timotius Pasang