Numerical Analysis And Experiment Of Nonlocal Friction In The Plastic Forming

Friction exists on the surface between tool and workpiece. It plays important role during metal forming processes. Friction model is one of the boundary conditions for the solution of mechanical problem of metal plastic forming. At present, only simplified friction model can be used to replace the actual friction distribution. Coulomb's friction model and Constant friction model are widely used simplified models. From the perspective of continum mechanics, contact surfaces are perceived as continual smooth surface instead of rough surfaces that are real surfaces of materials in contact. In mental plastic forming processing, the Coulomb friction law applied on each point of the contact surface is local, which means the friction stress at one point in a certain range of contact surface is only affected by normal stress at the same point. However, real metal surfaces are not smooth, and the contact surface is actually contacts between rough surfaces. In this case, the friction stress of a certain point is not only related to the normal stress of this point, but is affected by the other points' normal stress in a neighborhood of the point. This is called the nonlocal friction effect. Therefore, it is necessary to use nonlocal friction model as a replacement of Coulomb's friction model to analyse the nonlocal friction effect caused by the asperity structure on the contact surface on micro-level.The main content of the research paper is:Nonlocal friction model is applied as a replacement of Coulomb's friction model to study the distribution of pressure on the contact surface in the rolling process. A formula to caculate the pressure in the rolling process with nonlocal friction is obtained. What's more, the differences between the influences on the distribution of pressure on the contact surface casted by nonlocal friction model and Coulomb's friction model under different conditions is also studied.The nonlocal friction model theory proposed by Oden is first applied to use in the analysis of lubricating fluid mechanism in the cold extrusion process. A nonlocal lubricating model is established. Perturbation theory is used to get approximate solution. The friction user subroutine of ADINA has been studied. A new development of friction model is done based on the user subroutine of ADINA. Nonlocal friction model is introduced into ADINA.The influence of nonlocal friction model on the plastic forming process is studied both on a macro-level and on a micro-level by using initial numerical simulation. By applying the nonlocal friction model to the cylinder upsetting process the changing relations between the different height reduction and contact normal stress is found. The distribution of normal stress and friction stress on contact surfaces is also found. The influence of density distribution and height of the asperities on nonlocal friction is also studied here.A experimental system for friction test is designed. According the experiment, with the increase of stress and the area of the contact surface, friction coefficient become smaller. The pattern of how average normal stress changes on contact surfaces as the reduction increases is studied and the result is compared with the simulation results.All the research findings show that although using nonlocal friction model in replacement of Coulomb's friction model to analyse the friction in the plastic forming process has made the problem more complex, the findings of the analysis is more accurate and true. The application of nonlocal friction model is helpful for us to gain a deeper understanding of the friction mechanism in the metal plastic forming process. It is helpful for us to explore the value of the nonlocal friction model and extending the application range of it. It can serve as good theoretical bases for the application and development of the minor forming and processing technology. It can provide a promising future for the improvement and innovation of plastic processing technology.