Drawbead Restraining Force Modeling: Nonlinear Friction

A detailed investigation of influence of friction on drawbead restraining force modeling is presented in this paper. It is motivated by the need to accurately correlate line bead strengths, which are usually the output of an optimized draw development for controlling materials flow and achieving desired formability, and the physical drawbead geometries required for die face engineering. A plane-strain drawbead model with linear Coulomb friction is first established and the restraining forces corresponding to a range of bead penetration depths are obtained. The comparison of the simulation results with experimental data indicates that, while a larger Coefficient of Friction (COF) has better correlation for smaller bead penetrations and smaller COF does better for deeper bead penetrations, no single COF matches satisfactorily for overall range of bead penetration depths. Nonlinear friction models are subsequently proposed where the friction coefficient is dependent on the contact pressure, justified by a comprehensive review of theoretical and experimental evidences. Their applications to the drawbead modeling yield improved correlations for the restraining forces. The paper focuses on the investigation of the need for such nonlinear friction models, and other possible explanations behind the discrepancy between the test and the simulation are discussed, notably with the emphasis on the influence of materials models as a future study area.