چكيده به لاتين
In forming processes, knowing the acceptable amount of deformation which material can bear before necking is really essential. One of the beneficial tools in order to follow the forming behavior of metals is forming limit diagrams which are the proportion of principal strains in different statuses of forming. These diagrams can be obtained with experimental and theoretical methods. One of the well-known numerical methods is Marciniak-Kuczynski (M-K) model. In this method, presence of an initial defect such as thickness inhomogeneity is assumed as the necking factor. There are some parameters that affected the prediction of forming limit diagrams such as strain hardening power, strain rate, anisotropy coefficients, thickness of sheet, work hardening law and yield functions. These parameters can be considered through M-K model.
In this work, the effect of advanced yield functions like, BBC2008, Soare 2008, Plunkett2008 and Yld 2011 on the prediction of formability for different materials is studied theoretically and the results are shown that the ability of advanced yield functions is more than classical ones in prediction of the forming behavior of materials. Additionally, the yield functions’ numerical coefficients are obtained for some material through Genetic Algorithm (GA) and the results lead to accurate following of materials’ mechanical properties and the forming limit diagrams are extracted under the effect of these yield functions.
In another part of this work, forming limit diagrams of aluminum tubes are obtained through modified M-K method in which geometry inhomogeneity replaced with material inhomogeneity like strain hardening differences. The advantage of this procedure is that the sensitiveness of diagrams to variation of this parameter is less than sensitiveness to variation of geometry inhomogeneity. Additionally, the directional anisotropy coefficients of AA7020-T6 tube are calculated by using appropriate fixtures and tensile tests and using these coefficients in obtaining forming limit diagram of this tube theoretically.
Finally, in order to verification of numerical prediction, a comparison has been done between acquired results and experimental ones.
Keywords: Forming limit diagram, M-K numerical method, Advanced yield functions, Anisotropy coefficient.
