Forming Limit Of DP780 Under Nonlinear Strain Path

The conventional forming limit diagram(FLD)was obtained by theoretical calculation and experiments based on the consumption of linear strain path.However,the practical strain path of the formed part is always nonlinear in stamping processes.Changing strain path has a remarkable effect on the material behavior,especially on the hardening characteristic and forming limit.Therefore,the quantitative evaluation of metal forming under complicated loading is not only the important complement to metal forming theory,but also a great improvement to the application.This paper is focused on the issue of formability for DP780 steel sheet under non-linear strain path.The detailed content and major results are as follows:Based on the concept of traditional forming limit test,a two-punches design plan,which may achieve non-linear strain path,was proposed.The forming process was performed by Dynaform and the strain path of crack point shows the proposal tools can achieve the strain path consisted of two linear strain path in the crack point.According to the results of the numerical works,the experimental tools were designed and manufactured.The proper shapes of specimens and lubrication,which are the most important parts of the experimental program,were studied through trial and error.The experiment of forming limit of DP780 steel sheet under nonlinear strain path was conducted.The acquiring method for real-time strain path using DIC technique and the criterion for forming limit strain state were explored.The strain paths of three points from the simulation of the front rail using DP780 metal sheet was applied to the validation work.The result shows that the forming limit curve of DP780 under nonlinear strain path obtained from this paper was able to predict the crack phenomenon in the fillet of front rail,while the conventional forming limit curve was always failed to.