A Novel Approach to the Determination of Forming Limit Diagrams for Tailor-Welded Blanks

This paper presents the results of simulated hemispherical die stretching of laser-welded, low carbon steel (ST12 and ST14) blanks of various thicknesses. The simulations were designed to produce forming limit diagrams (FLDs) for the tailor-welded blanks. Multiple criteria, including the second time derivatives of major strain, thickness strain, and equivalent plastic strain extracted from the strain history of simulations, were used to accurately detect the start of necking in FLDs. This is to say that necking starts when the second derivative of the thickness strain, major strain or plastic strain reaches its maximum value. Knowing the onset of necking, one can measure the major and minor strains at the critical area and produce the corresponding FLD. Results from the proposed method and those from experimental tests are compared to demonstrate the efficiency of the proposed method.