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In this paper, a new method for determining the forming limit diagram (FLD) of thin sheet metals, called DIC-Grid method, is proposed based on digital image correlation (DIC) technique. It's assumed that there exists one virtual grid with an initial diameter of 2.5mm, which is usually the same dimension as the grid in traditional circular grid analysis, close to the crack of specimen, and the limit strain point on FLD is determined by the deformation of this virtual grid. The DIC-Grid method has been compared with traditional circular grid analysis and the standard ISO/FDIS 12004-2 in Nakajima tests. The results show that the forming limit strains obtained by the newly proposed method are more stable and precise. Furthermore, DIC-Grid method can avoid the measurement error which exists in the circular grid analysis. Meanwhile, it overcomes the shortcomings of time-consuming data processing and non-applicable for unrealistic strain distribution in the method of ISO standard.

The characteristics of China's automotive steel sheets and high strength steels (including bake-hardening, high strength IF and isotropic steel) developed recently by Baosteel for automotive outer panels are briefly reviewed. Among these steels, bake-hardening (BH) steel is often used to make outer panel parts, such as hoods, decklids, and front and rear doors. In this paper, we present results from an investigation of the effect of baking process (temperature and time) on the bake hardening index value of BH220 steel, and through evaluation of tensile specimens cut from a production hood and simulated part. The results show that BH value is more than 30 MPa when the baking time is more than 2 minutes (at a temperature of 170°C) or the baking temperature is more than 90°C (for baking time of 20 minutes). Measurements on an actual hood and simulated part showed that baking has no effect to enhance the yield strength of the panel, but it can enhance the dent resistance.