The Forming Limit Diagram (FLD) is an essential tool to assess sheet metal formability in sheet metal deep drawing. In FLDs/FLC (Forming Limit Curve) is representation of material formability limits at which material is not able to withstand higher deformation. In this work, different methodologies for FLC determination of mild steel DC01 are investigated. The Nakajima test is a well-known experiment for FLC determination but, however, contact conditions that can appear in the form of either friction or pressure may perturb the results. The former may cause the need for repetitions for different widths by altering the necking location, whereas the latter may be responsible for overestimation of the material formability. Another method for FLC determination without those effects is to use the cruciform specimen test under biaxial loading conditions. As the first step, Necking has to be induced at the center of cruciform specimen by thickness reduction or groove. Therefore, in this work, the cruciform geometry under biaxial loading is optimized using FEM through the comparative analysis. The goal of simulation was to induce the necking just in the thinner gauge, not adjacent zone. Thereupon, different loads on biaxial machine’s axes can provide different strain paths at the thinner gauge of the optimized specimen. After reviewing of various researches in connection with thickness reduction effect on sheet metal deformation, it was found that thickness reduction might not influence the material formability during uniaxial and plane strain tensions, but it was observed that there is gradual decrease of limit strains with decreasing the thickness for equi-biaxial tension.Manufacturing process like milling, cause small defects on material surface and local instability takes place at lower level of deformation. Besides, it was found that existence of through thickness normal stress leads to higher value of FLC. Deformation during Nakajima and cruciform experiments were captured by Digital Image Correlation (DIC) system. The execution and evaluation of those experiments are explained in detail and Time Dependent Method (TDM) was applied to detect the beginning time of instability. Experimental strain paths are compared with consideration of strain rates and, as it was expected, FLC from cruciform specimen is lower than Nakajima one, especially on the right hand side of FLD. The remarkable difference can be attributed to defects in the material and existence of through-thickness normal stress, which leads to overestimation of the material formability during Nakajima experiment.

成形极限图（FLD）是评估钣金深冲中的钣金可成形性的重要工具。在FLD / FLC（成形极限曲线）中，是材料可成形性极限的表示，在该极限下材料无法承受更高的变形。在这项工作中，研究了FLC测定低碳钢DC01的不同方法。Nakajima测试是FLC测定的众所周知的实验，但是，可能以摩擦或压力形式出现的接触条件可能会干扰结果。前者可能会通过更改缩颈位置而导致需要重复不同的宽度，而后者可能会导致材料成形性过高。没有这些影响的另一种确定FLC的方法是在双轴加载条件下使用十字形试样测试。第一步，必须通过减小厚度或开槽在十字形试样的中心引起缩颈。因此，在这项工作中，通过比较分析，使用有限元法优化了双轴加载下的十字形几何形状。模拟的目的是仅在较薄规格而不是相邻区域中引起颈缩。因此，双轴加工机轴上的不同载荷可以在优化试样的较薄规格上提供不同的应变路径。在回顾了有关厚度减小对钣金变形的影响的各种研究后，发现厚度减小可能不会影响材料在单轴和平面应变拉伸过程中的可成形性，但是观察到极限应变随着厚度的减小而逐渐减小。等双轴拉伸的厚度。像铣削这样的制造过程会在材料表面造成小的缺陷，并且在较低的变形水平下会发生局部不稳定性。此外，发现存在贯穿厚度的法向应力导致较高的FLC值。中岛和十字形实验期间的变形是通过数字图像关联（DIC）系统捕获的。详细解释了这些实验的执行和评估，并应用了时依赖方法（TDM）来检测不稳定的开始时间。将实验应变路径与应变速率进行了比较，并且可以预期，十字形标本的FLC低于中岛一，尤其是在FLD的右侧。明显的差异可归因于材料的缺陷和厚度方向法向应力的存在，