It is now possible using digital-image correlation techniques to precisely measure deformation within a developing necking instability, during forming-limit experiments. However, the current standards for measuring limit strains rely only on data outside of the instability. We propose exactly the opposite, to use just those deformations from the material where the instability will develop. Marciniak and Kuczynski experiments were performed on a drawing-quality steel and the entire deformation history recorded with a high-resolution photographic camera. The strain fields from these image were analyzed with the digital-image correlation program NCORR, concentrating on where the necking instability would form. The Merklein et al. (CIRP Ann Manuf Technol 59:295-298, 2010) and Hotz et al. (Key Eng Mater 549:397-404, 2013) temporal analyses were modified through an original smoothing technique to uniquely identify, through a correspondence of results, when the deformation acceleration rate begins to rapidly increase within the developing instability. This defines a limit strain. These results were compared to the standard Bragard-type determination specified in the norm International Standard ISO 12004-2:2008 (Metallic materials—sheet and strip: determination of forming-limit curves. Part 2—determination of forming-limit curves in the laboratory, International Organization for Standardization, Geneva, 2008). We found a close agreement in balanced-biaxial tension between our proposed technique and the standard Bragard-type analysis. However, in plane-strain and uniaxial deformation the standard analysis appeared to be excessively conservative, by as much as 40% for our steel.

现在，在成形极限实验期间，可以使用数字图像相关技术来精确地测量正在形成的颈缩不稳定性内的变形。但是，当前用于测量极限应变的标准仅依赖于不稳定之外的数据。我们提出了完全相反的建议，即仅使用材料中会产生不稳定性的变形。Marciniak和Kuczynski实验是在图纸质量的钢上进行的，整个变形历史都由高分辨率的照相相机记录下来。使用数字图像相关程序NCORR分析了这些图像的应变场，重点研究了颈缩不稳定性的形成位置。Merklein等。（CIRP Ann Manuf Technol 59：295-298，2010）和Hotz等。（Key Eng Mater 549：397-404，2013年），通过原始的平滑技术对时间分析进行了修改，以通过结果的对应关系唯一地识别变形加速度在发展中的不稳定性中开始迅速增加的时间。这定义了极限应变。将这些结果与国际标准ISO 12004-2：2008规范中规定的标准Bragard型测定（金属材料－片材和带材：成形极限曲线的确定。第2部分－实验室中成形极限曲线的确定）进行了比较。 ，国际标准化组织，日内瓦，2008年）。我们在我们提出的技术和标准的Bragard类型分析之间发现了平衡的双轴张力方面的紧密一致性。但是，在平面应变和单轴变形中，标准分析似乎过于保守，