The formability limits of bulk metal forming in principal strain space and in the effective strain vs. stress-triaxiality space are characterized by an uncertainty region in which cracks may be triggered by tension (mode I of fracture mechanics) or by out-of-plane shear (mode III). The problem in obtaining experimental data in this region has been known for a long time and the main objective of this paper is to present a new upset formability test geometry that can effectively contribute to the characterization of the formability limits of bulk metal forming parts subjected to biaxial tension. Alongside with this objective, this paper also presents an analytical expression for converting the fracture forming limit line corresponding to crack opening by mode III in principal strain space into a hyperbolic fracture limit curve in the effective strain vs. stress-triaxiality space. The overall methodology employed by the authors combines experimentation along with analytical and numerical modelling, and the contents of the paper is a step towards diminishing the actual lack of knowledge regarding failure by fracture in bulk metal forming parts subject to stress-triaxiality values beyond uniaxial tension. Results show that a new uncoupled ductile fracture criterion built upon combination of the integrands of the Cockcroft-Latham and McClintock criteria can be successfully used to model the physics of the bulk metal forming limits for the entire range of stress-triaxiality values corresponding to cracking on free surfaces.

大块金属在主应变空间和有效应变与应力-三轴空间中成形的可成形性极限的特征在于一个不确定区域，在该区域中，裂纹可能由张力（断裂力学的模式 I）或平面外触发剪切（模式 III）。在该区域获得实验数据的问题早已为人所知，本文的主要目的是提出一种新的镦粗成形性测试几何形状，该几何形状可以有效地有助于表征大块金属成形零件的成形极限。双轴张力。除了这个目标，本文还给出了一个解析表达式，用于将主应变空间中与模式III开裂对应的断裂形成极限线转换为有效应变-应力-三轴空间中的双曲线断裂极限曲线。作者采用的整体方法将实验与分析和数值建模相结合，本文的内容是朝着减少实际缺乏关于在超过单轴拉伸应力三轴性值的大块金属成形零件中断裂失效的知识迈出的一步.