Developing a proper local formability failure criterion is the key to the successful prediction of the local formability of Advanced High Strength Steels (AHSS) in computer simulations. Shear fracture, which refers to the fracture occurred in the die radius when a sheet metal is drawn over a small die radius, often occurs earlier than predicted by the conventional forming limit curve (FLC). As shown in a previous study using a laboratory Stretch-Forming Simulator (SFS), shear fracture depends not only on the radius-to-thickness (R/T) ratio but also on the tension/stretch level applied to the sheet during stretching or drawing. In the SFS test, a flat sheet is first clamped at the both ends then gradually is wrapped around the die radius as the punch moves downward. This process simulates the early stage of stamping when a sheet metal is initially stretched or drawn over a die/punch radius. However, shear fracture may not occur in this stage if the stretch/tension level is not high enough. In this study, the Bending under Tension (BUT) tester is used to evaluate shear fracture occurring in the later stage of stamping, after the sheet metal is totally wrapped around the die radius. It is demonstrated that shear fracture does occur in this deformation mode when a sufficient tension level is applied. Effects of forming conditions, such as forming speeds and lubrication on shear fracture, are also investigated. When compared to the results from the SFS, the data points failing at the die radius tangent point agree very well. It is observed that all data points above the tangent point failure line show shear fracture, while data points below this line show tensile failure (localized necking) regardless of the test methods used. This indicates that the tangent point fracture line can be used as the shear fracture failure limit. This failure criterion can be used in a computer simulation to simulate the shear fracture phenomenon in the entire deformation process involved in a sheet metal stretching or drawing over a die radius.
Fracturing in a tight radius during stretch bending has become one of the major manufacturing issues in stamping advanced high strength steels (AHSS), particularly for those AHSS with a tensile strength of 780 MPa or higher. Computer simulations often fail to predict this type of fracture, since the predicted strains are usually below the conventional forming limit curve. In this study, a laboratory stretch-forming simulator (SFS) is used to simulate the stretch bending of AHSS in stamping to develop a possible failure criterion for use in computer simulations. The SFS simulates the stamping process when sheet metal is drawn over a die radius with tension applied. Various sizes of die radius are used during the experiment, and the shear fracture phenomenon can be recreated using this test for a given material and gauge. It is found that shear fracture depends not only on the radius-to-thickness ratio but also on the tension/stretch level applied to the sheet. The experimental data show that a critical radius-to-thickness ratio for shear fracture exists for any given material and gauge, but this ratio is not unique and it depends upon the amount of tension imposed during the bending.
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