Experimental Study on Shear Fracture of Advanced High Strength Steels: Part II

Abstract

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.

Numerical failure analysis of a stretch-bending test on dual-phase steel sheets using a phenomenological fracture model

“…Shih and Shi (2008) and Shih et al (2009) performed full sets of stretch-bending tests with a Stretch Forming Simulator (SFS) on various AHSS strips, including the present DP780 steel. As shown in Fig.…”

“…During a SFS test, the strip is first clamped on both sides, and then the upper die moves down and stretches the blank into the die cavity, while both the tension level within the strip and the wrap angle / increase. It has been widely reported (Walp et al, 2006;Hudgins et al, 2007;Shih and Shi, 2008;Shih et al, 2009) that the failure modes in the stretch-bending operations are determined by the combination of R/t ratio and the applied tension level on the sheet. In a SFS test, the R/t ratio can be altered by replacing different lower dies, and the tension level on the strip is controlled by adjusting the clamping distance d, see Fig.…”

“…For this study, the test results from US Steel Corporation (Shih and Shi, 2008;Shih et al, 2009;Shih, 2009) on 2-in.-wide (50.8 mm) DP780 strips are utilized to validate our material modeling, especially the fracture predicting capability of the phenomenological fracture model proposed in Section 3. All strip specimens for this study were sheared perpendicular to the rolling direction from the same DP780 sheet.…”

“…These two parameters are usually utilized to determine a critical R/t ratio below which shear fracture on die radii will occur (Walp et al, 2006;Shih and Shi, 2008;Shih et al, 2009). In both experiments and simulations, the wrap angles / were calculated from drawing depth h at failure using trigonometry.…”

“…On the experimental side, Gotoh et al (1997) conducted a series of draw-bending tests on Cu-0, and showed a clear transition from shear fracture on die radii to tensile failure on the side wall as the R/t (die/punch radii to sheet thickness) ratio increases. Also, continuous efforts have been made in the forming industry to correlate failure stress (or other similar index) to R/t ratio and thus obtain empirical critical R/t ratios of shear fracture for various AHSS by conducting MDS or similar tests (Sriram et al, 2003;Walp et al, 2006;Shih and Shi, 2008;Levy and Van Tyne, 2009;Shih et al, 2009;Shih, 2009). On the analytical side, Bai and Wierzbicki (2008) derived close-form solution for both global force responses and local strain and stress state in the MDS stretch-bending test assuming plane strain 0020-7683/$ -see front matter Ó 2010 Elsevier Ltd. All rights reserved.…”