R&D Update: Shear fracture in bending advanced high-strength steel

Compared to conventional steels, forming AHSS leads to new challenges, such as shear fracture that may occur during stamping. As shown in Figure 1 , AHSS is more susceptible to shear fracture when the sheet material is stretched and bent simultaneously at sharp die radii.

The forming limit curve (FLC) is used routinely in sheet metal forming analyses to study the strain distribution in the deformed part and possible sheet material fracture. Recent studies have shown that FLC for stamping of AHSS is not always reliable in predicting fracture, especially in bending at tight die radii (see Figure 2 ).

Effects of Radius on Bindability

The advanced Steel Processing and Products Research Center (ASPRC) at the Colorado School of Mines conducted stretch forming test to evaluate the bindability of various grades of AHSS— including DP 600, TRIP 780, DP 780, and DP 980— and the fracture modes for bending with different punch radii—1 mm, 2.5mm, 5 mm, 7.5 mm, and 10 mm.

In these test, a strip specimen measuring 25.4 mm wide and 180 mm long was clamped with lock beads and bent by moving a punch at a speed of 21 mm per second. The punch displacement at fracture obtained for a given R/t ratio (the ratio of punch radius to sheet thickness) was compared for various grades of AHSS (see Figure 3 ).

In most materials, as the R/t ratio increased, the shear fracture occurring at the punch tip moved to the stretched portion of the sample. However, DP 980 samples all examined shear fractures at the punch tip, even with increasing R/t ratio.

Prediction of Shear Cracking in Bending

It is desirable to predict when cracks may occur in bending AHSS so that the dies and the stamping process can be designed to avoid fracture.

In a preliminary study, the CPF used finest elements analysis (FEA) together with various fracture criteria to correlate the state of strain and stress in bending with basic properties (yield stress, tensile strength, elongation) and the flow stress of AHSS. Researchers compared the FEA predictions, made by the commercial code DEFORM-2D, with experimental data given in the literature.

This preliminary investigation showed that, using the appropriate fracture criterion, it is possible to estimate when fracture occurs (see Figure 4 ) and predict the effect of punch radius on fracture in bending (see Figure 5 ).

Future Studies

The primary objective of the CPF research is to improve the existing fracture criteria for shear fracture and to evaluate bendability for AHSS under near stamping conditions. Researchers plan to conduct two different series of air bending and stretch bending tests. The FE simulations of these test will be used to evaluate bendability of AHSS and to establish a criterion of predicting fracture in bending various grades.