The Influence of the Through-Thickness Strain Gradients on the Fracture Characterization of Advanced High-Strength Steels

The development and calibration of stress state-dependent failure criteria for advanced high-strength steel (AHSS) and aluminum alloys requires characterization under proportional loading conditions. Traditional tests to construct a forming limit diagram (FLD), such as Marciniak or Nakazima tests, are based upon identifying the onset of strain localization or a tensile instability (neck). However, the onset of localization is strongly dependent on the through-thickness strain gradient that can delay or suppress the formation of a tensile instability so that cracking may occur before localization. As a result, the material fracture limit becomes the effective forming limit in deformation modes with severe through-thickness strain gradients, and this is not considered in the traditional FLD. In this study, a novel bending test apparatus was developed based upon the VDA 238-100 specification to characterize fracture in plane strain bending using digital image correlation (DIC). Three punches with tip radii of 0.2, 0.4, and 1.0 mm were used to demonstrate the influence of the bend severity on the fracture limit in plane strain tension. Moreover, the influence of the through-thickness strain gradient on equi-biaxial stretching conditions was also investigated using hemispherical punches with radii of 5, 10, 25, and 50 mm. It was observed that using smaller radius, Nakazima punches can help to mitigate necking and provide a near-ideal biaxial strain path until fracture.