On the fracture characteristics of advanced high strength steels during hydraulic bulge test

The advanced high strength steels (AHSS) are well recognized for their high-strength-to-weight ratio combined with good shock absorption properties. Moreover, these steels are also known for their complex forming and fracture characteristics. This study is aimed at investigating the fracture behavior of AHSS sheets under the influence of an equal-biaxial state of stress. The work was conducted on two important AHSS sheets -(i) DP980 and (ii) TRIP1180. A hydraulic bulge test was performed to cause the fracture under biaxial loading condition. The strains near the pole of the bulge specimens were measured using the digital image correlation technique. A detailed micro- and macro-scale characterization was performed to examine the fracture aspects of both sheets. The results clearly indicated the occurrence of shear localization during fracture. Moreover, the likely sources of micro-crack initiation were also revealed. Apart from fracture characterization, a comprehensive finite element study was done using appropriate constitutive equations. Finally, a simple fracture criterion was applied to predict the onset of fracture for both steels.