Failure Characteristics of Extra-Formable Steel Sheet Exhibiting Inhomogeneous Deformation Bands

Continued efforts in search of high strength materials with good formability have led to development of several novel advanced high strength steels (AHSS). The current study was performed on one such steel, i.e., extra-formable sheet, which has a good combination of both strength and formability. In this study, the forming and fracture characteristics of the sheet were analyzed using uniaxial tension, bulge, and hole expansion tests. The strain fields during each test were obtained using a digital image correlation technique. This material exhibited formation of Lüders and Portevin–Le Chatelier bands leading to an inhomogeneous plastic flow. In addition to experiments, a finite element study was carried out to model the plastic and fracture behavior of the sheet. For that, the yield and hardening functions of the material were described by Yld2004-18p and Hockett–Sherby, respectively. At last, a phenomenological fracture criterion was applied to predict the fracture in numerical mechanical tests. In this study, the effect of deformation mode on the occurrence of inhomogeneous plastic bands was evident. This finding was further used to decide a reference mechanical test for the calibration of hardening function. The adopted material models described the overall plastic behavior of the sheet reasonably well without capturing the non-uniform deformation bands.