| Dual Phase (DP) steels are a class of advanced high strength steel (AHSS) in increasing use for sheet formed automotive parts. In spite of attractive combinations of high strength, high ductility, and low cost, the widespread adoption of DP steels has been limited because practical die tryouts exhibit forming failures far earlier than predicted by standard industrial methods. These failures, often referred to as "shear fractures," occur in regions of high curvature and with little apparent necking, in contrast to "normal" or tensile fractures. Conventional wisdom attributes shear fractures to a postulated damage mechanism related to the special microstructure of DP steels.;In order to reproduce, characterize, and analyze such fractures in a laboratory setting and to understand their origin of the inability to predict them, a novel draw-bend formability (DBF) test was devised based on displacement control. DP steels from several suppliers with tensile strengths ranging from 590 to 980 MPa were tested over a range of rates and bend ratios (R/t, inner bend radius/sheet thickness). The new DBF test reliably reproduced three kinds of fractures identified as Type I, II, and III, corresponding to tensile fracture, transitional fracture, and shear fracture, respectively. These tests revealed a surprising result: the occurrence of shear fractures increased at higher deformation rates. This degradation of formability was shown to be principally a result of deformation-induced heating, which is greatly accentuated for AHSS because of their high plastic energy absorption and commensurate high temperature increases, up to 100 degrees C.;In order to understand and quantify the role of deformation-induced heating on plastic localization, temperatures were measured and simulated using a novel new empirical plasticity constitutive form describing the flow stress as a function of strain, strain-rate, and temperature. Designated the "H/V model", the new constitutive model consists of three multiplicative functions describing (a) strain hardening and its temperature sensitivity, (b) strain-rate sensitivity, and (c) temperature sensitivity. This form allows a natural transition from unbounded strain hardening at low temperatures toward saturation behavior at higher temperatures, consistent with many observations. Thermo-mechanical finite-element simulations using the H/V model confirmed its accuracy and the magnitude of the role on shear fracture. Failure types were predicted, as well as quantitative.;For most of the DP steels tested, heating induced by deformation was identified as the dominant effect in producing unpredicted fractures. This is a result of standard industrial techniques that do not take non-isothermal effect into account, in particular constructing forming limit diagrams from low-speed/isothermal testing, and use of isothermal finite element modeling to analyze industrial sheet forming operations. Microstructural damage can also contribute to shear fracture, but it was a secondary factor for all but one of the alloys tested, in one test direction. |
