DOI: 10.1002/adem.201300556 Characterization and Modeling of Failure Initiation in Bainite-Aided DP Steel ** By Ali Ramazani,* Yuling Chang and Ulrich Prahl This research work aims to characterize and model the failure initiation in bainite-aided dual-phase (DP) steel. Combined electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) measurements were applied to quantify the constituents (ferrite, martensite, and bainite) in the microstructure. Mini tensile test with digital image correlation (DIC) analysis was carried out and linked to local scanning electron microscopy (SEM) analysis to identify macroscopic failure initiation strain values. SEM measurements showed that the crack initiation occurs in martensite islands. A microstructure-based approach by means of representative volume elements (RVE) modeling combined with extended finite element method (XFEM) was utilized to model martensite cracking on mesoscale. The identified parameters were validated by comparing the predictions with the experimental results. 1. Introduction Since dual-phase (DP) steels show a combination of the high strength with good ductility, they are widely used in the automobile industry. [1] These good properties are attributed to the microstructure of DP steel, which normally consisted of hard martensite particles dispersing in a soft ferrite matrix. [2] Bainite can also be formed in DP steel due to improper industrial heat treatments. Since the mechanical and failure behavior of DP steels, it significantly depends on the microstructure of these materials. Presence of bainite in the microstructure, affects the mechanical behaviors of DP steels as well. [3] Many researchers investigated the effect of bainite content on the ferrite–bainite–martensite steels. [4–6] Sudo and Iwai [4] indicated that reducing the bainite content in ferrite–bainite– martensite steels causes a decrease of the ratio between the tensile and yield strength but an increase in the percentage elongation and strain hardening exponent. Kim et al. [5] suggested that small amounts of bainite in ferrite–martensite DP steels lead to an increased yield strength and ductility but it also decreases the tensile strength. Matlock et al. [6] investigated the effect of the microstructure on the mechanical properties of micro-alloyed ferrite–bainite–martensite steels and reported inferior tensile properties and toughness for these steels in comparison to conventional steels. Application of DP steels is restricted due to their complex failure behavior, which depends on the microstructure of these materials. Therefore, many research works are recently conducted to study failure behavior of DP steels. DP steel fails in a ductile manner, which can be divided into three stages: void nucleation, void growth, and void coalescence. Until now, mainly three observations in the fracture of DP steels have been proved: voids formation because of the brittle fracture of martensite; interface decohesion between mar- tensite and ferrite, and the ductile failure of ferrite matrix. [7–11] Even though at macroscopic scale, DP steel exhibits uniform and homogenous deformation, but due to its grain level inhomogeneity, the microscopic deformation is instable. Shen et al. [8] used a scanning electron microscope (SEM) equipped with a tensile straining stage to illustrate the inhomogeneous strain distribution between ferrite and the martensite grains in DP steels. According to their investigation, the ferrite phase starts to deform immediately and at a much higher rate than the delayed deformation of martensite phase, so the volume percentage of martensite in the DP steel will influence the failure mechanism. Maire et al. [12] investigated the failure behavior of DP steels based on the situ tensile tests. In their *[*] A. Ramazani Department of Materials Science and Engineering, University of Michigan, 2300 Hayward St., Ann Arbor, MI 48109, USA E-mail: ramazani@umich.edu A. Ramazani, Y. Chang, U. Prahl Department of Ferrous Metallurgy, RWTH Aachen Univer- sity, Intzestr.1 D-52072 Aachen, Germany [**] This research was carried out under Project number MC2.07293 in the framework of the Research Program of the Materials Innovation Institute M2i (www.m2i.nl). The mini tensile tests were carried out at MPIE Dusseldorf, the support of Stefan Zaefferer and his group is highly appreciated. DOI: 10.1002/adem.201300556 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com I ADVANCED ENGINEERING MATERIALS 2014, FULL PAPER