A study of microcrack formation in multiphase steel using representative volume element and damage mechanics V. Uthaisangsuk a,⇑ , S. Muenstermann b , U. Prahl b , W. Bleck b , H.-P. Schmitz c , T. Pretorius c a Department of Mechanical Engineering, King Mongkut’s University of Technology, Thonburi, 126 Prachautit Rd., Bangmod, Tungkru, Bangkok 10140, Thailand b Department of Ferrous Metallurgy, RWTH Aachen University, Intzestr. 1, D-52072 Aachen, Germany c ThyssenKrupp Steel Europe AG, Kaiser-Wilhelm-Strasse 100, D-47166 Duisburg, Germany article info Article history: Received 5 October 2009 Received in revised form 18 May 2010 Accepted 3 August 2010 Available online 1 September 2010 Keywords: Multiphase steels Representative volume element Damage curve Microcracks abstract Multiphase steels have become a favoured material for car bodies due to their high strength and good formability. Concerning the modelling of mechanical properties and failure behaviour of multiphase steels, representative volume elements (RVE) have been proved to be an applicable approach for describ- ing heterogeneous microstructures. However, many multiphase steels exhibit inhomogeneous micro- structures which result from segregation processes during continuous casting. These segregations lead to a formation of martensite bands in the microstructure causing undesirable inhomogeneities of mate- rial properties. The aim of this work is to develop an FE evaluation procedure for predicting a microcrack formation provoked by banded martensitic structures. A micromechanism based damage curve was applied as a failure criterion for the softer ferritic matrix in the microstructure in order to simulate the propagation of cracks resulting from the failure of martensitic bands. The parameters of the damage curve were determined by in situ miniature bending tests and tensile tests with notched samples. The presented approach provides the basis for an assessment criterion of the component safety risk of multiphase steels with inhomogeneous microstructures. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction A variety of high strength steels like low-alloy dual phase (DP) and transformation induced plasticity (TRIP) steels have been developed for the automotive industry with respect to the increas- ing demand for lighter, more deformable, and high energy absorb- ing materials [1,2]. These steels exhibit excellent mechanical properties as they combine high strength and good ductility com- pared to conventional steels of similar strength. This favourable balance of properties is owing to the existence of different phases in their microstructures. Modifications of the chemical composi- tion, thermomechanical processing or heat treatments allow vari- ous microstructure formations. Type, shape, size, fraction, and spatial distribution of the different phases are in charge of the overall behaviour [3–5]. Due to the multiphase microstructures, numerical FE simula- tion techniques using representative volume elements (RVE) as a submodel implemented to macroscopic global models have become a promising method for quantitatively correlating micro- structure and mechanical properties of materials. However, the RVE is usually generated assuming a homogeneous distribution of microstructural constituents. Hence, the effect of segregations and similar microstructural inhomogeneities has not yet been incorporated adequately in the RVE simulations. Indeed, it has been reported that segregations significantly alter the failure behaviour of multiphase steels. It was noted that multi- phase steels show a tendency to contain segregations as a result of high alloy contents and specific thermal cycles during the continu- ous hot dip galvanising process [6]. Assuming fast cooling condi- tions, there is not enough time for carbon diffusion and ferrite nucleation and thus no banded structure results. However, fast cooling only suppresses the formation of a banded microstructure, but the reason for banding, i.e. the microsegregation, cannot be re- moved [7]. In consequence, the bands will reappear when a speci- men with an inhibited banding is reheated and cooled down slowly. The effect of cooling rate on the formation of martensite bands has also been investigated by Thompson and Howell [8]. They noticed that banding is much less pronounced in samples ta- ken from the edge of a hot rolled sheet where the cooling rate is higher than in the centre of the sheet. It is furthermore possible to obtain banded ferrite/martensite microstructures by holding material in the austenite/ferrite two-phase region and subsequent fast cooling [9]. The effect of banded martensitic structures on mechanical prop- erties has also been investigated by several authors. Nevertheless, 0927-0256/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2010.08.007 ⇑ Corresponding author. Address: Department of Mechanical Engineering, King Mongkut’s University of Technology, Thonburi, 126 Prachautit Rd., Bangmod, Tungkru, Bangkok 10140, Thailand. Tel.: +66 2 470 9274; fax: +66 2 470 9111. E-mail address: vitoon.uth@kmutt.ac.th (V. Uthaisangsuk). Computational Materials Science 50 (2011) 1225–1232 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci