13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- The significance of two-phase plasticity for the crack initiation process during very high cycle fatigue of duplex steel Alexander Giertler1,*, Marcus Söker1, Benjamin Dönges2, Konstantin Istomin2, Ullrich Pietsch2, Claus Peter Fritzen2, Wolfgang Ludwig3, Hans-Jürgen Christ2, Ulrich Krupp1 1 Faculty of Engineering and Computer Science, University of Applied Sciences Osnabrück, 49009 Osnabrück, Germany 2 Faculty IV: Science and Technology, University of Siegen, 57068 Siegen, Germany 3 European Synchrotron Radiation Facility, 38000 Grenoble, France * Corresponding author: a.giertler@hs-osnabrueck.de Abstract The present paper reviews experimental results on the fatigue damage of grade 1.4462 austenitic-ferritic duplex steel in the very high cycle fatigue (VHCF) regime. Electrolytically polished miniature and bulk specimens have been fatigued in an ultrasonic fatigue testing machine while the surface is observed in-situ by an optical microscope. The pre-fatigued miniature specimens are investigated by synchrotron diffraction contrast tomography (DCT) to reveal three-dimensional crystallographic orientation data. These data are used for finite element modeling in combination with a material model accounting for elastic anisotropy and crystal plasticity to predict crack initiation sites. The bulk specimens are carefully analyzed by means of scanning electron microscopy (SEM) in combination with electron back-scatter diffraction (EBSD). Under VHCF loading conditions, slip band formation is limited to the softer austenite grains – in particular at twin boundaries. Once being formed, the bands generate high stress concentrations where they impinge the austenite-ferrite () phase boundaries, eventually, leading to crack initiation. The results are discussed by means of a numerical modeling approach that is based on (i) the finite element method (FEM) mentioned above and (ii) a crack initiation model proposed by Tanaka and Mura [1] and Chan [2]. Keywords Duplex steel, VHCF, short fatigue cracks 1. Introduction The fatigue life of metallic materials is determined by the plastic strain amplitude. The amplitude level determines the effective damage mechanism. For example, during LCF loading an early formation of slip bands can be observed, which leads to initiation and formation of long fatigue cracks [3]. In the HCF regime, the macroscopic strain amplitude is lower, but the microscopic strain amplitude can reach a critical value due to the elastic anisotropy of the microstructure, which results in the formation and propagation of microstructurally short fatigue cracks. In the VHCF range, the initiation and propagation of fatigue cracks is mainly influenced by the microstructure. Therefore, a large scatter can be observed in the fatigue life data. Life-determining factors are size, shape and distribution of non-metallic inclusions and the ability of grain and phase boundaries to block cyclic slip or microcracks. Duplex stainless steels exhibit a good combination of high strength, ductility and corrosion resistance [4]. Many applications imply cyclic loading, for instance the power transmission in off-shore boats. To ensure the reliability of such machines the knowledge about the fatigue behavior is essential. The assessment of the fatigue life is usually based on Wöhler data, but environmental effects, single overloads or the influences of varying microstructural impurities are often not taken into account, which leads to a non-conservative prediction. The present paper gives some evidences about the fatigue damage mechanism for ferritic austenitic duplex stainless steel in the VHCF regime and supports the hypothesis that phase boundaries act as effective barriers against fatigue crack initiation. 2. Experimental Procedure The behavior of microstructurally short fatigue cracks were investigated in the VHCF regime on the austenitic ferritic duplex stainless steel DIN 1.4462. Duplex steel exhibit good strength and ductility values paired with excellent corrosion resistance [4]. The chemical composition of the material and the heat treatment pa-
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