Materials Science and Engineering A 397 (2005) 84–91 Crack initiation and propagation close to the interface in a ferrite–austenite joint J. Besson a, ∗ , Y. Madi a , A. Motarjemi b,c , M. Koc ¸ak b , G. Martin d , P. Hornet d a Ecole des Mines de Paris, Centre des Mat´ eriaux, UMR CNRS 7633, BP 87, 91003 Evry Cedex, France b Institute of Materials Research, GKSS Research Center, Geesthacht 21502, Germany c Structural Integrity Group, TWI, Granta Park, Great Abington, Cambridge CB1 6AL, UK d EdF les Renardieres, Route de Sens - Ecuelles, 77250 Moret-sur-Loing, France Received 28 June 2004; received in revised form 21 January 2005; accepted 26 January 2005 Abstract The safety assessment of welded structures and joints still remains an important industrial problem. In this study, a simple diffusion bonded bi-material joint has been made in order to analyze the plastic mismatch effect. It consists of an assembly of ferritic and austenitic steels which are representative of nuclear pressure vessel components. Tests were performed on various specimens including smooth and notched tensile bars, Charpy V-notch specimens and single-edge notch bend specimens. Homogeneous and bimaterial specimens were tested. On deeply notched specimens, the effect of the distance between the notch root and the interface on fracture initiation and crack propagation direction was studied. Tests were modeled using elasto-plastic finite element simulations. These simulations were post-processed in order to determine crack initiation based on the Rice and Tracey criterion according to the “local approach of fracture” procedure. © 2005 Elsevier B.V. All rights reserved. Keywords: Mismatch effect; Welded joints; Ductile fracture; Rice and Tracey model; Finite element simulation 1. Introduction The structural safety assessment of welded structures (par- ticularly bi-material components) remains an important in- dustrial problem. The interaction between the welded parts, the weld metal and the heat affected zones makes the struc- tural integrity analysis difficult. Geometrical details of the weld may make the problem even more complex. Two common approaches to fracture that are usually used to analyze rupture of homogeneous elastic–plastic solids are the “global approach” (which started in the 70’s) and the “local approach” (which started in the 80’s). Applications to heterogeneous structures are more recent. The global ap- proach examines the changes in the crack driving force in terms of the J-integral [1], which may be related to the en- ergy release rate. Recently, the effect of the Q-factor [2,3], which accounts for scaling the stress triaxiality depending ∗ Corresponding author. Tel.: +33 01 60763000; fax: +33 01 60763150. E-mail address: jacques.besson@ensmp.fr (J. Besson). on the crack tip constraint, has been emphasized. The two parameters J–Q can be used to analyze the stress–strain field close to an interface between plastically dissimilar materi- als [4,5]. A three parameters approach (so called J–Q–M) has been recently proposed to analyze strength mismatched joints [6]. The need for a three parameters approach outlines the complexity of the situation to be analyzed. The second approach, referred to as local approach, has been developed in particular by the Beremin group [7,8] and considers the local stress and deformation contributions to the failure pro- cesses. This approach is able to deal with situations where no preexisting crack is present and to predict the location of crack initiation. In addition, it can be applied without any modification to welds and interfaces thus appearing as an at- tractive alternative to the global approach in these situations. An example of application to the brittle fracture of welds is given in [9]. Both approaches have been compared for homo- geneous materials in [10,11] and for heterogeneous structures in [5] showing that consistent results are obtained in case of cracked structures. 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.01.056