Issue 46

A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 306 Fatigue crack propagation in welded joints X70 Adel Deliou, Benattou Bouchouicha Laboratory of Materials and Reactive Systems LMSR, University Djillali, Liabes, Sidi Bel-Abbes, Algeria . del032003@yahoo.fr , benattou_b@yahoo.fr A BSTRACT . Structural failure assessment approaches take into account local parameters, specimen geometry, loading and material. In the case of welded joints, in addition to these parameters, consideration must be given to the effect of the heterogeneity of properties due to welding. The objective of our work is to study the fatigue crack propagation of welded joint in API X70 pipeline steel. This experimental study focused on welded joints in the different parts, base metal, weld metal and heat affected zone. The concepts of fracture mechanics are used to analyze the harmfulness of defects in welded joints and the main part of fatigue life falls on the crack propagation. The results obtained show that the fatigue crack propagation rate of cracks in the heat affected zone is delayed compared to the other zones. The effect of the microstructure and the quality of submerged arc welding of the studied X70 steel are significant. Tensile tests, hardness and measurement of energetically parameters complemented this work. K EYWORDS . X70; Mechanical behaviors; Fatigue crack propagation; Energy. Citation: Deliou, A., Bouchouicha, B., Fatigue crack propagation in welded joints in X70, Frattura ed Integrità Strutturale, 46 (2018) 306-318. Received: 16.08.2018 Accepted: 03.09.2018 Published: 01.10.2018 Copyright: © 2018 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION elding is one of the important assembly methods for manufacturing in steel structures. In current industrial practice, welded joints are an integral section of these complex configurations [1]. In general, fatigue behavior of welded materials is complicated by large variation in nature and properties of materials, presence of geometric defects and many factors intrinsic (inclusions, lack of penetration, gas pore), etc. [2-8]. Many studies show that, from such defects, the stage of initiation of the fatigue crack can be reduced [9] and that consequently a large part of the life of the welded joints subjected to fatigue propagation occurs [10]. For the design of these assemblies, it is interesting to use the crack propagation laws given by the Linear Elastic Fracture Mechanics [LEFM]. In order to estimate the life of the joints, by calculating the number of cycles needed to propagate a crack from these defects until the rupture [11-21]. Welded structures present a gradient of microstructure and mechanical behaviors from the weld metal to base metal. Important studies regarding the microstructural change and the mechanical behaviors of API X70 steel have been executed [22- 24]. Nanninga [25] examined the fatigue crack propagation of API X42 and APIX70 pipelines at a load ratio R = 0.1 under H 2 or N 2 environment. The presence of hydrogen shows an important damage compared to Nitrogen. The W