Étude du comportement en fatigue d'assemblages soudés par FSW pour applications aéronautiques

Abstract : Mass reduction of aeronautic structures is an important challenge for aeronautic industries. FSW is an efficient joining process to replace rivets : the mass reduction using this welding process could reach 20% for aircraft wings. This work carried out in collaboration with Dassault-Aviation was done in the framework of ADSAM project. The FSW process significantly modifies the microstructure and the local material properties, the overall mechanical response of the joint is largely governed by the response of the different zones of FSW joint. In the present study, the global and local mechanical responses under monotonic quasi-static tension and cyclic tension-tension (R=0.1) in low cycle fatigue regime (10⁴ to 10⁵ cycles) were carried out on FSW joints made of aluminum alloys (2000 and 7000 series). Full field strain measurements were carried out on FSW welded specimens using digital image correlation techniques assuming an iso-stress configuration (σlocal = σnominal ). Additionally tension tests parallel to the welding direction have been performed on micro specimens machined in each representative zone of FSW joints. These tests allow us to determine the local mechanical response (σlocal − εlocal) of the joint. Local mechanical properties were determined for each representative weld zone (nugget, TMAZ and HAZ). The full field anelastic strain localization scenario prior to fracture shows a good correlation with the microhardness profile along the FSW joints. Under monotonic loading, fracture was observed at the strain localization zones unlike under cyclic loading. In fatigue, the fracture zones are very dispersed for each weld configuration. SEM observations led to the identification of two crack initiation mechanisms:(i) fracture of intermetallic particles or (ii) decohesion between intermetallic particles and the matrix. Digital image correlation under cyclic loading shows an elastic shakedown of the studied materials in each zone of the joint around a non null mean strain. In terms of number of cycles to failure, fatigue test results on ACO treated notched specimens (representative of structure critical locations) show that the FSW process reduce up to 50% the faigue life of the base materials. The crack initiation was associated to ACO pits. The results of this study open interesting prospects for modelling the mechanical behavior of FSW joints and to assess their fatigue life.
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Younes Demmouche. Étude du comportement en fatigue d'assemblages soudés par FSW pour applications aéronautiques. Autre. Ecole nationale supérieure d'arts et métiers - ENSAM, 2012. Français. ⟨NNT : 2012ENAM0050⟩. ⟨pastel-00838617⟩



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