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Étude de l'influence des défauts de soudage sur le comportement plastique et la durée de vie en fatigue de soudures par friction-malaxage d'un alliage Al-Cu-Li

Abstract : With the aim of reducing aircraft weight, a friction stir welded Al-Cu-Li alloy (2198-T8) is considered by aircraft manufacturers for its use in fuselage or lower wing skin applications. The purpose of this study is to assess the fatigue behaviour of friction stir welds and the influence of some weld defects on fatigue properties for a target lifetime of 105 cycles. Base material, sound welds (produced via welding in one sheet), welds bearing a joint line remnant (due to the native oxide layer present on butt surfaces before welding), a kissing bond (due to a lack of tool plunge depth), and welds with an initial gap between the two butt surfaces, were tested. For the targeted lifetime, cross-weld specimens were plastically deformed during the first half of the first cycle. During cross-weld tensile tests, macroscopic properties were not altered by weld defects, although kissing bond and GAP defects acted as fracture initiation sites. In situ SEM tensile tests showed a threshold stress, beyond which the opening of the kissing bond is observed. The study of the tensile behaviour of the weld is enriched by a 3D finite element simulation. S-N curves showed that the joint line remnant did not generate a significant decrease in weld lifetime, whereas the kissing bond and GAP defects conducted to a decrease in the fatigue strength at 105 cycles by 17% and 28%, respectively. The study of initiation mechanisms revealed that those two defects were fatigue crack initiation sites only beyond a threshold value of stress. For higher values of stress, the fatigue crack initiation stage was shown to be reduced to the defect opening during the first half of the first cycle. The final weld fracture was studied through ductile tearing tests, revealing an evolution of defect harmfulness similar to the one found in S-N curves.
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Submitted on : Friday, July 20, 2012 - 1:37:13 PM
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  • HAL Id : pastel-00719638, version 1

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Thomas Le Jolu. Étude de l'influence des défauts de soudage sur le comportement plastique et la durée de vie en fatigue de soudures par friction-malaxage d'un alliage Al-Cu-Li. Matériaux. École Nationale Supérieure des Mines de Paris, 2011. Français. ⟨NNT : 2011ENMP0102⟩. ⟨pastel-00719638⟩

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