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Effet de l’humidité, de la température et de la fréquence sur la tenue en fatigue de l’alliage d’aluminium 7175 T7351

Abstract : In order to improve the degree of confidence in the fatigue-life prediction methods of aircraft parts, the understanding of the main physical mechanisms induced by environment-fatigue interactions is crucial. This PhD project targets the study of the environmental effects representative of flight conditions at altitude and on the ground on the fatigue strength of an aluminum alloy particularly used for fuselage reinforcement parts. The comprehension of the respective role of temperature, humidity and low frequencies on fatigue life and especially on the propagation of cracks in a 7175 T7351 alloy is therefore essential to improve the predictability of parts subjected to fatigue loading under various exposure conditions. The first part of this work consists, based on macroscopic results from high cycle fatigue and fatigue crack propagation tests, to establish the order of predominance of each individual studied parameter on the total fatigue life as well as on the resistance to fatigue cracks propagation. As it turns out, the over-tempered material does not show a significant sensitivity of the material's total life to the various environments explored. It has been observed that the decrease in frequency leads to an increase in the kinetics of cracks propagation, while the significant decrease in humidity leads to an increase in the resistance to cracks propagation. Whatever the considered environment, from inert to very humid air, the propagation mode corresponds to stage II with cleavage-like fracture. The fracture surfaces analysis reveals an effect of the environment on the marking of the striations, clearly visible in humid air, whereas they are difficult to detect under inert environment. In addition, a higher exposure rate to the environment achieved with a frequency of 10-4Hz has shown that in humid or very humid ambient air, the fatigue crack growth rates are generally similar. Nevertheless, the decrease in frequency in dry air would probably be responsible for a significant decrease in the resistance to fatigue crack propagation. Additional analyses were carried out in order to assess the effect of environmental exposure on crack tip plastic activity or the amount of trapped hydrogen. The comparison of these results made it possible to relate an increase in trapped hydrogen with a significant lattice rotation, particularly localized at the crack-tip, in a case of long exposure times to humid air. Results seems to plead for an action of hydrogen during the environmental fatigue coupling of the material in humid air of the HELP (Hydrogen Enhanced Localized Plasticity) and / or AIDE (Adsorption Induced Dislocations Emissions) types.
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Submitted on : Wednesday, September 7, 2022 - 3:52:11 PM
Last modification on : Thursday, September 8, 2022 - 3:40:12 AM


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  • HAL Id : tel-03771729, version 1



Sarah Saanouni. Effet de l’humidité, de la température et de la fréquence sur la tenue en fatigue de l’alliage d’aluminium 7175 T7351. Autre. ISAE-ENSMA Ecole Nationale Supérieure de Mécanique et d'Aérotechique - Poitiers, 2021. Français. ⟨NNT : 2021ESMA0018⟩. ⟨tel-03771729⟩



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