AA2618 aeronautical aluminum alloy has been largely used in the past, especially in well-known Concorde aircraft, developed during sixties decade. In more recent aircraft, this alloy has been largely replaced by others such as 7075 which present greater fatigue resistance. Forgotten for a time, AA2618 comes back in new aircrafts for structural parts submitted to fatigue loading at high temperature because of only a slight decrease of fatigue resistance of this alloy compared to room temperature fatigue resistance. In this paper, a complete fatigue characterization of 2618-T851 aluminum alloy is presented: fatigue tests under uniaxial tensile or torsion cyclic loadings, with mean tensile or shear stress have been realized; fatigue tests under combined tensile-torsion, in or out-of-phase have also been conducted as well as some combined tensile-torsion-internal pressure fatigue tests. All these tests covered 10 4-10 7 cycles range. At last, Crossland multiaxial fatigue criterion has been used and extended to median fatigue life domain to analyze these results.
Anodized Aluminum alloys are widely used in aeronautic construction due to their specific mechanical properties. However, anodization process often leads to a decrease of the fatigue resistance of the alloys. In order to identify and characterize the different mechanisms involved in the detrimental effect of anodization of 2618-T851 alloy on its fatigue life and to determine the impact of loading nature, several tests have been performed on specimens with different surface states at various stress ratios. It was found that roughness of machining has no effect unlike the stress ratio or mean stress in tensile tests. The tests on the pickled, anodized, impregnated and sealed specimens showed it was the anodic oxidation step which was the more detrimental for fatigue resistance under tensile loading comparing to the other steps. It has been also observed that no such detrimental effect occurred under torsion loading. Concerning the prediction of fatigue life, two critical plane-based analysis approaches have been used (Morel and Fatemi-Socie criteria) to make fatigue life prediction for uniaxial and multiaxial fatigue test. Comparisons showed that both criteria gives overestimated fatigue life for uniaxial tensile loading under compression mean stress and underestimated fatigue life for tensile-torsion in phase loading.
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