Assessment of fatigue life of a pre-corroded aircraft wing under drag alternating load

Abstract

The application of high strength aluminum alloys in aircraft structures is significant for the aeronautics industry. During life service, the aircraft wings undergo fatigue and the attack from corrosive chemicals of the surrounded atmosphere. The present research work studied the effect of pitting corrosion on fatigue life of the Onera M6 wing. An aluminum alloy 2024-T3 scale model (prototype) of the Onera M6 wing was built. The similarity analysis helped to calculate the dimensions of the prototype. A computational fluid dynamics (CFD) model was employed to estimate the drag coefficient in flight conditions and the airfoil’s center of pressure. The aircraft wing was corroded electrochemically in a 3.5% wt. NaCl solution at three different times 11, 33 and 66 h named low, medium and high corrosion, respectively. Then, a fatigue test was carried out on the aircraft wing subjected to an alternating drag cyclic loading. The real-time strain of the aircraft wing was measured through strain gauges and an oscilloscope during the fatigue test. The normal stress in the aircraft wing was calculated from the strain data measured experimentally. A curve of the number of loading cycles versus normal stress was built during the fatigue test for each corrosion condition. Then, a nonlinear curve-fitting was applied to the curve of number of loading cycles vs normal stress to estimate the fatigue life of aircraft wing in flight conditions. The results indicated that the fatigue life of the aircraft wing decreased up to 72% for the most severe corrosion condition. The aluminum alloy 2024-T3 presented a high corrosion rate during the electrochemical attack due to the rupture of its protective oxide layer. The most affected area by pitting corrosion was found in the extrados wall associated with higher curvature. The aircraft wing did not present cracking nor an increase in the size of the pitting corrosion by the effect of the stress concentration in fatigue testing.