Assessment on fatigue behavior of AL7475 T7351 subjected to natural corrosion, accelerated corrosion and artificial damages

icaf2023 Tracking Number 114

The typical in-service corrosion of helicopter components is due to improper material coupling or degradation / damage of the protection coating; corroded items increase with the size of the fleet, the exposure to aggressive environment and the ageing of the aircrafts. Into the mainstream of Leonardo Helicopters plan, for European Plan for Aviation Safety 2020-2024 on ‘ageing of the fleet’, this research activity is focused on experimental activities devoted to evaluate the fatigue material behaviour in presence of several kind of corrosion pits, either from accelerated and natural corrosion, with the final objective to identify a correlation with artificial defects made by Electrical Discharge Machining (EDM), when subjected to fatigue loads. More specifically, in the context of damage tolerance assessment, the analysis focuses on the evaluation of threshold to propagation of small cracks emanating from either corrosion pits or EDM notch; the crack growth threshold and the fatigue endurance limit are combined through the Kitagawa-Takahashi diagram, defining the area of non-propagating cracks. Kitagawa-Takahashi is usually based on the crack length; the fatigue strength of metallic materials in the presence of small defects are well predicted applying the √area parameter model proposed by Murakami and Endo. Experimental activities have been performed on Aluminium Alloy 7475 T7351 in alternate bending, in presence of a defect with √area = 0.445 mm, equivalent to a semi-circular flaw of 0.35 mm radius. Evaluated defects are EDM notch, accelerated corrosion pits from galvanostatic and salt spray techniques, natural corrosion from exposition in urban and in marine environments. An ad-hoc galvanostatic procedure has been developed to promote a localized corrosive electrochemical attack. EDM notch, well-known for shape, size and technological process, are considered as a base case study and accelerated corrosion pits were generated to obtain the same value of √area. The results of the activity show a strong correlation among the threshold to propagation from accelerated corrosion, natural corrosion and the artificial flaws obtained by EDM technique confirming the applicability of Kitagawa diagrams derived from EDM notches to describe the damage tolerance behaviour in presence of corrosion pits with equivalent size.