PSI - Issue 39

J.C. Ehrström et al. / Procedia Structural Integrity 39 (2022) 98–103 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Service Goal with a safety factor of 2, or justifies a DSG of 70000 cycles with a safety factor of 3. As a reference, the DSG of Airbus A220 is 60000 flights, Duprat (2019).

5. Discussion The FML test panel results show that very large inspection intervals can be achieved with this lower wing skin material while imposing higher fatigue stresses than for the 2024 reference. The 1g stress is increased by 20% and yet the inspection interval derived from the test is 45000 flights (with a safety factor of 3), that is 3 times longer than that of recent aircraft like Airbus A220. In addition, the density of the FML panel with 2024 metal layers is 2.58 kg/dm 3 , representing a 7% density benefit versus the bulk 2024 reference. The bonded stringers of the present test are in 2027 T351, but Al-Cu-Li stringers could have been used as well, with 5% density benefit. So, all together, the weight saving potential of the tested concept is approximately 25% on the wing cover, while allowing inspection intervals of 45000 flights. The test is performed on the most critical (FCG) requirement on the current metallic lower wings, according to several major OEMs, Hunt (1999). The question that comes then, in two aspects, is: (a) could a higher stress have been applied and further weight savings been claimed? (b) would the claimed benefit be significantly reduced because of other design drivers limiting the design? A global wing study based on a Finite Element Model of the total wing was used to assess the effect of FML lower wing concepts on the overall wing aeroelastic response and weight, Fabre et al. (2017). The platform is the CeRAS (Central Reference Aircraft Data System) CSR – 01, close to Airbus A320 and Boeing B737, and includes aerodynamic, landing gear and engine loads. The model specifically addresses the multiaxial loads inducing shear that might be limiting FML efficiency, depending on the lay-up. Multiaxial yield and blunt notch failure envelops are included. The result of the model is a large series of concepts resulting from a multifactorial genetic algorithm computation. Table 2 gives the weight results of the model for 2 concepts. The spars and upper wing are part of the model, but their design allowables are fixed. The weight saving on the lower cover: -19% and -33% encompass the -25% weight saving that result from the present study. Table 2 shows that, although the upper wing and spar weight are slightly modified, a beneficial impact of FML on the lower wing weight in the order of -25% can be achieved while meeting the other design requirements and increasing the inspection interval by a factor 3. This confirms the applicability of the present study at the levels of stress that were chosen for the test.

Table 2. Results from a previous global wing study versus present results. Case Upper cover

Lower cover

Spars

FCG rate Reference 67% slower 50% slower 67% slower

Baseline with 2024 lower wing

738 kg

978 kg

338 kg

Concept 1 with FML lower wing skin Concept 2 with FML lower wing skin

761 kg (+3%) 697 kg (-6%)

786 kg (-19%) 655 kg (-33%)

368 kg (+9%) 360 kg (+7%)

Current proposition with FML lower wing skin

NA

-25%

NA

6. Conclusion A 5-stringer bonded panel with a FML skin based on 0.8 mm sheet is tested in fatigue in order to validate the benefit of this material in damage tolerance: very large inspection intervals combined with a high fatigue stress level are achievable. The results justify a 20% increase in 1g – stress allowable versus bulk 2024 T351, with an inspection interval of 45000 flights, typically 3 times as much as a conventional interval for short/medium range aircraft. The total life of the panel in which a severe initial defect is cut (2 x 3.5 mm on both sides of an 8 mm diameter rivet), exceeding 220000 cycles, is consistent with a Design Service Goal of 90000 flight cycles.