PSI - Issue 39

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

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Fig. 2. Initial defect (upper left) and position in a view parallel to the stringers (left) and parallel to the rib, in the transverse direction (right).

The initial defect is cut by water jet. It has a 3.5 mm length on each side of a 8 mm diameter fasterner hole (Fig. 2). The FML skin is bonded in one step in an autoclave. The stringers are bonded onto the skin in a second step. All surfaces of the skin and stringers are anodized and coated with primer before bonding. 3. Testing conditions A 2500 KN servohydraulic testing machine is used to perform the test. Hydraulic grips are used to impose a controlled displacement that is periodically checked using the load cell signal and strain gauges to ensure the desired stress is maintained. The crack length is monitored by visual inspection and by Thermoelastic Stress Analysis, see Strain Solutions Ltd website in reference. The test aims at validating the behavior of a lower wing cover with a 1g stress ( σ 1g ) increased by 20% versus the 2024 T351 bulk reference, and providing the basis for increased inspection intervals. A relatively high 1g stress for 2024 T351 is 75 MPa, proposed by Torenbeek (2013). So, the 1g stress of the FML cover should be 90 MPa. A real lower wing will see a variable amplitude loading around this 1g stress. The exact spectrum used (for example the truncation level) has a significant impact on the number of flights that would be obtained, typically a factor 2 or more Schijve et al. (1984). Previous studies used constant amplitude loading to validate materials and design concepts, in particular the FML lower skin in a wing box section by Embraer: Mendonça et al. (2020). So the choice was made to test under constant amplitude loading. The question is then to determine the constant amplitude stress at R=0.1, σ max , that would represent the spectrum in the sense that the number of constant amplitude cycles would be similar to the number of flights for a real spectrum regarding crack growth. This is a common problem; the ratio: = 1 is called the spectrum factor (SF). The decision is to take SF = 1.5, so that σ max = 135 MPa, for σ 1g = 90 MPa. The first justification is to simply note as Ş en (2015) that for many aircraft the peak stress of spectra is approximately