PSI - Issue 36

Petro Yasniy et al. / Procedia Structural Integrity 36 (2022) 197–202 Petro Yasniy, Ihor Okipnyi, Oleksandr Dyvdyk et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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aircraft structures, particularly of cracks caused by material fatigue (Laviolette (2012), Burns (2012), Seligman (2015), Tajabadi (2016), Yasniy et al. (2020)). Surface plastic deformation is widely used to increase the lifetime of structure elements with functional and technological holes (FTI Process Specification 8101D. (1994), Lacarac et al. (2000), Zhang and Wang (2003), Yonezawa et al. (2020), Yasniy et al. (2017)). The positive effect of cold expansion holes in the structural elements on lifetime, microhardness, roughness, and residual compressive stresses is confirmed by Matos et al. (2007), Gopalakrishna et al. (2010), Yasniy et al. (2016)). (2008), Warner et al. (2014)). The effect of cold expansion on the crack growth of 2024T3 is investigated (Buxbaum and Huth (1987)). The initial crack sizes, as well as the type and parameters of loading (constant amplitude or flight spectrum) significantly affect the crack growth in plates near holes with preexisting cracks in aluminum alloys (Buxbaum and Huth (1987), Wanlin (1993), Bahloul et al. (2018)). 2. Statement of the objectives The main factors of cold expansion effect on fatigue cracks growth and residual lifetime of aluminum alloy plates with operational damage near the holes at constant amplitude loading are investigated. The aluminum alloy D16chT flat specimens with notch in the corner of the hole are tested on fatigue crack growth under uniaxial tension. Two types of specimens are used to investigate the cold expansion holes impact on the fatigue cracks growth. They are designed according to DSTU 2444-94, with the hole diameter 8, 10 and 12 mm: width 60 mm; length 120 mm; thickness 6 mm (Fig. 1).

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Fig. 1. Specimen for fatigue test: (a) sizes of specimen; (b), (c)- the crack front ( а is the crack length along the specimen thickness; с 1 - the length of the through crack on opposite specimen surfaces The first type are the specimens with notch and pre-crack in the corner of the hole perpendicular to the rolling direction without cold-expansion after pre-cracking The second type are the specimens with notch and pre-crack in the corner of the cold-expanded hole. The notch in the corner of the hole, with dimensions 0.5 x 0.5 mm is cut by Electronic Discharge Machining (EDM) with acc uracy ± 0.03 mm. After that, the specimen is subjected to cyclic loading at constant amplitude with the stress range Δσ = − = 147 MPa. The stress ratio is = ⁄ = 0.05 , the load frequency is 10 Hz (here σ min , σ max are minimum and maximum stresses). The size of initial crack on the specimen surface is ≈ 1 mm. The shape of the cycle is sinusoidal. Before the test, the areas of the crack growth surface are polished to the roughness R a = 0.05. To ensure the required accuracy of measuring the crack growth on the specimen hole surface, lines are made perpendicular to the