PSI - Issue 5

Nikolai Kashaev et al. / Procedia Structural Integrity 5 (2017) 263–270 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

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To investigate the variation of the effectiveness of crenellations with different materials, fatigue tests were carried out on flat and crenelated panels made of the two materials respectively. Square-shaped specimens (560 mm × 560 mm) representing parts of fuselage panels were cyclically loaded in two perpendicular directions, which is aimed at imitating the periodic fuselage load due to the repeated cabin pressurization and depressurization among flights. The specimens were cut from the metal sheets of both materials. The outer part of the specimens has the original thickness of the sheet, while the inner region (400 mm x 400 mm) was machined to be either flat (thickness: 2.9 mm, machined symmetrically from both surfaces) or crenellated (equivalent thickness: 2.9 mm, machined only on one side of surface). The geometry of the crenellation pattern is depicted in Fig. 1(b), which is in accordance with previous experiments (Lu et al., 2015b). The specimens were horizontally fixed in a biaxial servo-hydraulic testing machine with specially designed clamping device to ensure an even distribution of the exerted load (Fig. 1(c)). Constant amplitude cyclic loading was applied in both axes of the machine in phase ( F major, max = 112.4 kN, F minor, max = 56.2 kN) with load ratio R F = F min / F max of 0.1 and frequency of 3Hz. The fatigue crack was initiated at both tips of a 12 mm center notch (tip radius: 0.15 mm), which aligns parallel to the rolling direction of the sheet materials and perpendicular to the major loading direction. Every time the crack had an approximately 1.5 mm extension at both crack tips, the cyclic loading was halted and the positions of crack tips were recorded using a traveling camera under the specimen. The crack opening load was measured by a  5 clip gauge mounted at one crack tip, which is the same as used by Heerens and Schödel (2009). However, in contrast with the stationary clip gauge in their work, the clip gauge in this study was remounted to the new crack tip, every time when the crack extended 10 mm further (Fig.1(d)). During the remounting of clip gauge, two indents with 5 mm distance were firstly made above and below the crack tip. After fitting the two tips of the clip gauge into the indents, fast glue Z70 from the HBM Company was applied to both tips to ensure a good attachment to specimen surface. The glue was afterwards dissolved using a commercial acetone EMSURE® before the next remounting. During the fatigue test, in each load cycle 100  5 values and the corresponding instantaneous load were recorded with equal time interval. The data in uploading phases was used to extract the crack opening load using the compliance method according to ASTM E647. The 3D morphology of the fracture surface was measured using a VK-9700 Keyence color 3D laser microscope. The angle between the shear lip plane and the fracture plane was measured on the reconstructed 3D morphology model using the software VK Analyzer 2.5.0.1.

3. Results

3.1. Microstructure and microtexture

The grain structure of AA2139 and AA2198 are compared in Fig. 2(a-b). The AA2139 sheet shows slightly elongated grains in the rolling direction (Fig. 2(a)). The averaged grain sizes in the rolling direction and in the normal direction are 45.5µm and 24.3 µm respectively. In contrast, AA2198 exhibits highly elongated grains also referred to as pancake structure (Fig. 2(b)). The averaged grain size in the normal direction is 3.7 µm whereas in the rolling direction the grain size can exceed 1 mm. The aspect ratio is more than 400:1. Both materials also show distinct textures as shown in Fig. 2(c-e). AA2139 is characterized by a very weak <100>//RD fibre texture with a cubic texture component {0 0 1}<1 0 0> (Fig. 2(c)). No variation in terms of texture in the thickness direction was found. AA2198 shows very sharp Brass texture {1 1 0}<1 1 2> in the center of the panel (Fig. 2(d)), whereas near the sheet surface strong S texture components {1 2 3}<6 4 3> appears (Fig. 2(e)).

3.2. Fatigue tests

The results of the fatigue tests (Fig. 3(a)) show that, under the same loading condition, panels made of AA2198 generally have higher fatigue resistance than the panels of the material AA2139. The fatigue life improvement by crenellations in the material AA2198 is two times of that in the material AA2139.