PSI - Issue 60

Chinnam Sivateja et al. / Procedia Structural Integrity 60 (2024) 245–255 Sivateja et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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3. Process Description – Chemical milling and anodization Chemical etching was conducted in an etching bath of 10 % Nitric acid in water, at the temperature of the bath – 80 ± 5 °C, ensuring the etching only at the designated places by masking the rest area and after degreasing with Tri chloro-ethylene. The etching rate is approximately 0.9 mm to 1.0 mm/hour, and parts were washed in hot water (60 70°C) and dried afterwards. The standard chromic acid anodization process has been used to protect aluminium alloys against corrosion and act as the base for painting; the average thickness of the anodic coating measured is about 10±3 microns. 4. Testing Equipment and Methodology Tensile tests were performed on ten specimens of each type following the ASTM standard (see ASTM E8). The tests were conducted using a 50 kN servo-hydraulic computer-controlled machine at a 0.5 mm/min displacement rate. An extensometer with a gauge length of 25 mm and a travel of 12.5 mm was employed to measure strain. The specimens were loaded until fracture, and load-displacement data were continuously recorded. The percentage elongation was determined using the manual marking method, measuring the change in gauge length between two marked lines before and after fracture. A standard practice was followed for the fatigue tests, see ASTM E466, for conducting force-controlled constant amplitude axial fatigue tests of metallic materials. The tests were performed at a Stress Ratio (R) of 0.1 and a frequency of 7 Hz, using a computer-controlled 50 kN servo-hydraulic INSTRON test machine under stress control mode, as per the test plan. The fracture of the specimen into two or more pieces was considered the failure criterion. Four or five stress levels were chosen to generate the S-N curve, and at least two samples were tested for each stress level. Before initiating the tests, the surface roughness of the specimens was measured using an AEP technologies make surface tester with a tip radius of 2 microns. The average roughness value along the length of the gage section was measured at two lines (at a length of 25 mm) on the top surface and two lines on the bottom. Samples with 25 mm x 25 mm dimensions were prepared to evaluate residual stress using X-ray diffraction. The residual stress was measured using a Bruker AXS D8 Advance with Davinci Design with Co source, employing established Sin 2 (Ψ) methodology to determine changes in the d values of a single peak. Fractography and surface micrographs were acquired using a Scanning Electron Microscope (SEM) from the Zeiss make Evo 18 for detailed analysis. A thorough study was conducted on the macro surface features of both anodized and cladded specimens. Following fatigue testing, microscopic observations of the crack surface were performed to analyze fatigue features and measure crack size. 5. Results and Discussion 5.1. Surface morphology and roughness Surface appearance at the microscale level, often called surface texture or morphology, has been examined using SEM. Fig. 2 presents the surface features observed at the micron level. In line with expectations, the Clad Aluminum Alloy (CAA) exhibited a plain surface without distinct features, except for some dent-like structures due to clad rolling, as shown in Fig. 2(a). This outcome is consistent with the cladding process, wherein pure aluminium sheets are added to the 2024-T3 alloy sheets by rolling. As a result, the surface remains relatively smooth and featureless. However, in the case of CMAS-AA, a passive oxide layer is formed during the anodization process explained Keller – Hunter – Robinson model (Schneider and Fürbeth, 2022), and the morphology of this layer contains pores that contribute to the surface roughness (Eftekhari, 2008). Additionally, after the sealing operation, a conversion layer is formed, leading to the development of a rough surface due to the formation of microcracks (Wang et al., 2016). The SEM micrographs in Fig. 2(b) visually confirm micropores within the Conversion Coated Aluminium Surface (of CMAS-AA) measuring about 10 microns in size.