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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approximately 2-3 times decrement in fatigue life in chemically milled samples compared to electropolished samples (Sesana et al., 2019; Spear and Ingraffea, 2013; Sefer et al., 2016). This is because the chemically milled samples initiate cracks early from pits formed during the milling (Spear and Ingraffea, 2013), and it also can be related to the general detrimental effect of surface roughness on fatigue life. However, corrosion protection makes it compulsory to have anodized and sealed for life retention, sometimes though they are detrimental to fatigue life further. The existing literature proves that any surface process applied to aluminium alloys can reduce their fatigue strength (Suersh, 1998). This reduction creates a constant tradeoff between the aircraft's calendar life, influenced by corrosion prevention measures, and the technical life of the aircraft's structural integrity under fatigue loading. This study addressed the limited availability of fatigue data and the variation in manufacturing processes employed for chemical milling and anodization in the industry. The primary aim of this study is to develop stress-life curves for design purposes, focusing on two distinct cases. The first case involves using Cladded Al alloy (CAA) 2024-T3 sheet specimens with a thickness of 1.6 mm, as they are procured and commercially available. The second case involves Chemically milled, anodized, and sealed (CMAS AA) sheet specimens with a thickness of 1 mm. Given that several parameters, such as size, surface finish, and residual stresses can influence fatigue life, the study attempts to quantify these factors and establish correlations with the fatigue life of the specimens. By doing so, it seeks to provide valuable insights into the performance and reliability of corrosion protection methods and their impact on the overall static and fatigue behaviour of the aircraft's structural components. It also provides valuable insights and design values to designers. 2. Material and Test Specimens Two sets of static and fatigue samples, made of Aluminium 2024-T3 alloy, have been prepared for testing purposes testing a) Cladded Al alloy (CAA) 2024-T3 sheet specimen, 1.6 mm thickness (As procured and commercially available) b) Chemically milled, anodized, and sealed (CMAS-AA) sheet specimens, 1 mm thick. The thicknesses are chosen considering the typical sizes used in aircraft structural details and local thickness reduction requirements. These specimens were prepared following the guidelines of ASTM for tensile testing E8 (see ASTM E8) and E 466 (see ASTM E466) with an hourglass geometry (continuous radius between ends) for fatigue testing, as depicted in Fig. 1. All the specimens are prepared in the longitudinal direction (L) of rolling aligned with the loading axis. To maintain the integrity of the surfaces containing Cladding or anodization, the specimens were machined from a sheet using a saw-cutting process, followed by conventional milling. Care was taken during this milling process to prevent any alterations to the cladding or anodized surfaces. Additionally, the fatigue specimens' short-machined edges (thickness side) were meticulously buffed using emery paper to minimize the risk of fatigue cracks originating from the milled side edges. This process ensured the removal of any tool marks, scratches, or imperfections that could impact the test results. A total of ten samples were prepared for both tension and fatigue testing in each type of specimen. The fatigue specimens were selected to ensure that at least one stress level test was conducted at each decade of their operational life.

Fig. 1. Specimens dimensions used for (a) Tensile testing and (b) Fatigue testing (all dimensions are in mm)