PSI - Issue 14

Premkumar Manda et al. / Procedia Structural Integrity 14 (2019) 467–474 Author name / Structural Integrity Procedia 00 (2018) 000–000

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The damaged cooling duct is manufactured from an aluminum alloy. The thickness of the cooling duct indicates that it is made from sheet (0.15 to 6 mm thickness) material [Polmear, 2006]. The sheet material of the cooling duct has therefore been produced from cast ingot followed by various working conditions and heat treatments. Typical aluminum alloys AA1100, AA3003, Al-Mn-Mg and Al-Si are used for sheet metal applications [Davis et al., 1993; Rajan and Sharma, 2011]. These alloys are non-heat treatable and consist of a homogeneous solid solution with or without non-coherent precipitates. As a result, these alloys exhibit low strength and high ductility. The strength of these alloys can only be strengthened by strain / work hardening. Present work is thus concerned with the failure investigation of cooling duct. The detailed microstructural characterization along with chemical analysis and hardness has been utilized to explain the root cause of failure. 2. Experimental procedures Visual examination was carried out on cooling duct of fighter aircraft followed by photography in as-received condition. The chemical composition of the damaged cooling duct was determined using Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES). Hydrogen analysis was carried out using inert gas fusion technique. Microstructure of the damaged cooling duct was examined under Optical, Scanning Electron Microscopes (OM and SEM). The Electron Back Scattered Diffraction (EBSD) has been carried out to generate Inverse pole figure map in the z-direction (IPZ) of the cooling duct material. The back scattered electron (BSE) imaging mode based on atomic number contrast was employed in SEM for microstructural characterization. Electron probe micro analyzer (EPMA) line scan profiles were carried out to find the presence and enrichment of elements in the phases. Fracture surfaces of the damaged cooling duct specimens were examined in secondary electron (SE) imaging mode in SEM. Vickers bulk hardness (AFFRI hardness tester) values were measured on the damaged cooling duct using 2 Kg load for 15 secs. 3. Visual examination The photograph of the damaged cooling duct of fighter aircraft is shown in Fig. 1. The couplings marked ( 1 , 2 , and 3 ) in Fig.1 are connected with two generators ( 1 and 2 ) and one alternator ( 3 ) for cooling using ram air. The damaged area is shown in white rectangular box. The close view of the damaged area in other direction is displayed in Fig. 1b. The cracking observed is highlighted with ellipse. The cut pieces comprising close view of the crack regions are shown in Fig. 2.

Fig. 1. (a) The photograph of the damaged cooling duct; (b) The close view photograph (taken from Fig. (a) of the highlighted area.