PSI - Issue 36

M. Karuskevich et al. / Procedia Structural Integrity 36 (2022) 92–99 M. Karuskevich, T. Maslak, Ie. Gavrylov et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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182, 206, 207, and 210 aerorplanes that would require inspection for possible metal-fatigue cracking in the lower area of the forward cabin doorpost bulkhead, and any necessary repairs (Aero-news, 2015). The FAA said that following a report from an operator, investigation “revealed more than four dozen similar cracks” on affected aircraft. “This condition, if not detected and addressed, could result in failure of the wing strut attach point dur ing operation, which could result in loss of control ” it said. The FAA estimates that the AD affects 14,653 aircraft. The list of fatigue issues could be extended, but even these examples prove the requirements for the special attention for the aircraft primary structure fatigue. In the most significant extent, it is relevant to the all-metal aircraft structures. 3. Requirements for the method for fatigue monitoring The methods for fatigue monitoring for the Light Aircraft must differ from the systems which can be developed for and used on large transport planes. There is no space to accommodate complex engineering systems, consisting of the sensors, data transforming and recording devices, links between these components. As there are no processors, the indicator must be an autonomous, self-data-registering unit. As the light aircraft mass is a limited parameter, the indicator must be miniature and lightweight. The fatigue indicator must be sensitive to the operational loads contributing into the fatigue damage accumulation, as well as to be able to react on the loads after the certain number of flight cycles, which is less than the interval between inspections. The evolution of the damage parameter must be monotonic to avoid uncertainness in the interpretation of the measurements results. The informative parameter must be reflecting the nature of the fatigue damage phenomena by direct features of the fatigue process. All these requirements meet the Surface Relief Fatigue Indicator (SRFI). The indicator responds to the structural

loads by formation and evolution of the easily revealed indicator surface pattern. 4. Theoretical and phenomenological background of the proposed method

Fatigue process for some metals is accompanied by formation and evolution of the surface deformation relief (Man et al., 2009) – the pattern of surface extrusion, intrusion, persistent slip bands, Fig. 2. The deformation relief is a three-dimensional structure. The intensity of the deformation relief depends on the metal properties, stress level, number of loading cycles, parameters of loading.

Fig. 2. Surface relief extrusion/intrusion pattern (Man.et al., 2009).

The pure aluminium is a typical metal which is due to its plasticity react on deformation by the exhibiting surface relief. Pure aluminium is not used as constructional material, but it is often used for the protection of aluminium alloys against corrosion being clad layer of so-called Al-Clad aluminium alloy. The 2024-T3, 7075-T6 are well known examples of Al-Clad alloys used in the aviation industry. For aluminium layer of Al-Clad alloys, the geometry of deformation relief has been studied by the scan microscopy and the interference microscope. It was found to be equal approximately to 1 μm height for extrusions and 1μm depth for intrusions as a result of cyclical loading at the stress level 120 MPa that is close to the operational stresses in aviation bearing component made of aluminium alloys.