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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Despite the 3-D character of the surface deformation relief, the 2-D digital images of the relief are very much informative. Optical images of the deformation relief have distinguished areas of relief pattern and areas without signs of microplastic deformation. Cycle by cycle the number of grains involved in the damage accumulation process increases; simultaneously, the height of extrusions and the depth of the intrusion, as well as a number of persistent slip bands grow. The extrusion\intrusion structure of the deformation relief is used for the quantitative assessment of the accumulated damage of alluminium alloys under uniaxial (Karuskevich et al., 2012) and multiaxial (Karuskevich et al., 2016; Pejkowski et al., 2019) fatigue loading. The relief parameters were proposed and verified by numerous experiments. The primary damage parameter D considers the intensity of the surface relief, it is the ratio of the surface area with deformation relief pattern to the total investigated area. The additional damage parameter is a fractal dimension of the surface pattern. It describes quantitatively the shape of surface relief clusters. The “box -countin g” method was used for the calculation of some types of fractal dimensions, and the dimension Dp/s has been found as a more appropriate. The fractal dimension Dp/s is based on the ratio of the perimeter of the clusters to their area. It was proved that the remaining life of the component, made of a metal exhibiting surface relief could be calculated with an empirical relationship between the number of cycles to failure and relief parameters: Direct investigation of the components surface and estimation of the fatigue damage by surface relief is a realistic way for fatigue monitoring for Al-clad alloys, like D16AT, V95, 2024-T3, 7075-T6. Some bearing components of the primary structure are not able to exhibit surface relief after cyclical loading due to their physical properties, that’s why the attachable fatigue indicator has been proposed as an alternative to the direct investigation of the surface of the components. 5. Indicator’s design Indicator (sensor) looks like a micro specimen for fatigue tests, Fig. 3 made of Al-clad alloy. D16AT and 2024 T3 are proved to be appropriate materials for indicator manufacturing. The possibility to watch the surface relief on the V95 and 7075-T6 have been found as well. One of the principal requirements of the indicator attachment to the aircraft structural component is fastening of the indicator via regular structural bolts, preventing invasion into the structure, which can cause local reduction of the structural strength.

Fig. 3. Fatigue indicator dimensions.

Assembly of the indicator attached to the structural component (specimen for fatigue test) and the element’s dimensions are presented in Fig. 4. The total strain of the indicator is correspondent to the strain of the aircraft component at the base equal to the distance between the fastening points. The strain of the indicator is not a constant value along the indicator axis; it depends on the stiffness, which varies due to the different cross sections. The required level of strain and stress in the inspected cross section of the indicator can be achieved by the proper selection of the indicator’s geometry. This procedure is provided by the application of Finite Elements Analysis (FEA) using ANSYS software. Boundary conditions are shown in Fig. 4. The material of indicator and specimen is set to aluminium alloy. The assumed material model is bilinear elastic-plastic with Young modulus E = 68.3 GPa, Poisson ratio ν = 0.33 and hardening modulus H=2 80 MPa. Properties of the fastener elements correspond to steel. The assumed material model is linear elastic with E = 200 GPa and ν = 0.3. The structural component is fixed on one side and loaded with 10 kN force on the other side. It can deform only in the y direction. Bolt pretension is set to