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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was equal to 280 М P а; in indicator 2 it was equal to 26 0 М P а, in indicator 3 local stress was equal to 240 М P а. The relation of relief intensity vs number of cycles has been described by the log function, as well as it is done for numerous graphs of deformation relief evolution observed by the direct inspection of Al-clad fatigue specimens.

0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16

1

2 3

D

0

100

200

300

400

500

N, thousands cycles N, циклів

Fig. 9. Damage accumulation for three indicators of different sensitivity: 1 – indicator stress 280 М P а; 2 – indicator stress 260 М P а; 3 – indicator stress 240 М P а .

It should be mentioned here that saturation value of the damage parameter D depends on the material selected for the indicator manufacturing, thus to provide stability of the results the sheets of metal must be of the same series and

preliminary tests are required. 6. Indicator installation sites

The site for the fatigue indicator depends on the aircraft wing layout. Light aeroplanes are built according to the two arrangements: a) strut braced; b) cantilevered, with correspondent features of the loading of the wing structure. At this stage of the research and development, the activity will be focused on the cantilevered wing structure. For this arrangement the root area of the wing is most loaded; thus the fatigue indicator must be installed in this site. To provide an accurate estimation of the fatigue damage the preliminary calibrating tests of the indicators installed on the components are required. At the same time, comparative analysis of individual aircraft fatigue is possible on the base of the indicator state estimation even under the shortened preliminary tests. 7. Overstress indicator for light airplanes The practice of the fuses and mechanical safeguards application in the engineering structures and mechanisms for prevention of over regular regimes. Different types of safeguard devices are successfully used in the different fields of technics to prevent failure of the machines, equipment, and systems. Let us consider some featured examples: - Electrical fuse is an electrical safety device that operates to provide protection of an electrical circuit. Its main component is a metal wire that melts when current is too much, thereby stopping the current. - Crack indicator in the helicopter spar of the blade. The blade inspection method is a system to detect cracks in the spar of the blade. The method involves pressurizing of the rotor blade spar with nitrogen. If a crack is present, the pressure is lost, and an indicator juts above the external surface of the blade and becomes visible to the crew. - A shear pin is a sacrificial unit designed to break in the case of mechanical overload, preventing other, more expensive parts from being damaged. It is analogous to an electric fuse. It is not the only indicator but safety unit. But it informs the operator about the problem in the machine. Simplicity and reliability are the common features of the devices mentioned above. Design of proposed Overstress Indicator resembles the described above fatigue sensor. But, whilst the fatigue indicator reacts to repeating loads by the evolution of the sensor’s surface state, the overstress indicator reacts excessive strain by its complete failure. Similar to the fatigue sensor, the sensitivity of the indicator depends on the geometry: necked working section has higher strain than any others indicator’s section. The narrower the working cross section, the higher the local increase of stress.