PSI - Issue 52

Vitalijs Pavelko et al. / Procedia Structural Integrity 52 (2024) 382–390 Vitalijs Pavelko/ Structural Integrity Procedia 00 (2019) 000 – 000

384

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mm and connected with the sheet by rivet-joint. The stringer has a longitudinal technological joint. The two parts of the stringer are connected to each other and to the sheet by a rivet joint. In such a structure, an uneven distribution of shear forces on the rivets is inevitable, which is a potential cause of reduced fatigue strength. In this experiment, the task was to assess the fatigue strength of the panel, as well as the effectiveness of the monitoring system based on Lamb wave technology. For this purpose, two groups of five piezoelectric transducers PIC 151 of dimensions of 0.5x10x50 mm were installed on the surface of sheet along the line parallel to the axis of the stringer on each side. Cyclic loading of the panel at a frequency of 10 Hz was carried out with maximum stress in the cross-section of the panel of 150 MPa and minimum cycle stress of 50 MPa. At the stage of initiation of the fatigue crack, the technical condition of the transducers periodically (after 20 thousand cycles) was monitored by measuring their electromechanical impedance and comparing it with its value before the start of the tests (baseline). After 60000 cycles, EMI measurements showed that only three transducers remained fully operational. Five transducers were damaged (usually with several cracks) with a complete loss of functional performance. The remaining two transducers had no visible damage under the applied detection method used and partially retained the ability to respond to external excitation. However, the dynamic response to excitation the impact was significantly different from the baseline. The statistics of damages is shown in the table 1. In Fig. 3 shows transducer No. 5 of the right group before the start of the fatigue test (left picture) and after their completion (right picture). On the surface of the transducer after the tests, traces of seven cracks are clearly visible, crossing the sensor across the entire width.

Figure 1. Forced panel prepared for fatigue test and installed for the fatigue test to machine GRM-20 (left picture) and the view of panel after penetration test for finding out of cracks in PET.

Table 1. Statistics of PET damages Nr of PET 1

2

3

4

5

ID of group

1 b 8

2 s 1

1 b 6

2 b 6

1 s 1

2 b 7

1 g

2 s 1

1 g

2 b 6

Quality

Cracks amount

-

-

ID of group: 1 – left side vertical group; 2 – right side vertical group Quality: g - full operability; s - partial operability; b – full loss of operability Table 1 data gives possibility to estimate the probability of three different events after 60 kilocycles of alternative