PSI - Issue 59

Valerii Kobzar et al. / Procedia Structural Integrity 59 (2024) 344–351 Valerii Kobzar and Oleh Derkach / Structural Integrity Procedia 00 (2023) 000 – 000

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1 – beam specimen; 2 – lower die; 3 – spherical indenter; 4 – strain gauge; 5 – AE sensor. а

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Fig. 1. (a) Equipment for quasi-static punching. (b) Schematic diagram of the AE signal analyzer.

The parameter by which we will predefine the damage parameter, D , is the ratio of the AE count for partial damaged specimen to the cumulative count during quasi-static perforation of the specimen at a speed of less than 100 μm/sec. To characterize the damage through AE, the following measures were implemented. Bandpass filtering of the range where there is a maximum response from the damage of the CFRP fibres and matrix. This range was defined by Yu et al. (2006) as 80 to 300 kHz, where it was also proposed to use an oscillatory circuit with a central frequency of 230 kHz for filtering. In the present study, a 100 – 300 kHz bandpass filter was used. When choosing the lower limit of passband, two factors are taken into account: the preservation of destruction signals and the maximum possible elimination of friction signals. AE signals from friction are assumed to have a statistically lower frequency than fracture signals. The upper range is limited to reduce the impact of radio interference and isolate the useful signal. The choice of the threshold from below, providing false positive count less than 1% of pulses for completely damaged specimen. False positive counts were caused by artificial mutual friction between the matrix, sample and indenter and tapping on them. The time for counting false positive events was chosen to be at least as long as the sample quasi-static punching time. The conditions for the generation of false positive events were the assembled structure with the contact force within the elastic zone. The upper limit of the threshold is determined by maintaining a large number of pulses during quasi-static punching.