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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After choosing the threshold, it is fixed for a series of experiments. In Fig. 2, the shape of the AE signal and some of its characteristics used in the analysis are shown. These include the number of threshold crossings, the maximum amplitude of the signal, the duration of the pulse, and its rise time. The AE event at the sensor output corresponds to a burst of high-frequency signal. The amplified and filtered signal from the AE sensor is shown in Fig. 2(a). In Fig. 2(b), the main signals of the AE system are shown. The output of the comparator is fed to the input of the counter, as shown in Fig. 2(c). The output signal is the reading of the counter register. This number of registered pulses of discrete AE signals during certain time interval is denoted by N . Since the AE signals of individual events are mutually uncorrelated, the probability of simultaneous events is reduced by slowing down the experimental process (quasi-static punching of the specimen) to ensure correct AE events counting.

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Fig. 2. AE signal analysis: (a) amplifier output with filtering. (b) comparator output. (c) acoustic emission count.

It should be noted that for single AE events, the envelope of the burst has a near-exponential shape, representing the decay of acoustic waves in the specimen. Therefore, number AE counts for single burst is approximately proportional to the logarithm of the maximum burst amplitude of the AE signal. The cumulative AE count resulting from quasi-static punching (perforation test) through the specimen is denoted as N Σ . Therefore, the local damage of the specimen can be determined by the following formula: D N N   . (1) It is noteworthy that nearly the same N Σ was recorded during quasi-static punching of specimens with different numbers of CFRP layers, which were 8 layers for specimens with a thickness of 3 mm and 12 layers for specimens with a thickness of 4 mm. For damaged specimens ( D = 1) with cross-section of 3 × 20 mm, the cumulative AE count was approximately 2.5∙10 5 (Fig. 3). The termination of the quasi-static punching process was determined by a substantial drop in AE activity coinciding with the contact force reaching zero.