PSI - Issue 65

I. Shardakov et al. / Procedia Structural Integrity 65 (2024) 241–247 I. Shardakov, I. Glot, A. Bykov, I. Panteleev A. Shestakov / Structural Integrity Procedia 00 (2024) 000–000

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As for the AE signals recorded on the surface of the composite, their character is significantly different. From the moment the first crack forms in the concrete body of the reinforced beam (stage 16), the number of low-frequency signals increases sharply, their level significantly exceeds the level recorded on concrete, and does not decrease with a subsequent increase in load (Fig. 3c). Such low-frequency AE signals can be correlated with the occurrence of shear cracks. They can serve as a marker of the developing process of delamination at the composite-concrete boundary, which accompanies the formation of cracks in concrete. At the same time, the level and nature of changes in high-frequency signals recorded by the sensor on the composite are generally consistent with the data recorded by the sensor on concrete. All recorded AE signals can be divided into 2 types. In accordance with the approach used by Ohtsu, 2007, Ono, 2011, type 1 signals with rapidly increasing amplitude and high average frequency are usually correlated with the formation of tensile cracks (opening cracks). Type 2 signal – slowly increasing signal with low frequency, is associated with plane shear cracks. Below are diagrams reflecting the AE signals recorded during the loading process in the coordinates Average frequency (Count / Duration time) – Reverse rate of signal increase (Rise Time / Maximum Amplitude). Figure 4a shows the classification of signals obtained for a conventional concrete beam during the entire loading process. From the presented diagram it follows that most of the signals are correlated with sources such as opening cracks. Fig. 4b demonstrates the accumulation of signals of types 1 and 2 in the time interval corresponding to stages 14–27. Here we can distinguish a stage of slow accumulation of signals of both types (up to 13000 s) and the following stage of a sharp increase in signals of type 1. The first interval corresponds to loading steps without visible cracks on the surface of the beam, the second – to the stages of the appearance of the first visible crack and the activation of crack formation. The growth rate of the of AE signals generated by shear cracks remains constant throughout all stages of loading and is significantly lower than for signals of type 1. a b

Fig. 4. Generation of AC signals when loading a conventional beam: (a) classification of AE signals by mode, (b) – total count of AC signals of mode1 (blue) and 2 (red)

When analyzing deformation processes in the beam reinforced with a composite, the nature of the distribution of signals in the high-frequency and low-frequency ranges is significantly different. In the diagrams Fig. 5 and 6, these two groups of data are highlighted in different colors (high-frequency range is blue, low-frequency is green). The sensor installed on concrete recorded signals, the bulk of which belonged to type 1. The count of type 2 signals is significantly lower, and low-frequency signals predominate among them. For signals corresponding to opening cracks, an increase in emission is noted at stages correlated with the formation of the first cracks in concrete (13000s). For signals generated by shear cracks, an increase is observed at significantly later loading steps (18000 s and onwards). This may indicate that the sensor installed on the beam body responds mainly to processes occurring specifically in concrete, that is, it detects the formation of opening tensile cracks in concrete. This conclusion indirectly confirms a comparison of Figures 6a and 7a, in which the distribution maps of AE signals and the accumulation diagrams of type 1 signals are qualitatively consistent.