Issue 53

D. Wang et alii, Frattura ed Integrità Strutturale, 53 (2020) 236-251; DOI: 10.3221/IGF-ESIS.53.20 245 macroscopic phenomena, the cumulative number of events at all measuring points rocketed up, with growth rates of 82.8, 98.2, 62.5, 65.0, 34.7, and 94.1 %, respectively. The growth rates at 8# and 15# both surpassed 90 %. Hence, in the heating phase, the area of plate-column nodes was still more active and more severely damaged than the other places. The relatively large cumulative number of events at 7# signifies the generation of many diagonal cracks. In the third phase, the fire was turned off, and the furnace cooled down. In this case, the cumulative number of events rose rapidly for a while by less than 10 % and then stabilized. (a) 7-channel sensor (b) 8-channel sensor (c) 11-channel sensor (d) 12-channel sensor (e) 13-channel sensor (f) 15-channel sensor Figure 13: Variation in the cumulative number of events of S3 with time and furnace temperature. Event rate, energy rate and b-value The event rate reflects the change of events measured per unit time, providing an indicator of crack density. The AE energy (unit: mv ∙ μs) is not an energy in the physical sense. This parameter is defined as the area under the envelope of the signal waveform, reflecting the signal intensity. Generally, the square of the amplitude and duration of AE signals can both serve as energy parameters. The internal damage or surface cracking that emit acoustic waves are often accompanied by energy generation. The change of energy measured per unit time, i.e. energy rate, can reflect the degree of cracking and internal damage of the concrete. The b-value, an important parameter in seismic analysis, was originally used to characterize the relationship between seismic frequency and magnitude. With the proliferation of AE techniques, b-value has been adopted to analyze the AE signals of building structures under fire. The relationship between the number of events N and the amplitude A can be expressed as: 0 100 200 300 400 500 600 0 2000 4000 6000 8000 10000 12000 14000 Cumulative number of events Mean furnace temperature Ignition 105min Flameout 315min Cumulative number of events (each) Time (min) Temperature (℃) 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 0 100 200 300 400 500 600 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 105min 315min Cumulative number of events (each) Ignition Flameout Cumulative number of events Mean furnace temperature Time (min) Temperature (℃) 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 0 100 200 300 400 500 600 0 2000 4000 6000 8000 10000 12000 105min 315min Cumulative number of events (each) Ignition Flameout Cumulative number of events Mean furnace temperature Time (min) Temperature (℃) 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 0 100 200 300 400 500 600 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 105min 315min Cumulative number of events (each) Flameout Ignition Cumulative number of events Mean furnace temperature Temperature (℃) Time (min) 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 0 100 200 300 400 500 600 0 2000 4000 6000 8000 10000 12000 14000 105min 315min Cumulative number of events (each) Flameout Ignition Cumulative number of events Mean furnace temperature Temperature (℃) Time (min) 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 0 100 200 300 400 500 600 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 105min 315min Cumulative number of events (each) Flameout Ignition Cumulative number of events Mean furnace temperature Temperature (℃) Time (min) 0 100 200 300 400 500 600 700 800 900 1000 1100 1200