Issue 53

D. Wang et alii, Frattura ed Integrità Strutturale, 53 (2020) 236-251; DOI: 10.3221/IGF-ESIS.53.20 241 Figure 6: Infrared monitoring device. D EPLOYMENT OF AE SYSTEM AND SENSORS his paper adopts the Micro-II multi-channel digital AE system with a PCI-E acquisition card. As shown in Fig. 7a, fifteen AE sensors were deployed on the plate surface to record the AE signals at different positions, considering how plate cracking affects the generation and propagation of AE signals under fire. The working principle of AE is illustrated in Fig. 7b. Tab. 1 describes the setting of AE system and sensors. (a) AE mesuring points (b) Working principle of AE Figure 7: Layout of AE measuring points and working principle of AE. Sensor Sampling frequency (kHz) Threshold (dB) Preamplifier (dB) Waveform setting Number of sensors (each) R6-A resonance type 20~100 40 40 Sampling rate (MSPS) Length (number of points) 15 Table 1: Setting of AE system and sensors. P LATE DAMAGE PHENOMENA Macro phenomena rom 0 to 1 min at room temperature, as the load applied by the jack gradually reached 50 kN, there was no obvious cracking or deformation on the plate surface. From 14 to 15min, the load increased from 50 to 100 kN. During this process, five fine cracks appeared almost simultaneously on the plate surface on the east and west long sides, respectively. The cracks were symmetrically distributed about the central axis of the long sides, with a spacing of 15-18 cm. The cracking positions basically overlap those of steel bars on the plate surface. Besides, cracks 1# on both east and west sides propagated to the center of the column. The crack width on the edge of the plate fell between 0.20 and 0.40 mm. From 26 to 28 min, the load further grew from 100 to 150 kN. During this process, two new cracks emerged on the east long side, and one on the west long side. At 30 min, two more cracks were observed on the east long side, and five on the west long side. The crack development was accompanied by a crisp sound similar to that of grains falling on the ground. T F