Issue 53

D. Wang et alii, Frattura ed Integrità Strutturale, 53 (2020) 236-251; DOI: 10.3221/IGF-ESIS.53.20 244 As shown in Fig. 12, at 125 min (20 min after ignition), several hot spots formed in the longitudinal cracks near the center of the plate surface, indicating that the longitudinal cracks near the center were wider than those in other places. At 135 min (30 min after ignition), hot spots were clearly visible on several large longitudinal cracks and individual diagonal cracks. The radiating contour of cracks on the plate was faintly visible, and the cracks on the north and south sides developed in a uniform manner. At 150 min (45 min after ignition), there were lots of water stains on the plate surface. Through our infrared thermal monitoring device, the hot spots were clearly visible on each crack, and the cracks were radiating from the center on the plate surface. Moreover, the hot spots were plump and hot in the longitudinal cracks along the long sides. This means the longitudinal cracks are wide, and much heat is transmitted to the plate surface. By contrast, there were few low-temperature hot spots in diagonal cracks and transverse cracks, indicating that these cracks are relatively narrow. At 180 min (75 min after ignition), the hot spots in diagonal cracks were as plump as those in longitudinal cracks, and new short diagonal cracks formed at the corner of the plate. At 225 min (120 min after ignition), the temperature at other positions of the plate gradually approached the temperature in the cracks. At 315 min (flameout), the shape of the cracks of the plate surface was illegible. AE DATA ANALYSIS here are two major types of AE parameters: basic parameters and characteristic parameters. Basic parameters refer to the frequency and time domain parameters directly measured by the AE device. Basic parameters can be further divided into three categories: cumulative count parameters, statistical parameters, and rate of change (ROC) parameters. The most common cumulative count parameters include the cumulative number of events, total energy, total ring count, and total amplitude count. Typical examples of statistical parameters are amplitude distribution, frequency distribution, and duration distribution. The ROC parameters demonstrate the change of an AE parameter per unit time. Being quantities of states, popular ROC parameters like event rate and energy rate are closely correlated with the internal change of materials. The AE parameters reflect the differences between various states or projects. There is no uniform standard for the selection of such parameters. Instead, the AE parameters should be chosen to fully manifest the AE process and state, according to the scope and purpose of each research. The b-value is one of the most commonly chosen AE parameter. Based on the macroscopic phenomena and AE signals, this paper selects such four parameters as cumulative number of events, event rate, energy rate and b-value to disclose the time-varying AE features. Since the sensors are deployed symmetrically, the data collected by six (7#, 8#, 11#, 12#, 13# and 15#) out of the fifteen sensors (Fig. 7a) were selected for further analysis. For the lack of space, specimen S3 was taken as an example in the following discussion. Cumulative number of events The AE event is defined as a local change in the material that emits acoustic waves. The total count of all events in a process is called the cumulative number of events. This parameter reveals the time of crack emergence and the degree of crack development. The maximum cumulative number of events demonstrates the development and density of cracks at the measuring point, providing a yardstick of source activity. Fig. 13 shows the variation in the cumulative number of events at each measuring point of S3 with time and furnace temperature. Obviously, the entire process can be divided into the following three phases. The first phase is the staged loading at room temperature. In the first 72 min, the cumulative number of events surged up at each measuring point. The fastest growth rate (124 events/min) appeared at 8# at the center of the plate, with a cumulative number of events of 8,899. The cumulative number of events was relatively large (8,000) at 12#, 13# and 15#, which were arranged along the long side. The above results show that the central nodes of the plate were relatively active in the staged loading at room temperature, resulting in a high density of cracks. This agrees well with the macroscopic phenomenon that ring cracks continuously emerge in the area of these nodes. In addition, the nodes along the long side were more active than those along the short side, which is in line with the macroscopic phenomenon that longitudinal cracks mainly develop along the long sides. In the second phase, the fire was ignited and the furnace temperature started to rise. In this case, the cumulative number of events increased sharply and then stabilized. The sharp increase occurred between 106 and 143 min when the furnace temperature jumped from to 868 °C. In this phase, numerous diagonal cracks emerged, and the longitudinal cracks formed under room temperature continued to expand, causing the concrete at the bottom of the plate to spall. Echoing with T