Issue 53

D. Wang et alii, Frattura ed Integrità Strutturale, 53 (2020) 236-251; DOI: 10.3221/IGF-ESIS.53.20 248 (1) Room temperature stage I From zero to one 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. As shown in Fig. 14, the event rates of measuring points were around zero events/min, and the peak event rate, 53 events/min only, was captured by 13#, reflecting the rarity of cracks inside or on the plate. As shown in Fig. 15, the energy rates of measuring points were rather low at about zero mv ∙ μs/min. Thu s, the cracks released a very small amount of energy. As shown in Fig. 16, the first valley was observed for the b-values in all channels. The b-values at most measuring points were around 1.7-1.8, except that (1.55) at 13#. The values were much higher than those in the subsequent loading stages at room temperature. This means the crack width in this stage was much narrower than that in other stages. That is why the cracks were not observable in this stage. (2) Room temperature stage II From 14 to 15 min (second loading stage), the load increased from 50 to 100 kN. During the loading and holding, five fine cracks appeared in the east and west of the plate, respectively. From 26 to 28 min (third loading stage), the load further grew from 100 to 150 kN. During the loading and holding, four fine new cracks emerged in the east, and six on the west of the plate. From 38 to 39 min (fourth loading stage), the load increased from 150 to 175 kN. In this process, four fine new cracks emerged in the east and the west of the plate, respectively, and the first ring crack formed on the top of the column. From 49 to 50 min (fifth loading stage), the load continued to rose from 175 to 200 kN. During this process, two new cracks emerged in the east and one on the west of the plate. As shown in Fig. 14, with the emergence of new cracks on the edges, the penetration of cracks and the appearance of ring crack on the top of the column, the event rates of the measuring points surged up to the peak values, and then plunged in the holding period, exhibiting as serrated curves. The event rates were uniformly distributed across the different loading stages. In the second loading stage, the peak event rate (541 events/min) was recorded at 7#; in the third loading stage, the peak event rate (591 events/min) was recorded at 8#. In the fourth loading stage, the peak event rate (456 events/min) was recorded at 8#; in the fifth loading stage, the peak event rate (409 events/min) was recorded at 12#. As shown in Fig. 15, the energy rates at measuring points rose sharply to the peak values, and nosedived in the holding period, exhibiting as serrated curves. The peak energy rates differed greatly from measuring point to measuring point. Relatively high peak values were observed at 12# and 13# through the five loading stages. In the second loading stage, the peak energy rate (568,205 mv ∙ μs/min) was recorded at 13#; in the third loading stage, the peak energy rate (754,844 mv ∙ μs/min) was recorded at 12#; In the fourth loading stage, the peak en ergy rate (302,332 mv ∙ μs/min) was recorded at 12#; in the fifth loading stage, the peak energy rate (189,839 mv ∙ μs /min) was recorded at 12#. As shown in Fig. 16, the b-value at each measuring point hit the valley in each loading stage, and the valley values were close to each other, ranging from 1.45 to 1.50. Therefore, the cracks developed rapidly in all loading stages. (3) High temperature stage I Right after the ignition at 105 min, cracks appeared quickly along the short sides on the north and south of the plate, so did diagonal cracks. Meanwhile, the cracks widened obviously at plate-column nodes. From 114 to 118 min, several popping sounds were heard consecutively, due to concrete spalling. As shown in Fig. 14, the event rates at all measuring points reached the peaks again; all are far greater than the peaks at room temperature. The highest peak value was measured by 12#: 1,108 events/min. This means, with the rising furnace temperature, the specimen became more active in AE under the coupling of load and temperature. As shown in Fig. 15, the energy rates at all measuring points returned to the peaks after ignition. The highest peak energy rate (165,293 mv ∙ μs/min) appeared at 12#. The results indicate that new energy was released by new cracks and concrete spalling. As shown in Fig. 16, the b-values at all measuring points hit the valleys quickly after ignition. The valley values were between 1.40-1.45, lower than those under room temperature. Next, the b-values gradually rebounded. It can be seen that cracks widened quickly and diagonal cracks emerged rapidly at the beginning of the fire; with the growing furnace temperature, it is increasingly difficult for new cracks to emerge and for old cracks to widen. (4) High temperature stage II At 121min, the furnace temperature was 763 °C. Four sounds of concrete spalling were heard; lots of water stains appeared in the cracks on plate surface, and seeped towards the short sides; water vapor escaped from the relatively width cracks. One concrete spalling sound was heard respectively at 131 and 133 min. At 140 min, the furnace temperature rose to 857 °C. The plate height was distorted at the top of the column. At 252 min, the furnace temperature was 1,078 °C, and the water stains on the plate surface were basically evaporated. At 315 min, the fire was turned off.