4.2 Case study 2: AE activities at position G The second example is for the crack monitoring of position G of Figure 2. The elapsed time and the fatigue cycles along with AE monitoring and visual inspection results at this position are shown in Table 2. Figure 6 shows the AE hit rate from 139.9 to 237.8 elapsed fatigue hours. AE activity or crack initiation started from 148.4 elapsed fatigue hours or 534 thousand fatigue cycles, then continuous AE activities lasting for over 12 hours were observed, there is also long resting period that no or very rare AE activity was observed after the crack initiation. Since three sensors, numbered as 1, 8 and 2, were installed in position G, crack locations are able to be picked up with the linear location algorithm applied to the AE hits detected by the three sensors. Figure 7(a) shows a distribution of the located crack events along with the linear coordinate of the sensors, it shows two clustered location groups around the sensor 8, one is in between sensors 1 and 8 and another one is in between sensors 8 and 2. These two clustered location groups indicate that two cracks exist in these two areas. However, as is seen in Table 2, visual inspection couldn’t find cracks in the following 21 days which it challenged the findings of the acoustic emission. Table 2. Remarks of crack monitoring alone with elapsed fatigue time & cycles at location G Date Time of the Day Elapsed Time (Hours) Fatigue Cycles (Thousands) Remarks 16:07 139.9 504 AE monitoring started. 2/Aug 22:35 148.4 534 AE activity that indicates crack initiation was observed in the first time. 9:30 159.3 574 Visual inspection didn’t find crack. 3/Aug 13:47 163.6 589 Visual inspection didn’t find crack. 4/Aug 8:50 182.6 658 Visual inspection didn’t find crack. AE activity was relatively quiet in this period. 8:45 206.5 744 Visual inspection didn’t find crack. Significant AE activities with locations were observed in next few days which indicate long lasted continuous fatigue crack development. 5/Aug 13:07 210.9 760 Visual inspection didn’t find crack. 6 – 23 /Aug AE kept indicating crack activities, but no crack was found with visual inspection at this location in this period. 24/Aug 9:00 662.8 2880 After applying higher load and faster fatigue frequency up to 2 Hz, visual inspection eventually found two cracks in 220mm and 15 mm long respectively. With continued fatigue cycles, although AE kept indicating crack activities, no crack was found at this monitoring area for over 21 days after the crack initiation was detected with AE.
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