Before finishing the 30 day fatigue test, accelerated fatigue test parameters, e.g. higher load and faster fatigue test frequency up to 2 Hz, were applied to the structure to speed up the damage process. Then two cracks were eventually visualized in about 22 days after AE detected crack initiation. A photo of the crack location after the overall 30 day fatigue test is shown in Figure 7(b), one crack in 220mm long is located at the web in between sensors 1 and 8, another crack in 15mm long is located along with the weld of the rib in between sensors 8 and 2, these two cracks are located at the same positions that AE technology has reported in about 22 days ago. This illustrates that either the microscopic initial crack was too small to be visualized or the crack was not in the surface of the visible area in early stage of fatigue crack growth. However, AE not only detected the crack initiation and growing process, but also it located crack positions in so many days before visual inspection found the cracks. The visual inspection results eventually proved the effectiveness of the AE technology for dynamic monitoring of fatigue crack process of large structures. Figure 6. AE hit rate at position G from 144.4 elapsed fatigue hours Figure 7. AE crack event location and indications of the cracks found (a) Distribution of AE event location (b) Photo of the monitored area (c) Sketch drawing of crack location 8 1 2 220mm crack 15mm crack
RkJQdWJsaXNoZXIy MjM0NDE=