*Determining the Characteristics of Acoustic Emission in the Fatigue Crack Growth of Aluminum… DOI: http://dx.doi.org/10.5772/intechopen.99360*

the test in 30 to 33 minutes. In diagram of specimen NO. 2, this event also occurred in the approximate cycle of 20 to 28, which is from 3 to 4:30 minutes.

For example, in specimen NO. 9, which was also examined in the signal amplitude vs. standard cycle diagram, in standard cycle NO. 15, which is the beginning of changes, it has a cumulative count of 10259, and in standard cycle NO. 20, which is the end of these changes, it has a cumulative count 23893. The slope of the change in the chart is 2870, which is 4 times more than before the start of the experiment when the slope is 703. In all specimens, this sharp increase in slope is noticeable. As mentioned, where the count reaches its maximum value, the rate of the internal events of the material is also at its maximum, in all specimens, the maximum value of the count is in the same range of changes. According to the above, examining the diagrams and the slope of different parts of the diagram, it can be concluded that crack initiation occurs when the slope of the diagram increases sharply. This increase in slope varies from 4 times to 16 times the slope before cracking between the tested specimens.

**Table 2** shows maximum acoustic emission count and cycle which signal change start.

For better understand the simultaneity of increasing the signal amplitude and increasing the slope of the cumulative count diagram, the amplitude and cumulative count versus standard cycle diagrams plotted. **Figure 11** shows acoustic emission signal amplitude and cumulative count vs. standard cycle diagram for all 9 specimens.
