**3.1 Bending fatigue test results (crack initiation)**

## *3.1.1 Acoustic emission signal amplitude versus standard cycle diagram*

The first plotted diagram is signal amplitude vs. normalized cycle. This diagram shows the signal amplitude in each standardized cycle. This diagram is important because it determines the recognizability of the signals and is also directly proportional to the magnitude of the event that occurred at the source [3, 10].

After examining the signal amplitude vs. standard cycles for all specimens, it was determined that in each specimen in a different cycle, the signal amplitude starts to increase sharply, then this amount reaches its maximum, and then begins to decrease. The amplitude of the start signal changes from 80–96% of the maximum signal amplitude in the specimens.

This increase in signal amplitude in the diagrams indicates the occurrence of an event within the test specimen. For example, the signal amplitude vs. normalized cycle diagram for specimen NO. 9, which is randomly selected from 9 specimens to explain in detail is shown in **Figure 7**. In the specified part, the signal amplitude in the standardized cycle 15 reaches 52 dB after the increase, then this value increases again until it reaches its maximum value in cycle 18, which is 54 dB, and then it starts to reduce. **Figure 8** shows acoustic emission signal amplitude vs. standard cycle for all 9 specimens.

**Figure 7.** *Acoustic emission signal amplitude vs. standard cycle diagram of specimen NO. 9.*

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

**Figure 8.** *Acoustic emission signal amplitude vs. standard cycle diagram for all 9 specimens.*

**Table 1** shows the maximum signal amplitude and amplitude which signal change start in all 9 specimens.
