# Validation of Acoustic Emission Waveform Entropy as a Damage Identification Feature
## 5 Validation and Discussion
### Fatigue Test  
DIC enabled the correlation of the damage mechanism (i.e., the onset of plastic zone formation) with AE activity. The AE data and DIC images were synchronized by starting both measurements at the same time so that they capture data simultaneously. Figure 17 illustrates important DIC images of the strain map (\(\epsilon_{yy}\)). Figure 17a represents the reference image. This image shows the state of the strain in the specimen before the onset of loading. Figure 17b shows that the strain maximizes in the middle of the specimen, and this image was taken after 1000 s from the start of loading. It is clear from Figure 17c, taken at 21556 s, and Figure 17b that there was no noticeable difference between them. This indicates that the surface strain from 1000 s to 21556 s remains stable. Figure 17d, taken at 22911s, shows the concentration of strain at the specimen edge, forming a plastic zone (indicated by an arrow). A significant formation and coalescence of a micro-crack may have resulted in this plastic zone. An explanation of this circumstances is provided in reference [47]. Figure 17e, taken at 23250 s, shows an enhancement of the plastic zone found in Figure 17d (indicated by an arrow). This plastic zone area becomes clearly highlighted in Figure 17f, taken at 23590 s (indicated by an arrow). Figure 17g, at 23928 s, shows a visible fatigue crack from the plastic zone seen in the earlier images. The time of occurrence of the plastic zone and fatigue crack is cross validated with AE activity. The correlation of traditional features and AE entropy of the filtered events, captured from the fatigue specimen, is shown in Figure 18. Figure 18a-f represents the variance of rise-time, duration, count, energy, peak amplitude and AE entropy for the entire length of the test. It is evident from Figure 18c-f that at the initial and final stage (at the beginning and end of the test), there was a significant increase in count, energy, peak amplitude and AE entropy. The initial and final stage was accompanied with an AE entropy above 10. The initial increase in these traditional features and AE entropy is due to the rapid yielding (i.e., caused by the onset of loading). The maximum engineering stress exerted on the specimen at each cycle of loading (including the first cycle) was 480 MPa. This is beyond the yield stress of the material. As a result, yielding was introduced in the specimen in the first cycle of loading. Yielding in austenitic stainless steel has been reported to be accompanied by the sharp increase in some traditional AE features (e.g., count and peak amplitude). The increase in the traditional features and AE entropy at the final stage is due to the fracture of the specimen. It is clear from these results that like the traditional features, AE entropy is also sensitive to the yielding and fracture of austenitic stainless steel. It is a well-known fact that fatigue crack initiation leads to plastic zone formation in the specimen, and the crack propagates until the final fracture [48]. The DIC, as a secondary monitoring technique used in this test, was able to identify the onset of plastic zone formation. The onset of plastic zone formation in the specimen, identified by DIC, is marked with a red dashed line in each of the graphs of Figure 18. It can be observed in Figure 18c-f that there was no noticeable increase in count, energy, peak amplitude and AE entropy at the onset of plastic zone formation. In fact, apart from yielding and fracture, there was no noticeable increase in these traditional AE features and in AE entropy. Figure 17: DIC Images of specimen 3: (**a**) reference stage, (**b**) at 1000 s, (**c**) at 21556 s, (**d**) at 22911 s, (**e**) at 23250 s, (**f**) at 23590 s and (**g**) at 23928 s.  
Figure 18: Variation of AE features for the fatigue specimen: (**a**) rise-time, (**b**) duration, (**c**) count, (**d**) energy, (**e**) peak amplitude and (**f**) AE entropy. The plastic zone is known to produce continuous AE waveforms with higher rise-time [49].