# Validation of Acoustic Emission Waveform Entropy as a Damage Identification Feature
## 6 Conclusions  
This research addresses one of the major drawbacks of the state-of-the-art AE monitoring technique and improves it so that the possibility of AE implementation for monitoring the primary wall is increased. The improved technique is based on an AE feature, which is a measure of randomness of the AE waveform, calculated using Renyi's entropy. The new technique is referred to as AE entropy in this research. AE entropy reduces human involvement with the data acquisition system and has the potential to be implemented in the commercial AE data acquisition system. The following points summarise the key findings of this study:  
1. The evaluation process adopted in this study suggests that AE entropy is independent of user-defined acquisition threshold and extracts the collective information from a waveform. This independency of AE entropy arises as it is directly calculated from the discrete voltage values, which are both in and outside the threshold. 2. The evaluation process also suggests that AE entropy is independent of the user defined HDT. This is mainly because the computation of AE entropy takes into account the discrete voltage values in a waveform even after the expiration of HDT. This independency of AE entropy is an important property, as the loss of information from sub-waveforms occurring at different time intervals within a waveform can be avoided. 3. In the tensile test, correlation AE entropy possessed the same trend as most of the widely used traditional AE features. From the onset of loading to just after yielding, there was plenty of AE activity due to dislocation movement; however, it did not have dominant AE entropy, whereas the fracture of the specimen was accompanied by a small number of AE activities, but it had dominant AE entropy. Figure 21: Original and de-noised waveforms. In the fatigue test, the yielding was accompanied by a dominant increase in correlation AE entropy. This suggests that AE entropy has the potential to provide indication regarding the yielding, which is considered to be a dominant damage mechanism in the primary wall. Since AE entropy is sensitive to yielding, it can be successfully implemented to monitor the primary wall against this damage mechanism. At the formation of plastic zone (identified by DIC), there is no noticeable increase in the AE entropy. This suggests that it may not be a suitable technique for the identification of plastic zones formed due to repeated cyclic loading. 5. Compared to the yielding of the tensile specimen, the yielding of fatigue specimen was accompanied by dominant correlation AE entropy. This was due to an enhanced average velocity of the moving dislocation (\(v_{\textit{nr}}\)), as a result of the higher strain rate (\(e\)) of fatigue specimens. Duration of the cyclic loading adopted in this study was comparable to the duration of the sloshing impact. Therefore, a strong sloshing impact has the potential to cause deformation in the primary wall with a high strain rate (than that of the strain rate of tensile test), which can produce dominant AE entropy. 6. Both the cumulative count and AE entropy increased noticeably before the damage manifests into plastic zone formation. At the formation of a plastic zone (identified by DIC), there was a sharp increase in both of these cumulative features. A similar trend in these cumulative features suggest that the traditional analysis of cumulative count can be replaced with cumulative AE entropy in the experimental investigation of fatigue damage evaluation. Despite the potential of cumulative AE entropy to identify the fatigue damage evolution, it cannot be used to monitor the primary wall primarily because the sloshing impact on the primary wall is separated by zero pressure intervals. As a result, there might not be a noticeable change in the cumulative feature if the duration of zero pressure values increase during plastic zone formation. 7. The majority of the disorderness in a waveform arises from micro-structural damage.