# Validation of Acoustic Emission Waveform Entropy as a Damage Identification Feature
## 5 Validation and Discussion
### Correlation Between the Coupon Specim and Real Structures under Operation  
In a LNG cargo tank, sloshing is the only unpredictive load that results in its structural degradation. Sloshing studies conducted in accordance with the American Bureau of Shipping and DVN.GL guidelines suggest that the maximum and mean duration of the sloshing impact were 200 ms and 11.7-27.4 ms, respectively [29; 52]. It is also suggested in reference [52] that the sloshing impacts are separated by zero pressure intervals in the order of a few seconds, and the interval between the sloshing impacts are not consistent. Figure 20 shows the loading spectra adopted in fatigue testing of this  
Figure 19: Comparison of the cumulative trend for the fatigue test. research. The specimens were subjected to the same loading spectra from the start of the test until fracture. It is evident from this figure that the duration of the respective load (i.e., from one minimum load point to the next) was 200 ms, which is comparable to the maximum duration of the sloshing impact experienced by a LNG cargo tank during voyage. However, it can be observed from this figure that there was no delay between the subsequent load. In other words, there was no separation between the subsequent loads, as experienced by the real LNG cargo tank. Despite the potential of cumulative AE entropy in providing valuable information of damage (i.e., plastic zone formation) discovered during fatigue tests of this study, it cannot be implemented as a condition-monitoring technique on the primary wall of the cargo tank. This is primarily because the loading condition of the coupon specimen adopted in this study was continuous in nature without any delay between the subsequent load. Hence, there may not be a noticeable increase in the cumulative AE entropy, during plastic zone formation especially, if the duration of zero pressure intervals between sloshing impact increases during plastic zone formation as compared to the other stages. Considering that the cumulative AE analysis may not be a suitable condition monitoring technique for the primary wall, the only other technique that can be explored is the correlation analysis of AE features. It can be observed in Figure 18f that during the onset of plastic zone formation, there was no noticeable increase in AE entropy. In fact, it can be observed in Figure 18c-e that there was no noticeable increase in either count, energy or peak amplitude at the onset of plastic zone formation. It is mentioned in reference [53], that the rate of AE activity increases with the formation and growth of the plastic zone. However, the correlation AE analysis (e.g., count, energy, peak amplitude and AE entropy) did not provide any information regarding the formation of the plastic zone. This indicates that although the rate of AE event generation was higher, the individual strength of AE events did not change noticeably. In austenitic stainless steel, plastic zone formation is known to produce AE events with weaker features as compared to the ones at fracture [46]. This is because the amount of material yielding in and at the edge of plastic zone is not sufficient to produce AE events with high strength. Since the AE entropy during the formation of plastic zone is lower as compared to the ones at fracture and onset of yielding, it suggests that AE events during these periods are less disordered. It can also be suggested that the lower strength AE events during this period is responsible for its low disorderness. It is clear that the plastic zone formation due to repeated cyclic loading will not be identified by AE entropy and traditional features. The plastic zone precedes crack initiation in material subjected to fatigue loading. Since the AE entropy is not sensitive to plastic zone formation, a warning prior to the crack initiation in the primary wall of the cargo tank cannot be successfully achieved. It is clear in Figure 18f that the initial yielding (at the onset of loading) was associated with a noticeable increase in AE entropy.