## 3 Results and Discussion  
The total duration of the AE monitoring period was 72 days. Taking into account the findings of the reproducibility tests, the AE acquisition threshold was originally set at 45 dB. However, due to the large unsustainable level of events captured, mostly noise, the threshold was initially increased to 55 dB then finally to 60 dB within the first 24 h of the experiment. The data recorded in this experiment included the number of AE hits (number of emissions detected by the sensors) and AE time domain features, including the number of counts, energy, rise time, time of arrival, and duration, in addition to the load and displacement measured by the load cells. Sensors S1L3 and S4L6 failed during the first days of the experiment for unknown reasons and were not replaced. Only the data from S2L4 and S3L5 were considered in the present analysis. The first time the pre-set limit displacement was exceeded occurred after 4,333,424 cycles. After the water tank was fully drained, a complete inspection of the chain using visual inspection and magnetic particle inspection (MPI) revealed a \(\sim\)5 mm crack located at the weld area in Link 3 (Figure 7a). No other indication of cracking was found, and the experiment was resumed. At 4,923,552 cycles, close to the fatigue life (\(5\times 10^{6}\) cycles when similar loading conditions are applied), the experiment was interrupted. A MPI inspection revealed that the same crack in Link 3 had grown to \(\sim\)120 mm (Figure 7b). A complete inspection of the chain did not reveal any other cracks. Figure 6: AE reproducibility was verified as the tank was being filled with water (**a**) before the onset of the experiment (**b**). Figure 7: Crack indications after magnetic particle inspection (MPI) at (**a**) 4,333,424 cycles (5 mm) and (**b**) 4,923,552 cycles (120 mm). The crack of interest was revealed 31 days before the end of the experiment. It should be noted that a few interruptions occurred at the beginning of the experiment. Fortunately, these were during the early stages of the test and the information collected after day 41 can be considered accurate. For these reasons, only the AE data that were acquired during the last 31 days were processed. The key AE signal features were examined in an effort to establish correlations that would link the AE activity to the fatigue crack evolution. Some of the relationships considered were:  
* Peak amplitude (dB) vs. load (kN): It was desirable to establish a correlation between the AE amplitude of the events detected and the loading state of the chain links;
* Number of hits vs. average frequency (kHz): The frequency analysis was performed in order to measure the characteristic central frequency and characterise the damage evolution;
* Cumulative energy vs. number of hits: The cumulative AE energy is associated with the cumulative energy of the AE hits. An increase in the cumulative energy's slope may be related to the evolution of the damage [38]. The AE signal features detected during the experiment were studied in detail. An example of the peak amplitude vs. load plot is shown in Figure 8. As can be seen in the figure, the events detected by the sensor attached to Link 5 are exclusively located within certain load ranges: 3150-3250 kN, 3700-3780 kN, and 3800-3850 kN. The corresponding events detected in Link 4 cannot be distinguished due to their uniform distribution. The location pattern of the red events without a clear connection to any cracking suggests they are most likely due to a mechanical noise, possibly caused by the test setup or surface friction where the links meet. It is usually accepted that in fatigue cracking, the AE signals generated close to the high end of the loading range are related to crack growth. [PERSON] et al. [22] showed that any correlation attempt was unsuccessful when the complete loading range was considered. Attributing the top 10 and 5% of the load range to crack growth improved the correlation to AE significantly. [PERSON] et al.