## 2 Experimental Procedure
### Hardware Selection and Sensor Deployment  
The AE data acquisition was performed using a Vallen AMSY-6 (Vallen Systeme GmbH, Icking, Germany) digital multi-channel AE-measurement system (ASIP-2 dual channel acoustic signal processor). Different AE sensors with different resonance frequencies and frequency bandwidths were tested: Vallen-VS150-WIC-V01 (resonant frequency 150 kHz, bandwidth 100-450 kHz), VS375-WIC-V01 (resonant frequency 375 kHz, bandwidth 250-700 kHz), and VS900-WIC-V01 (resonant frequency 350 kHz, bandwidth 100-900 kHz). It has been found [33; 34] that chain failure most likely occurs at the point of the intrados (KT point) and crown positions, due to higher localised stresses in these areas (Figure 2). Therefore, on each link, the sensor was placed 10 cm away from the weld on an accessible side (Figure 3b). Each sensor was equipped with an integrated 34 dB pre-amplification. Four sensors were used on Links 3, 4, 5, and 6 (Figure 3a, and Figure 3c): one VS150, one VS375, and two VS900. A water-based couplant was used to facilitate the transmission of the sound signal between the transducer and the link's surface. Links 1 and 7 were not monitored in the present setup. The sensors used in this study had already been calibrated according to the American Society for Testing and Materials standard, ASTM E1106 [35], prior to the experiment to ensure the reliability of the collected signals. Since the experiments were to be performed with the chain submerged in water, the reproducibility of the AE sensors response was verified in air and underwater, according to ASTM E976 [36], by carefully breaking a 0.5 mm pencil lead against the link's surface (the Pencil Lead Breakage (PLB) test). PLB, also known as the Hsu and Nielsen [37] pencil lead break, is a well-established technique and has long been used as a method to artificially generate reproducible AE signals [36]. Additionally, to verify the sensor coupling, the pulsing function was applied. Each sensor was used as a signal generator that sends signals to be intercepted by the rest of the sensors. These details are briefly summarised in Table 1.  
Figure 1: Test rig illustration: the chain is fixed at one end (right) and the loading (strain) is applied at the other end (left). Figure 2: Chain link potential failure locations. Figure 4 shows the results of the PLB calibration in air. Sensor 1 (S1), located on Link 3 (L3) and referred to as S1L3, and sensor S3L5 both correspond to the VS900 broadband AE sensor and seemed to receive higher dB levels of \(\sim\)88 dB. For sensor S2L4 and sensor S4L6, corresponding respectively to VS150 and VS375, the dB levels received were approximately 10 dB lower at around 78 dB. From the figure, it is clear that the AE wave propagated through the links--the dB levels dropped to \(\sim\)64 dB in the first adjacent link, \(\sim\)54 dB in the second, and \(\sim\)45 dB in the third (Figure 4a). Signal reproducibility was also demonstrated using the pulsing function. Each sensor generated four consecutive signals of peak amplitude equal to 81 dB (top rectangle in Figure 4b), which were intercepted by the rest of the sensors. As can be seen in Figure 4b, the wave was attenuated to a minimum of \(\sim\)54 dB in the furthest sensor.