## 3 Materials and Methods
### Instrumentation  
Fatigue experiments were performed with loading ratio of 0.1 and loading frequency of 5 Hz. A servo-hydraulic Materials Testing System (MTS) machine retrofitted with an Instron 8800 controller was used to perform the fatigue tests. The maximum applied load varied between 8 and 15 kN. The theoretical maximum applied stress can be estimated by dividing the load by the gauge length cross-sectional area, 57.15 mm\({}^{2}\), and multiplying by the stress concentration factor, 2.61, yielding estimated applied stresses between 480 and 685 MPa. The strain around the edge notch was measured with an Epsilon 3542 extensometer with a gauge length of 25 mm. To prevent sliding or rubbing on the specimen, rubber bands were used to fasten the extensometer to the specimen's face. An optical microscope with an attached time-lapse camera captured images every 25 cycles to monitor the \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline
**Element** & **Al** & **Zn** & **Mg** & **Cu** & **Cr** & **Fe** & **Mn** & **Si** & **Ti** & **V** & **Zr** & **Other** \\ \hline Composition (wt \%) & 89.7 & 5.7 & 2.6 & 1.4 & 0.20 & 0.15 & 0.08 & 0.06 & 0.02 & 0.01 & 0.01 & 0.05 \\ Material Property & Ultimate Strength (MPa) & \multicolumn{6}{c}{Yield Strength (MPa)} & \multicolumn{6}{c}{Elastic Modulus (GPa)} \\ Property Value & \multicolumn{6}{c}{587} & \multicolumn{6}{c}{538} & \multicolumn{6}{c}{67.8} \\ \hline \end{tabular}
\end{table} 
Table 1: Composition and mechanical properties of AI7075-T6. Figure 4: Experimental setup showing the specimen, AE sensors, mechanical damping apparatus and crack growth monitoring system. initiation of a crack and its growth from one side of the specimens. Crack initiation was recorded as the first instance a crack was visible on this one face and, because the cracks could have initiated on the other face not monitored by the microscope, there is some uncertainty in the crack initiation time and small crack length. However, it is assumed that once the crack reached 1 mm as measured on the monitored specimen face, the crack is of sufficient length to have propagated through the specimen thickness. AE signals were recorded with a PCI-2 based AE system supplied by the MISTRAS Group (Princeton Junction, NJ, USA). Two resonant Micro30s AE sensors with a frequency range of 150-400 kHz and resonant frequency of 225 kHz were used. A sampling rate of 1 MSPS was used to capture the signals. According to the Nyquist-Shannon sampling theorem, frequencies up to 1/2 of the sampling frequency meaning up to 500 kHz were distinguishable. However, signal frequencies at the top of the frequency range of the sensor are considered unreliable at this sampling rate. If other AE sensors with higher operating frequency ranges were to be used, the sampling frequency would have to be increased accordingly. The sensors were mounted to one side of the specimen 23 mm above and below the center of the notch. Ultrasonic gel was used as a couplant, and electrical tape fastened the sensors to the surface. The AE signals passed through a 40 dB preamplifier before reaching the data acquisition module where AEwin software then plotted and extracted the AE signals. The AE acquisition system also received load and extension data as analog inputs from the testing machine. This feature enabled the AE signals to be paired with the applied load and is crucial to post-process filtering described in Section 3.3. Other user-defined settings that control how the AE signals are collected are summarized in Table 2. These parameters were selected based on pencil lead break tests [41], a common standard that produces repeatable artificial AE waves with similar characteristics to damage-related AE signals. The AE sensors and crack growth monitoring system are shown in Figure 4.  
It should be noted that the values of entropy from AE signals would change if any of the acquisition parameters were altered. However, the trends in entropy measures between the AE signals throughout a fatigue experiment would not change.