## 3 Materials and Methods
### Noise Reduction
This method assumes that the AE signals are sporadically received as microcracks form and will most likely occur when a structure is applied with maximum stress [4; 10; 12; 26]. An example of the expected AE signal behavior with no apparent noise signals is plotted in Figure 6a for experiment S8. This graph shows the AE signals as points at the associated load and time and meets both of the expected criteria. The cumulative hits over time for this experiment is depicted in Figure 6d where a majority of the hits are shown to be acquired near specimen fracture as expected. However, other experiments showed both the expected AE behavior and another behavior that is believed to be unwanted noise. This unwanted behavior is characterized by AE signals continuously occurring at mid-range loads forming clusters when plotted against their associated load and arrival time. The noise behavior also seems to evolve throughout the fatigue tests where numerous noise signals are suddenly detected after a substantial amount of the specimens' fatigue lives are depleted and then are not evident after a few cycles. These clusters suggest either that strain energy is released  
Figure 6: AE signals at their associated loads versus time for three experiments: (**a**) S8 has no apparent AE noise clusters; (**b**) S4 has defined clusters that are assumed to be due to noise and can be filtered; (**c**) another experiment with undefined clusters of noise signals that cannot be filtered with ease; (**d**) cumulative AE hits over time for S8 with no apparent AE noise clusters; (**e**) cumulative AE hits over time for S4 with filterable noise; and (**f**) cumulative AE hits for an experiment with unfilterable noise. consistently away from the maximum load or that mechanical noise is generated consistently at particular points of the loading cycle. The latter is assumed to be more likely where this noise is believed to be an unfortunate attribute of the hydraulic system that powers the fatigue testing machine, and therefore, these signals were excluded. The scatter plot of applied load at the instant of AE signals for one such test, S4, is pictured in Figure 6b. The cumulative AE hits for this experiment is depicted in Figure 6e. This plots shows that the AE hits assumed to be from the noise clusters create a jagged cumulative hits trend and account for about 40% of the acquired signals. Too many noise signals were recorded for other tests, and the expected AE behavior and noise could not be confidently differentiated. Figure 6c shows the applied load at AE hits versus time for one such test as an example of noise that proved impossible to separate from the assumed damage-related AE hits. Subsequently, the cumulative AE hits over time is shown in Figure 6f where the total hits are over 10 times more than an experiment with no apparent noise, and the trend is close to linear rather than exponential towards the end of the experiment. In the end, the filtering method was kept consistent in that clusters of AE signals at low loads were removed, and tests that would have required questionable noise filtering were not used in the analysis. Further information about the noise reduction processes can be found in [31].