## 2 Methods and Materials
### Experimental Setup
#### 2.2.1 Testing Machine and the Noise Reducing Damper  
Servo-hydraulic machines, which are most commonly used in fatigue experiments, are known to produce background vibrations on the actuator and a test piece, arising primarily from servovalves [77; 78]. These vibrations often cause uncontrollable fluctuating and load-dependent noise at the output of the AE sensor attached to the test specimen, giving rise to false signals, both continuous and transient, which fall in the frequency and amplitude range comparable to damage-related acoustic emissions, thus making it challenging to remove them in the post-mortem analysis.  
To reduce the noise-related artefacts in AE data, several methods have been probed in the research community. These include: (i) passive mechanical dampers installed between the frame, actuator, and the test specimen [77]; (ii) guard sensors and sensor arrays locating the AE signals from the region of interest (e.g., close to the crack tip) [79; 80; 81; 82; 83]; (iii) load gating and partitioning the signals with respect to the load (e.g., acquiring the signals which appear only close to the peak load) [84; 85]; (iv) setting the high amplitude discrimination threshold [86; 87]; and (v) post-processing filtering methods including criterion-based feature selection [88], information entropy-based approach [78], and a variety of hierarchical clustering procedures in an attempt to identify outliers according to distributions of specific features [89].  
As will be seen shortly, passive damping is universally efficient and is undeniably recommended for any experimental setup based on servohydraulic or electrodynamic frames. In fact, as has been correctly pointed out by [PERSON] et al. [78], neither of the other measures taken individually or in any combination can be considered 100% reliable and universal. We share this opinion, supported by our extensive laboratory experience. Even a very appealing technique of locating AE sources within ROI according to signal arrival times between two or more sensors suffers from substantial drawbacks. To be more specific, it cannot be used for small samples, it fails to work with continuous signals, which are common due to plastic deformation of ductile materials, and, besides, the acquisition system is often overloaded and ceased to operate under cyclic loading due to intensive pulse flow from the area outside ROI. The noise signals and damage-related signals can hardly be distinguished by their frequency spectra, thus making frequency-based filtering ineffective. Needless to say that setting the high amplitude threshold does not help much to solve the machine noise problem as a lot of valuable low-amplitude acoustic information is missed in this way.  
Therefore, the present experimental setup is based on the 10 kN electrodynamic Instron E10000 Electropulse universal testing machine. Compared to servo-hydraulic testing frames, a family of modern electrodynamic machines are considerably more silent. Nonetheless, the examination of several electrodynamic machines has shown that they are not completely noise-proof in the dynamic regime and a damper-clevis assembly shown in Figure 2 was installed between the actuator and the specimen grip to reduce the machine noise captured by AE. The clevis joints have a small contact area which helps to reduce the noise. A mechanical damper, inspired by [PERSON] and [PERSON]'s early work [77], was made as a laminated structure consisting of numerous aluminium and felt interfaces. The sketch and the photograph of the damper are shown in Figure 2. The signal on the actuator, the signal on specimen without the damper, and the signal on the specimen with the damper installed during cyclic loading of a non-deformable \"dummy\" specimen are compared in the same figure. One can see that the noise measured on the specimen with the damper was equivalent to the electric background noise at the sensor output.