## 2 Methods and Materials
### Experimental Setup
For AE recording, the PC-controlled system based on 18 bits PCI-2 (MISTRAS, Princeton, NJ, USA) data acquisition board with a data streaming capability (see [67] for technical details) was used. The signal from the sensor output is amplified by 60 dB by the low-noise wide-band preamplifier 2/4/6 with the built-in band-pass 30-1200 kHz filter. The background noise level measured peak-to-peak was of 32 \(\upmu\)V or less at the sensor output (depending on the sensor used). The MISTRAS F50\(\alpha\) 200-800 kHz wideband sensor was used. Molykote 33 silicone-based grease was used as the coupling medium, and the sensor was mounted using a 3D-printed polymeric mechanical clamp. The background noise level at the sensor output was measured with the specimen mounted at zero load as of 3 \(\upmu\)V rms (of 31 \(\upmu\)V peak-to-peak) in the tensile setup and of 2.6 \(\upmu\)V rms (of 27 \(\upmu\)V peak-to-peak) in the FCG setup. The AE recording was performed continuously at 2 MHz sampling rate. The preference to use the waveform streaming acquisition mode is given based on the strong arguments unfolded in [68, 69, 70, 71, 72]. In brief, AE during plastic deformation and fracture of structural materials appears as a random sequence of arbitrarily spaced individual pulses having different waveforms and amplitudes depending on the properties of the emitting source. The amplitude of AE bursts can vary by several orders of magnitude in the dynamic range exceeding 100 dB. Pulses with a high SNR (e.g., \(>\)10 dB) are easily detectable by simple hit-detectors based on the amplitude threshold, which are traditionally built-in in commercial AE apparatus, where a hit is marked when the pre-set amplitude threshold is crossed by the signal [67]. As has been reviewed in [68, 70], and more recently in [71], this method suffers from many irrecoverable drawbacks when low-amplitude signals are to be resolved on the background of electric noise or when the low-amplitude transients overlap and form a continuous signal. For instance, if the trigger level is set sensitively low, false alarms are recorded due to fluctuations of noise, which, as stated above, is particularly important in dynamic fatigue setups. Unidentified false triggering burdens the data analysis and can potentially result in misinterpretations and wrong decision making. For example,a common practice in the AE field of counting the AE activity as the number of AE hits per unit of time suffers particularly strongly from the arbitrary settings of the amplitude threshold. Low-amplitude continuous signals, comparable to the background electric noise, are the most common signatures of plastic deformation due to dislocation slip [24], which not only dominates the strain hardening behaviour during the monotonic tensile or compressive test but is of crucial significance for the evolution of the microstructure in the plastic zone ahead of the propagating fatigue crack. Moreover, it is plastic work that determines the heat dissipation in the cyclic plastic zone [73; 74; 75; 76]. In addition, a wealth of powerful signal processing techniques have been developed recently for unveiling and handling low-amplitude signals buried in the background noise, provided the waveform is continuously recorded in a sufficiently wide frequency range (see [69; 71] and literature therein). Thus, it is for these reasons the use of thresholdless high-speed data streaming is uncompromising when it comes to the need to investigate the low-amplitude dynamic events in the AE flux. All measuring devices have to be synchronised and triggered depending on the desired timing and loading conditions, which might be challenging as the definition of moment of interest (MOI) might vary for different devices. To this end, a homemade PC-controlled triggering unit was used for precise temporal synchronisation of both cameras with the current load. Tracking and logging of the external load signal and the load-controlled trigger signal (MOI) were performed by a user-programmable USB digital oscilloscope PicoScope-4284A.