## 2 Description of the Experimental Full-Scale Test  
This section summarizes the experimental full-scale test conducted to monitor the fretting fatigue phenomenon in axles, along with a description of the used AE acquisition equipment. As stated above, this test and its results are the same already used for developing and proposing the different post-processing approach described in the previous work by the authors [22] on the topic of interest and, moreover, they were originally part of a broader research program whose full details are available in [40; 41; 42]. A summary of the main points, relevant to the present research, is reported in the following. A full-scale specimen (Figure 1) made from EA4T steel grade (quenched and tempered 25 CrMo4 steel) was tested according to the suggestions of standard EN 13261 [43] for press-fitted assemblies to characterize fretting fatigue damage. To simulate typical fretting damage found in real railway axles, four artificial notches, each 350 \(\upmu\)m deep, were machined using Electrical Discharge Machining (EDM) 9 mm from the edge of the press-fit seat. These notches were spaced 75\({}^{\circ}\) apart circumferentially to assess the propagation behavior of pre-existing defects under the press-fit [44]. The test was performed using the \"Dynamic Test Bench for Railway Axles\" (BDA) available at the Department of Mechanical Engineering, Politecnico di Milano. This bench,  
Figure 1: Full scale specimen and artificial notches. a Vitro test rig, applies a three-point rotating bending condition to the specimen. The test operated at a rotational speed of 509 rpm, typical for freight applications. Due to the bench's geometry and configuration, the specimen could not accommodate a real press-fitted full-scale wheel, so a specially designed wheel hub (bush) was used to simulate it. The test was conducted over a total of 22 \(\times\) 10\({}^{6}\) fatigue cycles. A stress of 108 MPa was applied for the first 15 \(\times\) 10\({}^{6}\) cycles, after which the stress was increased to 135 MPa until the test's completion to induce the final fracture. This approach was used to ensure the initiation and propagation of fretting fatigue damage [45]. The adopted AE acquisition system (Vallen Systeme AE control unit AMSY-6 with eight channels) continuously monitored the specimen using a single AE sensor (Vallen Systeme VS150-M, Vallen Systeme GmbH, Offenbach am Main, Germany) with a resonance frequency range of 100-450 kHz. Due to spatial constraints, just one sensor was positioned at one of the two free ends of the full-scale specimen using a custom-made mount, which also held a pre-amplifier (Vallen Systeme AEP4, Vallen Systeme GmbH, Offenbach am Main, Germany, 34 dB). The sensor coupling was periodically verified with a Pencil Lead Break (PLB) test [46]. The signals generated by the PLB test displayed a distinct burst waveform with a maximum amplitude of 70 dB. The acquired signals were transmitted from the rotating parts to the AE acquisition system by a sliding contact (Michigan Scientific S4, Michigan Scientific Corporation, Grand Rapids, MI, USA) and the number of applied fatigue cycles was also recorded. To verify that the contact did not adversely affect the signal quality, the aforementioned periodic Pencil Lead Break (PLB) tests were conducted before and after signal transmission through the slip ring. These tests showed consistent waveform characteristics and energy levels, confirming that the sliding contact did not introduce noticeable signal distortion or attenuation. AE monitoring during the test was managed using Vallen Systeme AE-Suite Software R2017.0504.1. The proper functioning of the acquisition chain was checked using, again, the PLB test, and the finally adopted acquisition parameters are detailed in Table 1. The acquisition threshold was set to 69 dB to capture signals similar to PLB bursts, while minimizing the collection of unnecessary background noise.