## 4 Experimental Validation  
### Test Rig and AE Data Acquisition  
One can find more details of the experimental setup and durability test in [17]. To evaluate the performance of the proposed method, a rolling contact fatigue test was carried out in this section. The test rig, designed at SINTEF Industry (Trondheim, Norway), consists of four roller bearings, as illustrated in Figure 7a. The test specimen is in the central position, supported by another three rollers. Each roller is supported by two needle bearings (type SKF NA 6914-zw). To monitor damage associated with the rolling contact fatigue, the broadband WD (MISTRAS, Princeton, NJ, USA) sensors were mounted on the housing of the needle bearing supporting the test roller. A close-up view of the sensors and their location on the rig is presented in Figure 7b. The streaming AE signals were recorded periodically at fixed time intervals, and each data file contains 2 s of streaming AE waveforms sampled at 2 MHz using the Kongsberg HSIO-100-A (Kongsberg Maritime, Trondheim, Norway) high-speed acquisition module. At the beginning of the test, the recording time interval was set at 60 min. When the first damage was confirmed by periodic ultrasonic inspections of the test roller, the recording time interval was reduced to 20 min to obtain more AE realizations containing information about the faults. At the end of the experiment, 2471 AE records were qualified for the analysis. Figure 8 displays the amplitude of the raw AE signal against contact fatigue cycles. An appreciable change in the AE amplitude is observed for the first time after 4.6 \(\times\) 10\({}^{7}\) fatigue cycles.  
To monitor and visualize the development of incipient damage, ultrasonic inspections were performed periodically during the test using the Olympus OMNISCAN SX (Olympus, Tokyo, Japan) phase array ultrasonic tester (PAUT). According to the PAUT inspection results, the health condition of the test specimen was divided into five stages, as indicated by different colors in Figure 8. The number of records for each state is presented in Table 1. The short-time Fourier spectrograms of several randomly chosen AE signals, which are typically observed during the five stages of the damage propagation, are presented in Figure 9. It can be seen that the fault signatures at the early stage of the damage (corresponding to 0.5 mm and 1 mm length of the internal crack) are still invisible to the naked eye. As the faults grow up to 1.5 mm, some high-frequency components induced by AE bursts emerge gradually, and the number of AE bursts increases with the damage development.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Health Condition** & **Number of Records** & **Number of Fatigue Cycles** \\ \hline No damage & 542 & \(2.8\times 10^{7}\) \\
0.5 mm crack & 377 & \(3.6\times 10^{7}\) \\
1 mm crack & 809 & \(4.8\times 10^{7}\) \\
1.5 mm crack & 718 & \(6.5\times 10^{7}\) \\
2 mm crack & 25 & \(6.6\times 10^{7}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: The number of AE records for different stages of damage propagation.  
Figure 8: Display the raw AE signals according to the fatigue cycles.  
Figure 7: Display of the rolling fatigue test rig. (**a**) Photographic image and schematics of the geometry of supporting rollers and the testing roller, and (**b**) a close-up view of the setup instrumented with sensors.