## 4 Experimental Investigation  
To check the accuracy of the proposed method, an experiment was conducted on the rotor bearing test rig. The FCBB was equipped in the test rig, and the system was driven by a motor, as shown in Figure 13. Test bearings were inch type R16 bearings. The test parameters are given in Table 2.  
Figure 12: Empirical mode decomposition diagram of simulated fault vibration signal.  
Figure 13: MFS-MG mechanical fault comprehensive simulation experimental bench.  
In order to obtain the effect of subsurface mesoscale cracks on running performance of FCBB, two samples of healthy FCBB(zirconia) were set to run continuously at radial force of 300 N and operating speed of 1800 r/min for 50 h. Then, the two faulty bearings were tested through ultrasonic testing equipment and industrial computerized tomography (industrial CT), respectively. The obtained faulty bearings were placed on the \"MFS-MG mechanical fault comprehensive simulation experimental bench\" for testing. The results are displayed in Figure 14.  
As shown in Figure 14, the nondestructive testing methods provided the internal morphology of the FCBB. After continuously running for 50 h, some subsurface mesoscale cracks started to appear in FCBB, and are seen in ultrasonic results as dots. In the industrial CT results, a mesoscale crack with a depth of about \(d\) = 2.0 mm is seen clearly.  
The tested bearing was then installed in the test rig again to obtain the dynamic response with mesoscale cracks, and displacement sensors were arranged on the pedestal to collect the signals. The signals were transmitted to the data collector for further processing. The vibration signal is shown in Figure 15a, and the corresponding simulation result is given in Figure 15b.  
As shown in Figure 15, at the crack length \(d\) = 2.0 mm, periodic small wave peaks appear in the time-domain signal of the test bearing, which is consistent with the simulation results. The existence of small wave peaks in the test signal determines the existence of subsurface mesoscale cracks in the outer ring of the bearing, and the accuracy of the proposed model is thus verified. However, the amplitudes of both the overall vibration signal and the small wave peaks are influenced by the background noise and other vibration sources,  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Name** & **Numerical Value** \\ \hline Spindle speed \(\omega_{\text{s}}\) (r/min) & 1800 \\ Radial load \(F_{\text{r}}\) (N) & 50 \\ Sampling frequency (Hz) & 3000 \\ Outer diameter of bearing outer ring (mm) & 50.8 \\ Inner diameter of bearing inner ring (mm) & 25.4 \\ Bearing width (mm) & 7.144 \\ Number of rolling elements & 9 \\ Contact angle (\({}^{\circ}\)) & 0 \\ \hline \hline \end{tabular}
\end{table}
Table 2: FCBB outer ring subsurface mesoscale crack test parameters.  
Figure 14: Failure morphology by nondestructive testing methods.  
and the recognition of the small wave peaks is also affected. The collected vibration signal is transformed into frequency domain through FFT to obtain the frequency components, as shown in Figure 16.  
As shown in Figure 16, \(f_{0}\) is the rotational frequency. The amplitude corresponding to the frequency of small wave peak is more prominent, which is the same as the simulation results in Figure 11. Thus, it is concluded that the frequency component signifies the existence of mesoscale cracks; it is easier to recognize and is more decisive for the quantization of subsurface faults.