## 4 Results and Discussion
### Crack Signal Initiation Mechanism  
A comparison of the time-domain waveforms and time-frequency spectrums of the AE signals in the crack-free and crack-propagation stages is shown in Figure 12. The time-domain signals show a significant increase in the amplitude of the AE signal from approximately 3 V to 10 V during the crack extension. In the absence of crack propagation, the impact signals are predominantly clustered in the low-frequency band around 35 kHz at the 80-100\({}^{\circ}\) and 260-280\({}^{\circ}\) stages. As the crack propagates rapidly, noticeable impact signals appear at 80-100\({}^{\circ}\) and 260-280\({}^{\circ}\). A phase characterised by the coexistence of sudden impact signals and continuous AE signals of increasing amplitude appears at 310-360\({}^{\circ}\). This segment is mainly concentrated in the 70-150 kHz frequency band, as shown in the red dashed box in Figure 12d. The occurrence of these signals corresponds to the phase angle associated with the rod load commutation and the subsequent change in the rod load. During the operational cycle, the piston rod experiences cyclic tensile and compressive loads, which exert pressure on the crosshead pin towards one side of the small bearing  
Figure 11: Fracture occurring when the piston is running towards the TDC. Schematic diagram of the secondary collision at the cylinder head end **(a)**; photographs of the fracture morphology of the prefabricated fracture rods **(b)**. Figure 12: Time-domain AE signals and time–frequency spectrum. Time-domain AE signal in the stage of no crack propagation **(a)**; time–frequency spectrum of the AE signal in the stage of no crack propagation **(b)**; time–domain AE signal in the stage of rapid crack propagation **(c)**; time–frequency spectrum of the signal in the stage of rapid crack propagation **(d)**. of the connecting rod, creating a clearance on the opposite side to allow for the entry of lubricating oil and to achieve cooling, as shown in Figure 13a. The reversal of the piston rod load must be sustained for a particular period to ensure sufficient lubrication. In Figure 13b, impacts R1 and R2 occur when the piston rod load is at zero, leading to the switch of the contact position of the bearing and pin, generating an impact event. The stress cloud illustrates that, at impact R1, the bearing and pin contact position switches from the head end to the crank end, imposing an impact force on the piston rod in the compression direction. Conversely, at impact R2, the contact position changes from the crank end to the head end, resulting in an impact in the tensile direction on the piston rod, which serves as the primary trigger for the extension of cracks in the piston rod. The fatigue cracks gradually develop and propagate as the number of cycles increases, and there is a clear tendency for the amplitude of the AE signal to increase at the R2 impact position. In addition, during the fatigue process, the material is subjected to stress changes through alternating cycles of loading and unloading. Even in regions where the point of maximum stress has not been reached, the material may encounter small stress concentrations induced by cyclic loading. These concentrations may be sufficient to initiate and propagate cracks before the point of maximum stress is reached. Consequently, events in the AE signal indicative of crack propagation appear in the high-frequency band when the piston rod is under stress and the piston has not yet reached the TDC (\(\theta=360^{\circ}\)), as shown in Figure 12d. In light of the previous analysis, the AE signals are subjected to bandpass filtering in the range of 70-150 kHz. Figure 14 shows the sub-band signals at various stages.