## 4 Results and Discussion
### Damage Identification  
Piston rod fracture induces a high-frequency AE signal, primarily due to the rapid release of stored elastic energy within the material during fracture. The combination of the high fracture rate, low ductility, and rapid crack propagation contributes to steep strain gradients in the crack tip path, resulting in the generation of high-frequency stress waves. These events are associated with the formation of microcracks and the release of stored elastic energy in the form of high-frequency stress waves. The deformation or fracture of metals initiates various microstructural changes, including the formation [55] and propagation of dislocations [56], cracks [57; 58], and other defects, which cause rapid fluctuations in the local stress field within the material. Consequently, this phenomenon leads to the propagation of stress waves throughout the material. The signal analysis results from the previous section suggest that, during the load reversal of the piston rod in a compressor, a significant crack signal is triggered when the piston rod experiences an impact along the tensile direction generated by the contact between the crosshead pin and the bearing during the reversal direction. Both this impact force and cyclic tensile loading can initiate crack propagation, which can be effectively detected by the AE signal. Following the above analysis, the initial AE signal was subjected to filtering, with a focus on retaining signal band components between 70 and 150 kHz. For a single-cycle AE sub-band signal, distinct calculations are performed for AE counts, energy, peak-to-peak values, and waveform factors. Figure 15 depicts the changes in the various AE parameters concerning the number of fatigue loading cycles. In Figure 15a,b, the AE counts and energy exhibit heightened sensitivity to the rapid crack propagation. A marginal increase in their fluctuation ranges is observed within the initial 20,000 cycles, indicating the initiation of a limited number of microcracks. The AE counts experience a doubling of the peak fluctuation from less than 150 to 300, while the peak AE energy rises from 15 to 25 during this early stage. As the crack enters the phase of rapid expansion (post-20,000 cycles), both the AE counts and energy values demonstrate a pronounced escalation, attributable to the emergence of numerous high-amplitude impact components associated with crack expansion. Before 25,000 cycles, fluctuations in their growth patterns suggest some discontinuity in the initial stages of crack extension. Beyond 25,000 cycles, with the formation of macroscopic cracks, the cross-sectional area supporting the load diminishes, intensifying the stress concentration and accelerating the crack extension. Consequently, the AE counts and energy exhibit sustained increases. Linear fitting estimates indicate that, during the rapid crack expansion phase after 20,000 cycles, the growth rate of the AE counts is approximately 20 times that before  
Figure 14: Sub-band AE signals at different stages (70–150 kHz). No crack propagation (**a**). The early stage of crack propagation (**b**). The stage of approaching fracture (**c**). 20,000 cycles, and the growth rate of the AE energy is approximately 30 times, highlighting their increased sensitivity as indicators of crack expansion. On the other hand, the peak-to-peak and waveform indices of the signals show increased sensitivity during the rapid propagation phase of the crack. As shown in Figure 15c, the amplitude of the AE signal increases significantly from less than 5 to the 15-20 range as crack propagation intensifies. The impulse index, a dimensionless parameter used to detect the presence of impact in the signal, shows an early response, revealing a burst-type signal due to crack formation around 10,000 cycles. In the later stages, the impulse index decreases slightly as the crack expansion component of the signal continues to increase due to tensile loading.