## 4 Conclusions  
In this research, fatigue crack growth experiments for G20 Mn5 QT and related butt welds were performed using CT specimens, and the corresponding AE signals were collected. By comparing and analyzing the AE behaviors, the following conclusions could be obtained:  
1. The AE activities vary greatly in different fatigue crack growth stages. The AE release activity of G20 Mn5 QT and butt welds reaches a peak at the end of fatigue crack stage I and decreases subsequently, demonstrating a relatively low level in the first half of stage II. The AE release activity returns to an increasing trend in the middle of stage II. The regrowth point for G20 Mn5 QT occurs earlier than that for the weld. Moreover, stage II of the welds can be further divided into two sub-stages based on the AE activity; however, G20 Mn5 QT does not exhibit such behavior.
2. By introducing the K-means cluster algorithm, the AE signals for both G20 Mn5 QT and the weld can be divided into three clusters: cluster 1 pertains to the burst type with a short rise time and duration and is related to tensile crack fractures during fatigue crack growth; cluster 2 pertains to the continuous type with a short rise time and longer duration and is related to cyclic plastic deformations before the crack tip; and cluster 3 signals are similar to the burst type albeit with a long rise time and duration, and they are related to shear cracks.
3. Fatigue crack growth is a combination of cyclic plastic deformations before the crack tip, tensile crack fractures, and shear crack fractures. Crack tip advancement can be regarded as the discontinuous process of the critical area brittlely rupturing as a large number of tensile crack fractures and rare shear crack fractures with fatigue damage due to cyclic plastic deformation reach critical status. The fatigue damage accumulation process releases around half of the total energy. The energy released by a single tensile crack fracture is quite small, but a large number of tensile crack fractures make the total accumulated energy slightly less than the energy released by fatigue damage. The energy released by a single shear crack fracture is larger than that released by a single tensile crack fracture, but the total accumulated energy is the least.
4. Shear crack fractures in fatigue crack growth are induced by local multi-states of stress and strain caused by casting or welding defects. Shear crack fractures demonstrate discontinuous occurrences and pertain to the release of large average energies. The material weakening caused by the defects of G20 Mn5 QT is greater than that caused by the defects of the weld.  
Figure 17: Scanning electron micrographs of crack surfaces: (a) shear bands of G20 Mn5 QT cast steel; (b) shear bands of butt weld.  
**Author Contributions:** Conceptualization, Q.L. and Z.Z.; methodology, G.L.; software, Y.X.; formal analysis, Q.L.; writing--original draft preparation, Y.X.; writing--review and editing, Q.L., J.X. and G.L.; supervision, G.L.; funding acquisition, J.X. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Natural Science Foundation of China (No. 51408408) and the Tianjin Natural Science Foundation (16)CQNJC07400).  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author due to privacy.  
**Conflicts of Interest:** Authors [PERSON] and [PERSON] was employed by the company Shandong Electric Power Engineering Consulting Institute Corp., Ltd. The remaining authors declare that the re-search was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.