## 3 Results and Discussion
### AE Behaviors and Fatigue Crack Growth Mechanism  
After confirming that cluster 1 is related to tensile crack fractures, cluster 2 is related to cyclic plastic deformations, and cluster 3 is related to shear crack fractures, the behaviors of each cluster are analyzed to clarify the mechanism of fatigue crack growth further for both G20 Mn5 QT cast steel and related butt welds. The dissimilarities in the AE signals among different clusters are reflected mainly in terms of the parameters of rise time, duration, and ringing count (see Figure 13). It can be noted that these three parameters increase with the increase in the cluster number. Therefore, AE signals released from shear crack fractures exhibit the largest rise time, duration, and ringing count; AE signals released from cyclic plastic deformations have the second-largest rise time, duration, and ringing count; and the AE signals released from tensile crack fractures have the smallest rise time, duration, and ringing count. Moreover, the corresponding three characteristics for the weld are larger than those pertaining to G20 Mn5 QT. This phenomenon has also been noted for metal 316 LN [39]. Generally, an AE signal with a larger rise time, duration, and ringing count carries a larger amount of energy. Therefore, shear crack fractures have the maximum average energy carried by an AE signal, cyclic plastic deformations have the second-largest average energy carried by an AE signal, and tensile crack fractures have the least average energy carried by an AE signal. Figures 14 and 15 show the cumulative hit and energy of each cluster for both G20 Mn5 QT and welds. In Figures 14 and 15, the X-axis denotes the stress intensity factor range, and the Y-direction indicates the percentage of each cluster for all signals at the end of the experiment. For G20 Mn5 QT cast steel, the increase in the AE hit is primarily due to the tensile crack fractures, whose contribution is much greater than that of clusters 2 and 3. However, cluster 2 has the largest contribution with regard to the increase in AE energy, and the corresponding contribution of cluster 1 is the second largest, being only slightly less than that of cluster 2. At the end of the test, clusters 1, 2, and 3 are noted to contribute 73.24%, 23.19%, and 3.57% to the cumulative hit and 39.56%, 45.35%, and 15.09% to the cumulative energy, respectively, as shown in Figure 14. At fatigue stage I and the beginning of stage II, both the hit and energy exhibit a fast growth rate. Figure 11: Correlation plot between RA and AF. values generated by clusters 1 and 3 of the weld are slightly less than those generated by clusters 1 and 3 of G20 Mn5 QT. Based on the analysis discussed previously, fatigue crack growth is a combination of cyclic plastic deformation, tensile crack fractures, and shear crack fractures. Generally, AE signals pertain to stress waves released by deformations or crack fractures, and the signals' energies detected by the AE sensors are positively associated with the energies released by the deformations or fractures [57]. Cyclic plastic deformations before the crack tip release the maximum amount of energy, which is basically equal to the sum of the energies released by the tensile crack fractures and shear crack fractures. However, tensile crack fractures  
Figure 14: Cumulative AE of different clusters for G20 Mn5 QT: (**a**) hit; (**b**) energy. Figure 13: Wave characteristics for each cluster: (**a**) G20 Mn5 QT; (**b**) weld. Figure 15: Cumulative AE of different clusters for the weld: (**a**) hit; (**b**) energy. have the maximum number of occurrences, and this number is considerably larger than those for the others. Furthermore, fatigue crack propagation can be regarded as a discontinuous crack tip advancement caused by fatigue damage accumulation before the crack tip, which has also been observed in an annealed, commercially pure iron (JIS-C2504) [58].