## 3 Results and Discussion  
### Fatigue Crack Growth and AE Activity  
The fatigue crack growth rates of the G20 Mn5 QT and butt welds were compared. Figure 5 shows the relationship between the fatigue crack length and the fatigue load cycles. Figure 6 shows the relationship between the fatigue crack growth rate \(\Delta{}K\) and the stress intensity factor range \(\Delta{}K\). It was noted that compared to the G20 Mn5 QT, the weld requires a larger number of load cycles to reach the same crack length. The fatigue crack growth rate of the weld is less than that of G20 Mn5 QT at low values of \(\Delta{}K\); however, the corresponding rates tend to be similar at high values of \(\Delta{}K\). The results obtained by fitting the fatigue crack growth rate curves according to the famous Paris law [42] (see Equation (3)) on double logarithmic coordinates can be represented as in Equation (4) (R\({}^{2}\) = 0.97) for G20 Mn5 QT and Equation (5) (R\({}^{2}\) = 0.95) for the weld.  
\[\log_{10}(da/dN) =m\log_{10}(\Delta{}K)+\log_{10}C \tag{3}\] \[\log_{10}(da/dN) =3.07\log_{10}(\Delta{}K)-8.34\] (4) \[\log_{10}(da/dN) =3.78\log_{10}(\Delta{}K)-9.52 \tag{5}\]  
According to the fatigue crack growth rate (FCGR) curves, the fatigue crack growth process can be divided into three stages, as shown in Figure 6. The transition from stage I to stage II pertains to crack initiation progressing to stable crack growth. The transition from stage II to stage III corresponds to the crack demonstrating unstable crack growth. The AE activities of G20 Mn5 QT and the weld vary considerably at different fatigue crack growth stages. The differences between \(\Delta{}K\), AE hit release rates, and energy release rates for G20 Mn5 QT and the weld are shown in Figure 7. For G20 Mn5 QT, the hit and energy release rates reach the first peak at the end of stage I and later decline rapidly. For the weld, the hit and energy release rates also increase in stage I, but they reach the first peak at the boundary between stage I and stage II and decline gradually compared to the corresponding values for the case of G20 Mn5 QT at the beginning of stage II. In stage II, the AE activity remains stable and relatively low level at first; however, it resumes growth in the middle of stage II as the fatigue crack propagation approaches stage III. The regrowth point for G20 Mn5 QT appears earlier than it does for the weld. In stage III, the fatigue crack rapidly grows to the point of ultimate failure, and the hit and energy release rates become discrete and unstable.  
Figure 5: Fatigue crack length vs. load cycles.  
Therefore, the cumulative hit and energy of G20 Mn5 QT demonstrate rapid accumulation in stage I and later demonstrate a stable increase in the first half of stage II. In contrast, the cumulative hit and energy of the weld indicate a high increase rate at the beginning of stage II, as shown in Figure 8. An obvious inflection point on the cumulative curve of the weld in stage II can be observed, and it divides stage II into two sub-stages: IIa and IIb. However, the G20 Mn5 QT material does not demonstrate such a behavior involving sub-stages. Moreover, the AE activity of the weld is higher than that of G20 Mn5 QT when comparing the cumulative AE hit and energy, as shown in Figure 8. This phenomenon was also reported in some studies focusing on the base metal and weld [17].  
Figure 6: Fatigue crack growth rate vs. stress intensity factor range.  
Figure 7: AE hit and energy release rates.