## 2 Experimental Section
### Fatigue Crack Growth Tests  
All the specimens were tested on an electrohydraulic servo fatigue testing machine under ambient temperature. The setup of the test is illustrated in Figure 4. The tests were carried out under sinusoidal cyclic loads with a maximum load of 18 kN and a load ratio of 0.5. In this test, the load frequency of 20 Hz was used to prefabricate the fatigue crack, and the load frequency of 4 Hz during the main phase of the fatigue test was applied to facilitate the precise observation of fatigue cracks. The stress intensity factor range \(\Delta K\) could be calculated via Equation (1) [40].  
\[\alpha=a/W \tag{1}\] \[\Delta K=\frac{\Delta P}{B\sqrt{W}}\cdot\frac{(2+\alpha)}{(1-a)^{3/2}} \left(0.886+4.64\alpha-13.32\alpha^{2}+14.72\alpha^{3}-5.6\alpha^{4}\right),\]  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & **C** & **Mn** & **Si** & **S** & **P** & **Ni** & **Fe** \\ \cline{2-8}  & **wt. \%** & **wt. \%** & **wt. \%** & **wt. \%** & **wt. \%** & **wt. \%** & **wt. \%** \\ \hline G20 Mn5 QT & 0.190 & 1.340 & 0.400 & 0.008 & 0.014 & 0.600 & Bal. \\ ERS0-6 & 0.077 & 1.450 & 0.870 & 0.013 & 0.012 & 0.010 & Bal. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of G20 Mn5 QT base material and ER50-6 weld wire.  
Figure 1: Microstructures: (**a**) G20 Mn5 QT cast steel; (**b**) welds.  
where \(a\) is the crack length, which represents the distance between the fatigue crack tip and the loading centre line; \(W\) is the geometry parameter of the specimen (=100 mm in this study); \(\Delta K\) is the stress intensity factor range; \(\Delta P\) is the load range; and \(B\) is the specimen thickness (=5.5 mm in this study).  
The fatigue crack length was measured using two different methods (see Figure 4). One was the visual measurement method, in which a high-magnification digital camera was employed, and a scale ruler was engraved on the surface of the specimen parallel to the crack. The test machine and the camera began operation simultaneously. The camera captured images of the crack tip every 4 min (960 load cycles). The crack length in the images was then measured. The other method was the compliance method. A COD extensometer was employed to measure the crack opening displacements every 400 load cycles, and the crack size was calculated based on Equation (2) [40].  
\[\mu_{\chi}=\left[\left(\frac{E\
u B}{P}\right)^{1/2}+1\right]^{-1} \tag{2}\] \[a/W=1.0010-4.6695\mu_{\chi}+18.460\mu\chi^{2}-236.82\mu\chi^{3}+1 214.9\mu\chi^{4}-2143.6\mu\chi^{5}\]  
where \(E\) denotes the modulus of elasticity; \(\
u\) is the crack opening distance; \(B\) denotes the specimen thickness; and \(P\) is the load. The values measured using each method were  
Figure 3: Geometry of CT specimen.  
Figure 2: Specim machining zone.  
compared and plotted in the same figure and subsequently fitted into a crack length-fatigue load cycle curve.  
The fatigue crack growth tests were terminated at a crack length of 55 mm as at this point, the crack had already reached the unstable growth stage. After the fatigue crack growth tests, a scanning electron microscope (SEM) was employed to observe the fracture surfaces of all specimens.